The Higgs Boson: So, what?

Professor Bill Murray

Professor Murray is a joint academic with Warwick University. He was the ATLAS Higgs Convener in the run-up to Higgs discovery 2012 and then Physics Coordinator. More recently he has coordinated the ATLAS Tracking in Dense Environments group and is currently Higgs and Diboson Searches Convener. He also engages with future collider projects. ​


Professor Murray also works at the Rutherford Appleton Laboratory understanding the interactions and properties of the Higgs boson using the ATLAS detector at the Large Hadron Collider at CERN and he searches for new physics, especially dark matter.


The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope Professor Murray, and my readers will forgive any mistakes and let me know what I got wrong.

In this lecture Professor Murray talked about the Higgs Boson.


Candidate Higgs boson events from collisions between protons in the LHC. The top event in the CMS experiment shows a decay into two photons (dashed yellow lines and green towers). The lower event in the ATLAS experiment shows a decay into four muons (red tracks)

The Higgs boson is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory.

Professor Murray has been based at CERN for about 10 years and over this time he was particularly involved with the discovery of the Higgs Boson. Since the discovery of the Higgs in 2012 he has been attempting to understand it.

In his talk he aimed to explain how the Higgs was discovered, why particle physicists were looking for it and what they have learned about it.


The above image is a slide about the Standard Model that the Rutherford Appleton Laboratory produced towards the end of the 20th century. It shows all the different particles that were believed to exist at the time.

The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, and not including the gravitational force) in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

In the first half of the twentieth century physicists had recognised that atoms, which are the smallest unit ordinary matter that form chemical elements, had a nucleus consisting of protons and neutrons with electrons going around it.


In chemistry and physics, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines an isotope’s mass number (nucleon number).

Until the 1960s, nucleons were thought to be elementary particles, not made up of smaller parts. Now they are known to be composite particles, made of three quarks bound together by the strong interaction. The interaction between two or more nucleons is called internucleon interaction or nuclear force, which is also ultimately caused by the strong interaction. (Before the discovery of quarks, the term “strong interaction” referred to just internucleon interactions.)

A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons. Protons and neutrons are examples of hadrons.

In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10−15 m (1 femtometre), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 1038 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. (below left)


Murray Gell-Mann (September 15, 1929 – May 24, 2019) was an American physicist who received the 1969 Nobel Prize in Physics for his work on the theory of elementary particles (above right)

George Zweig (born May 30, 1937) is a Russian-American physicist.

Initially it was thought there were only two types of quark, the up and down.

A proton was made up two up quarks and one down quarks. A neutron was made up of two down quarks and one up quark


We now know there are six of them. As well as the up and down there is the charm, strange, top and bottom quarks. The top quark is in fact the heaviest quark with a mass equal to a gold atom.

It could, in fact be argued that there are 18 quarks because they come in one of three different “colours”. A proton and a neutron must have one of each “colour” in their nucleus.

The “colour charge” of quarks is completely unrelated to the everyday meaning of colour. The term colour and the labels red, green, and blue became popular simply because of the loose analogy to the primary colours.

The electron is a fundamental particle and belongs to a group of particles called leptons.

The first lepton identified was the electron, discovered by J.J. Thomson and his team of British physicists in 1897. (below left)


Sir Joseph John Thomson OM PRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered.

In 1930, Wolfgang Pauli postulated the electron neutrino to preserve conservation of energy, conservation of momentum, and conservation of angular momentum in beta decay. (above right)

The name lepton comes from the Greek meaning “fine, small, thin” and was first used by physicist Léon Rosenfeld in 1948.


Léon Rosenfeld (14 August 1904 in Charleroi – 23 March 1974) was a Belgian physicist

We now know there are six leptons. As well as the electron and electron neutrino, there is the muon, muon neutrino, Tau and tau neutrino, which was discovered in 2001.

Particle physicists are fairly convinced that there won’t be a fourth row, but they have no idea why there are three rows.

Quarks and leptons are classed as fermions.

Fermions are sometimes called matter particles, because they are the particles that make up most of what we think of as physical matter in our world, including protons, neutrons, and electrons.


Fermions were first predicted in 1925 by the physicist Wolfgang Pauli, who was trying to figure out how to explain the atomic structure proposed in 1922 by Niels Bohr. Bohr had used experimental evidence to build an atomic model which contained electron shells, creating stable orbits for electrons to move around the atomic nucleus. Though this matched well with the evidence, there was no particular reason why this structure would be stable and that’s the explanation that Pauli was trying to reach. He realized that if you assigned quantum numbers (later named quantum spin) to these electrons, then there seemed to be some sort of principle which meant that no two of the electrons could be in exactly the same state. This rule became known as the Pauli Exclusion Principle. (below left)


Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922.

The image above right is the Bohr model of the hydrogen atom. A negatively charged electron, confined to an atomic orbital, orbits a small, positively charged nucleus; a quantum jump between orbits is accompanied by an emitted or absorbed amount of electromagnetic radiation.

Bohr model – Wikipedia

In 1926, Enrico Fermi and Paul Dirac independently tried to understand other aspects of seemingly-contradictory electron behaviour and, in doing so, established a more complete statistical way of dealing with electrons. Though Fermi developed the system first, they were close enough and both did enough work that posterity has dubbed their statistical method Fermi-Dirac statistics, though the particles themselves were named after Fermi himself. (below left)

image (above right)

Paul Adrien Maurice Dirac OM FRS(8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century.

So, a fermion is a type of particle that obeys the rules of Fermi-Dirac statistics, namely the Pauli Exclusion Principle. These fermions also have a quantum spin with contains a half-integer value, such as 1/2, -1/2, -3/2, and so on.

The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously.

As well as the fermions there are the Bosons. They are force particles.

In quantum mechanics, a boson is a particle that follows Bose–Einstein statistics. the elementary bosons are force carriers that function as the ‘glue’ holding matter together. This property holds for all particles with integer spin (s = 0, 1, 2, etc.)

An important characteristic of bosons is that there is no restriction on the number of them that occupy the same quantum state.

Examples of bosons are fundamental particles such as photons, gluons, and W and Z bosons (the four force-carrying gauge bosons of the Standard Model).

Photons are the force carriers of the electromagnetic field.

W and Z bosons are the force carriers which mediate the weak force.

Gluons are the fundamental force carriers underlying the strong force.

An electromagnetic field (also EM field) is a classical (i.e., non-quantum) field produced by accelerating electric charges. It is the field described by classical electrodynamics and is the classical counterpart to the quantized electromagnetic field tensor in quantum electrodynamics. The electromagnetic field propagates at the speed of light (in fact, this field can be identified as light) and interacts with charges and currents. Its quantum counterpart is one of the four fundamental forces of nature. The photon is the carrier particle for electromagnetism. It is a massless particle that we see as light. It’s what’s responsible for us being able to see.

In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction also participates in nuclear fission. The W and Z bosons are the carriers of the weak nuclear force, which is quite which is important for the sun to shine. For the sun to shine, you have to turn four protons into two protons and two neutrons of the core of a helium and the weak force is the only thing that can do that.

In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10−15 m (1 femtometre), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 1038 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton. The gluon is the carrier for the strong nuclear force and binds together the components of the atom.

In quantum statistics, Bose–Einstein (B–E) statistics describe one of two possible ways in which a collection of non-interacting, indistinguishable particles may occupy a set of available discrete energy states.

The idea was adopted and extended by Albert Einstein in collaboration with Bose. (below left)


Albert Einstein (March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics) (above right)

Satyendra Nath Bose FRS (1 January 1894 – 4 February 1974) was an Indian mathematician and physicist specialising in theoretical physics.

So, all these forces are important and physicists have understood them quite well. But one of the things that has puzzled them is that the W and Z bosons that are responsible for the weak nuclear force is are heavy, unlike the photon and the gluon.


The graviton is not included in the standard model because physicists, at the moment, have no idea how gravity works.

They understand the theory of special relativity, but they can’t actually fit it into the standard model yet. Maybe there is such thing as a graviton. At the moment they have no way of knowing.

In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein’s original treatment, the theory is based on two postulates:

The laws of physics are invariant (that is, identical) in all inertial frames of reference (that is, frames of reference with no acceleration).

The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.

So far there has been no mention of the Higgs. There was no mention of it on the “20th century Standard Model” slide. This was the model that physicists had accepted and were using from 1973 to 2012. But this model had a contradiction. All the particles, such as the W and Z bosons, were supposed to be massless, but this was clearly not true.

It was very difficult to explain why they had a mass. So, in 1964 Peter Higgs proposed a new field and particle which was, hopefully, going to solve the problem. What was his suggestion?


The above link is a copy of his paper.


Peter Ware Higgs CH FRS FRSE FInstP (born 29 May 1929) is a British theoretical physicist, Emeritus Professor in the University of Edinburgh, and Nobel Prize laureate for his work on the mass of subatomic particles.

Peter Higgs’ paper was quite short. He wrote about broken symmetries and the masses of gauge bosons.

In particle physics, a gauge boson is a force carrier, a bosonic particle that carries any of the fundamental interactions of nature, commonly called forces, which were mentioned earlier. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge bosons—usually as virtual particles. All known gauge bosons have a spin of 1.

In physics, a symmetry of a physical system is a physical or mathematical feature of the system (observed or intrinsic) that is preserved or remains unchanged under some transformation.

It is a very important idea in physics; it lies behind basic laws such as the conservation of energy, where secondary school students are taught that the energy cannot be created or destroyed. The total energy into a system equals the total energy out.

Arguably the most important example of a symmetry in physics is that the speed of light has the same value in all frames of reference.

If a system is symmetric under some change, it means it looks the same after the change… so the change is in some sense no change.

The idea of broken symmetry is just as important as the idea of symmetry. If the standard model was symmetric the W and Z bosons would be massless like the photon, but something is breaking that symmetry and the W and Z bosons have mass.

The two W bosons are verified mediators of neutrino absorption and emission. During these processes, the W± boson charge induces electron or positron emission or absorption, thus causing nuclear transmutation.

The Z boson mediates the transfer of momentum, spin and energy when neutrinos scatter elastically from matter (a process which conserves charge).

These bosons are among the heavyweights of the elementary particles. With masses of 80.4 GeV/c2 and 91.2 GeV/c2, respectively, the W and Z bosons are almost 80 times as massive as the proton – heavier, even, than entire iron atoms. The fact that they had mass while photons didn’t was a problem so some mechanism was required to break the symmetry, giving them mass

Ironically the W and Z bosons hadn’t even been discovered in 1964, when Professor Higgs proposed the mechanism, first put forward in the symmetry breaking paper mentioned above, that fulfilled the role. It required the existence of another particle, now known as the Higgs boson.

His short paper of just one and a half pages sparked off a 50 year search, and it’s amazing to notice a couple of things in the article. There’s one sentence in which he says

“It may be expected that one of these gauge fields will acquire mass” and that’s all he says about the Higgs boson. That’s the prediction of the Higgs boson being made there.


He does add a little footnote in the paper in which which he says that actually, his theory isn’t really a proper quantum mechanical theory, it’s only a classical theory. It doesn’t respect quantum mechanics, but it’s okay he says, because these other gentlemen. Brout and Englert have already produced the quantum mechanical version.

image (below left)


Robert Brout (June 14, 1928 – May 3, 2011) was a Belgian theoretical physicist who made significant contributions in elementary particle physics. (above right)

François, Baron Englert (born 6 November 1932) is a Belgian theoretical physicist and 2013 Nobel prize laureate.

Nobody really knows why it is called the Higgs boson and not the Brout or Englert particle.

So, what is this this Higgs field like?

Professor Murray likes the analogy of a fish tank, which contains lots of weeds, rocks and fish in it


If you take out the weeds and the rocks. You can see how many fish there are other more clearly the fish will probably call this an empty tank, but it still has water in it.


The Higgs field is rather like the water the fish are swimming through. Without it everything would be different but we can’t get out of it. If we could we would be able to move much more rapidly. Electrons would stop having mass and they would shoot into things, flying off in different directions. And, of course, we would all fall apart so we don’t want to get out of this field. The problem is that we don’t really know what this field is like.

The Higgs field explains the mass of the W and Z bosons, which are the carriers of the weak force. That is what it was designed to do. But it also describes the masses of the matter particles

So, the field explains the mass of the w in tempo songs. That’s what it was designed to do. But Professor Murray uses the word “described” deliberately. It doesn’t predict what they are. It just allows physicists to dial up a number, an uncertainty and control it. So, they can explain more of what they see.

Masses of matter particles are proportional to their interaction with the Higgs boson. The more massive the particle the greater the amount of the Higgs field it drags around. Or to put it another way the mass of the particle is simply the amount of the Higgs field it is dragging around with it.

So, what does the model predict? How do physicists know that it was right or not?

What the model tells physicists is that the W and Z bosons are massive and predicts the ratio of their masses. It doesn’t actually tell them the absolute value. But if one is measured, it tells them what the other one should be. This was the major prediction of the model. The model also states that the photon and the gluon law should be massless.

The model also states that the Higgs particle should be massive but have no spin. This in contradiction to all the other particles that have been mentioned so far, that do have spin.

In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.

In physics, angular momentum is the rotational equivalent of linear momentum (product of mass x velocity). It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a closed system remains constant.

This gyroscope in the image below remains upright while spinning due to the conservation of its angular momentum.


So, this massive spin-less Higgs boson interacts with nearly all particles in proportion to their mass, but its mass is unknown.

The model says that the vacuum should be filled up with these Higgs particles, a “sea” of weak charge and that the Higgs field should be everywhere.

The predictions about the massive W and Z bosons and the mass-less photon and gluon had been known for a long time. Their existence has been tested and proved.

Evidence of the Higgs particle and the fact that a vacuum is filled with a “sea” of them is a bit weaker.

A paper was published in 1976 which included various calculations.



The above graph shows branching ratios of the Higgs boson for different values of its mass. The curved are calculated from decay rates mentioned in the paper. It shows some of the possible decays.

The paper included an apology to experimentalists that the authors had no idea what the mass of the Higgs was and that they didn’t want big experimental searches to find out.

So, this was the theoretical landscape in the 1970s. Physicists didn’t really know what to do, where to go and didn’t particularly think it was worth putting too much effort in. But by the 1980s big experimental searches were becoming more sensible, more plannable and a 27 km tunnel below the French-Swiss border was drawn out and then dug to build and electron-positron collider (matter-antimatter) to measure the W and Z bosons and also to search for the Higgs boson.

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1 e, a spin of 1/2 (the same as the electron), and has the same mass as an electron. When a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more photons.

The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed.

It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP collided electrons with positrons at energies that reached 209 GeV. It was a circular collider with a circumference of 27 kilometres built in a tunnel roughly 100 m underground and passing through Switzerland and France. LEP was used from 1989 until 2000. Around 2001 it was dismantled to make way for the Large Hadron Collider, which re-used the LEP tunnel. To date, LEP is the most powerful accelerator of leptons ever built.


Construction of the LEP was significant undertaking. Between 1983–1988, it was the largest civil engineering project in Europe.

When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45 GeV each to enable production of the Z boson, which has a mass of 91 GeV. The accelerator was upgraded later to enable production of a pair of W bosons, each having a mass of 80 GeV (the mass of a proton is about 1GeV). LEP collider energy eventually topped at 209 GeV (200GeV is about the energy required to make one proton) at the end in 2000. At the end of 2000, LEP was shut down and then dismantled in order to make room in the tunnel for the construction of the Large Hadron Collider (LHC). It had discovered the W and Z boson but not the Higgs

Meanwhile, in America, they were planning to make an even larger device. The Superconducting Supercollider. The president of the time, Ronald Reagan invited US physicists to be bold.

The Superconducting Super Collider (SSC) (also nicknamed the desertron) was a particle accelerator complex under construction in the vicinity of Waxahachie, Texas.


Its planned ring circumference was 87.1 kilometres with an energy of 20 TeV per proton (40TeV collisions) and was set to be the world’s largest and most energetic collider with the aim of finding the Higgs. After 22.5 km of tunnel were bored and nearly two billion dollars were spent, the project was cancelled by congress in 1993 (21st October) due to budget problems. This was a huge blow to US particle physics, and half of the physicists left science.


Ronald Wilson Reagan (February 6, 1911 – June 5, 2004) was an American politician who served as the 40th president of the United States from 1981 to 1989 and became a highly influential voice of modern conservatism. Prior to his presidency, he was a Hollywood actor and union leader before serving as the 33rd governor of California from 1967 to 1975.

The United States Congress or U.S. Congress is the bicameral legislature of the federal government of the United States and consists of the House of Representatives and the Senate. The Congress meets in the United States Capitol in Washington, D.C. Both senators and representatives are chosen through direct election, though vacancies in the Senate may be filled by a governor’s appointment. Congress has 535 voting members: 100 senators and 435 representatives, the latter defined by the Reapportionment Act of 1929. In addition, the House of Representatives has six non-voting members, bringing the total membership of the Congress to 541 or fewer in the case of vacancies.

Europe was left looking for the Higgs at that point.

But before the SSC was closed there were a lot of arguments about it and a distinguished US physicist Leon Lederman published this book


The God Particle: If the Universe Is the Answer, What Is the Question? is a 1993 popular science book by Nobel Prize-winning physicist Leon M. Lederman and science writer Dick Teresi.

The book provides a brief history of particle physics, starting with the Pre-Socratic Greek philosopher Democritus, and continuing through Isaac Newton, Roger J. Boscovich, Michael Faraday, and Ernest Rutherford and quantum physics in the 20th century.

Particle physicists have been haunted by the title ever since. People keep asking them why is it called the God particle


Leon Max Lederman (July 15, 1922 – October 3, 2018) was an American experimental physicist who received the Wolf Prize in Physics in 1982, along with Martin Lewis Perl, for their research on quarks and leptons, and the Nobel Prize in Physics in 1988, along with Melvin Schwartz and Jack Steinberger, for their research on neutrinos.

Lederman explains in the book why he gave the Higgs boson the nickname “The Goddamn Particle”:

This boson is so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive, that I have given it a nickname: The Goddamn Particle. Why God Particle? Two reasons. One, the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing. And two, there is a connection, of sorts, to another book, a much older one…

So back in Europe there was still the electron-positron collider (LEP), which by about the year 2000 had gradually raised its energy up to 208 GeV. The particle physicists were hoping to produce the Higgs and Z boson together. Colliding an electron and positron to produce a Z and Higgs boson.

e+e –> ZH


In theoretical physics, a Feynman diagram is a pictorial representation of the mathematical expressions describing the behaviour and interaction of subatomic particles. The scheme is named after American physicist Richard Feynman, who introduced the diagrams in 1948.


Richard Phillips Feynman ForMemRS (May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superfluidity of supercooled liquid helium, as well as his work in particle physics for which he proposed the parton model. For contributions to the development of quantum electrodynamics, Feynman received the Nobel Prize in Physics in 1965 jointly with Julian Schwinger and Shin’ichirō Tomonaga.

There needs to be enough energy produced in the collisions for both the Z and the Higgs to be produced.


The Z had a mass of 91GeV so a Higgs could be produced if its mass is less than 115GeV

So, the LEP would be able to discover the Higgs if its mass was less than 115. Would it be enough?

Well, there was a lot of excitement in the summer of the year 2000 because physicists started to see events.


Above is a false coloured computer interpretation of two or three events.

An electron and positron have collided and there four “coloured” jets of particles coming out in different directs from the point of the collision. This could be interpreted as a Z and a Higgs one of them turning into pair of quarks and the other turning into a pair of particular quarks (pair of b quarks).

Had a Higgs boson with a mass mH ~ 115GeV been produced or was it a pair of Z bosons?

Well, particle physicists do what they always do. They calculate the statistical probability of seeing what was seen using some different hypotheses. Is the particle a Higgs or is the particle not a Higgs?

The image below shows (a) Display of a Higgs candidate event from the four-jet category, recorded by the ALEPH experiment and (b) Plot showing the combination of the reconstructed Higgs mass distributions (Higgs candidate masses) for all channels and all LEP experiments


The above shows the result of some statistical analysis of all the data available at the time,

The yellow area (background) of the graph is what is predicted on this mass spectrum for known production of Z bosons

The red area is what physicists expect to see if there was a Higgs particle as well with a mass of 115GeV. They would expect to see three Higgs events. And there are three events at 119, 118 and 114.4GeV?

ALEPH was a particle detector at the Large Electron-Positron collider (LEP). It was designed to explore the physics predicted by the Standard Model and to search for physics beyond it.

But the probability of seeing something like that just by random fluctuations was 9% (~1.7 sigma). Also, there was so much background so the results weren’t very convincing evidence for the Higgs.

The data didn’t show convincingly that there was something new there. The only definite statement that could be made was that if the Higgs was present its mass must be greater than 14.4GeV as every lower mass had been examined.

Then, of course, the LEP was closed in 2000 leaving great uncertainty.

Meanwhile Fermilab in the US was running a 6km ring Tevatron collider.

Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics. Since 2007, Fermilab has been operated by the Fermi Research Alliance, a joint venture of the University of Chicago, and the Universities Research Association (URA). Fermilab is a part of the Illinois Technology and Research Corridor.


The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as Fermilab), east of Batavia, Illinois, and is the second highest energy particle collider ever built, after the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.28 km ring to energies of up to 1 TeV, hence its name. The Tevatron was completed in 1983 at a cost of $120 million and significant upgrade investments were made during its active years of 1983–2011.

The main achievement of the Tevatron was the discovery in 1995 of the top quark — the last fundamental fermion predicted by the Standard Model of particle physics and in about 1998 it was getting ready to look for the Higgs boson.

The Tevatron was colliding protons with anti-protons and the energy achieved was 2 TeV (2000GeV). This was ten times what the LEP could achieve, but less than the LHC’s 14TeV. However, it would have been able to create Higgs bosons

The problem with proton collisions is that they are very messy. Essentially two bags of quarks are colliding and a lot of debris emerges. The Higgs will rarely emerge and when they do they will very quickly decay into pairs of known particles.

A lot of collisions and a lot of data would be required

20fb-1 was estimated to give a 5s clear proof that a particle was the Higgs boson

A barn (symbol: b) is a metric unit of area equal to 10−28 m2 (100 fm2). Originally used in nuclear physics for expressing the cross-sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is approximately the cross-sectional area of a uranium nucleus. The barn is also the unit of area used in nuclear quadrupole resonance and nuclear magnetic resonance to quantify the interaction of a nucleus with an electric field gradient. While the barn never was an SI unit, the SI standards body acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use in particle physics.

The inverse femtobarn (fb−1) is the unit typically used to measure the number of particle collision events per femtobarn of target cross-section, and is the conventional unit for time-integrated luminosity. Thus, if a detector has accumulated 100 fb−1 of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data.

Sigma (s) is a measurement of the confidence in a result. 5s means you are 100% sure of your results.

In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values.

20 was 200 times more than the Tavatron, when the top quark was discovered.

So, the big challenge was to increase the data rate of the LEP by a factor of 200. This was expected to take from 2000 to 2007.

This was never achieved. Antiprotons are very hard to make and by 2011 the LEP had only delivered 11fb-1 and their results weren’t decisive enough. Luckily the LHC was up and running by then as the Tevatron was closed


Weekly and total integrated luminosity over Tevatron Run II (2001-2011).

Before the Tevatron closed it did provide a hint of the Higgs boson


Complicated plot showing the chance of seeing what was done as a function of mH, if no Higgs. The local background p-value is shown as a function of Higgs boson mass for the combined CDF and D0 searches in all channels. The solid lines show the observed values (the probability of seeing what you did see), the dark short-dash lines show the expected value, and the dark- and light- shaded bands indicate the one and two standard deviation probability regions in which the values are expected to fluctuate.

The solid black line dipped at 120GeV in the ZH, WH of H decaying to two b quarks mode. The probability is 1 in a 1000 at 3s. However, 1 in a 1000 is not conclusive proof, it could still be chance. No actual proper discovery at that point.

So, the LHC was then built to replace the electron collider (the 27km LEP tunnel was reused for the proton-proton collider) and extra caverns were dug.


The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres in circumference and as deep as 175 metres (ATLAS is about 100m below ground) beneath the France–Switzerland border near Geneva.

The collider has four crossing points (ATLAS, CMS, LHCb and ALICE are the four sites for experiments), around which are positioned seven detectors, each designed for certain kinds of research. The LHC primarily collides proton beams, but it can also use beams of heavy ions: lead–lead collisions and proton–lead collisions are typically done for one month per year. The aim of the LHC’s detectors is to allow physicists to test the predictions of different theories of particle physics, including measuring the properties of the Higgs boson and searching for the large family of new particles predicted by supersymmetric theories, as well as other unsolved questions of physics.

The LHC is designed to give 14 TeV collisions (7TeV from each proton). So, seven times more than the Tevatron in the United States

A lot of collisions. Bunches of protons will cross at 40 megahertz. This means that bunches of protons will pass through each other 40 million times a second. Protons are very easy to produce.


To make the protons”, physicists inject hydrogen gas into the metal cylinder -Duoplasmatron – then surround it with an electrical field to break down the gas into its constituent protons and electrons. This process yields about 70 percent protons.


Every time the proton bunches pass through each other 23 protons will smash into another 23 protons. If you multiply the “40 million times a second” with this 23, you get about 800 million pairs of protons colliding with each other per second. This would give 300fb-1 which is clearly a much larger amount of data than before.


What are the challenges in the LHC? One of them is the energy. It holds the record for accelerator energy. Nobody has produced a higher energy accelerator.

The target collision energy at the LHC was 14000 GeV (1GeV is the energy required to make a proton). This implies that the protons themselves each have an energy of 7000 GeV which implies a time dilation of 7500 because they are travelling close to the speed of light i.e. 99.999998%c (because of this they could travel to Alpha Centauri in five hours.

Time dilation is a difference in the elapsed time as measured by two clocks due to a relative velocity between them or to a difference in gravitational potential between their locations.

The lifetime of particles produced in particle accelerators appears longer due to time dilation. To a proton travelling very close to the speed of light, time would appear to be passing normally. Proton time would seem strange only to an observer outside the LHC, for whom 1 second for the proton would appear to last about 2 hours.

The protons move round the 27km tunnel 10,000 times per second. That is 1.1 million passes at any given point in a hundred seconds. At each one of these points there are eight high voltage cavities (see the image below)


Each of the high voltage cavities gives the protons an energy kick as they pass them. But because they’re passing them a billion times every couple of minutes, they don’t actually need to be given that much of a kick in order gain the huge energy

This is the great advantage of a circular ring. A relatively modest accelerating unit gets used a million times and produce enormous energies. So that is, in fact, the easy bit.

The challenge is making the protons path bend. Circular motion means constantly reversing direction. At one point the protons are moving in one direction then 22,000 seconds later they are travelling in the opposite direction. So, 22,000 times a second these protons have to be reverse directions at nearly the speed of light. The centripetal force is huge,

A centripetal force is a force that makes a body follow a curved path. Its direction is always at right angles to the motion of the body and acts towards a fixed point of the instantaneous centre of curvature of the path.


Because F = mass x acceleration a huge force means a huge acceleration,

but because a = v2/r the bigger the ring the smaller the acceleration. That is why a large ring is desired. Despite the large ring the acceleration is still 1017g (allowing for relativity). If the human body were to experience the value of 1017 x the value of gravity it would be completely squashed.

So, how is the path of the proton bent. Well, when a charged particle is moving in a magnetic field it experiences a transverse (side-ways) force.


The necessary force on each proton travelling at almost the speed of light will be:


To generate this force there are 1232 magnetic dipoles located on the eight arcs, each one having a magnetic length of 14.3 m, giving a total implied length:

1232 × 14.3 = 17618 m

Precisely, we can calculate the so-called “bending radius”:




Fc = 4 x 10-10 N

on each proton.

Curving the beam’s path is achieved by the magnetic field of dipole magnets. This is because the magnetic force exerted on charged particles is always perpendicular to their velocity, perfect for curving the trajectory.

The magnetic field in every dipole generates a vector B on each pipe opposite in direction to that of the other pipe. This makes the forces over both proton beams act in the same direction towards the centre of the ring.


Since the centripetal force Fc is also the magnetic force:

Fc = Bqv where m is the magnetic field strength, q is the proton charge and v is taken as the speed of light

B = Fc /(q x v) = 4 x 10-10/(1.6 x 10-19 x 3 x 108) = 8.3T

(100000 times Earth’s magnetic field)

If the LHC had been made of conventional magnets, it would have needed to be 120 km long to achieve the same energies and its electricity consumption would have been phenomenal.

Some comparisons:

The LHC requires 8.3 T (T = Tesla, the unit for magnetic field strength)

The Earth’s magnetic field at the equator is 0.00003T

A fridge magnet is ~ 0.01T

The absolute record for a rare earth permanent magnet is 5T

So, electromagnets need to be used.

A current flowing through a wire produces a magnetic field. If protons are going to be a few cm from the wire a current of 1MA is required to produce a magnetic field of about 8T.


Actually, the 8T is produced with 12000 amps by turning a single wire into a coil made up of 80 turns.

The protons past through the core of these magnets, where the field is strongest. Each of the electromagnets (see above) is 15m and there is 1232 of them in the tunnel, around the circumference of 27km plus a few other magnets which have different jobs.

How do these magnets get cooled?


The power lost due to resistance is I2R, where I is the value of the current and R is the resistance of the conductor that makes up the electromagnet.

If the resistance was 1W with a current of 12000A then the power lost/deposited in the magnet would be 144MW.

20km of magnets are needed and under normal conditions would result in an enormous electricity bill. The only way to solve this problem is superconductivity.

Superconductivity is the magic property where the resistance of certain materials drops to zero when they are cooled.

LHC magnet coils are made of copper-clad niobium-titanium cables.

Niobium-Titanium super conducts at low temperatures. The LHC magnets are operated at 1.9 K above absolute zero (~ -300oC). The lower the temperature the greater the current flowing. 1.9K easily allows 12000A to flow without the power loss.

The only energy bill is for cooling the cables/magnets as there is zero resistance. The cooling is provided by liquid helium.


The above image shows a view inside the tunnel. You can’t see much of the magnets because they are inside a vacuum case to keep them cold as a vacuum is a great insulator. The image also shows the curvature of the ring.

At four points around the ring the two vacuum tubes containing the protons cross and experiments are place at those positions.

One of those is the ATLAS experiment


Computer generated cut-away view of the ATLAS detector showing its various components. 360° Panorama ATLAS detector

(1) Muon Detectors

Magnet system:

(2) Toroid Magnets

(3) Solenoid Magnet

Inner Detector:

(4) Transition Radiation Tracker

(5) Semi-Conductor Tracker

(6) Pixel Detector


(7) Liquid Argon Calorimeter

(8) Tile Calorimeter

ATLAS (A Toroidal LHC ApparatuS) is the largest, general-purpose particle detector experiment at the Large Hadron Collider (LHC), a particle accelerator at CERN (the European Organization for Nuclear Research) in Switzerland. The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators. ATLAS was one of the two LHC experiments involved in the discovery of the Higgs boson in July 2012. It was also designed to search for evidence of theories of particle physics beyond the Standard Model.

The experiment is a collaboration involving roughly 3,000 physicists from 183 institutions in 38 countries.

The above link is a time lapse film of the creation of the ATLAS experiment inside the tunnel begun in 2004. It is a string of photos taken once a day for a few years.

Bits and pieces of equipment were gradually assembled and you get a sense of scale form the occasional glimpses of people.

This cavern, apparently, is the biggest thing dug to such a depth without any pillars.


The above image shows one of the magnets being installed. The magnets are used to bend the paths of the charged particles.


Another magnet going in.

The video shows the rig of eight magnets being put in over time. Those magnets define the field geometry of the ATLAS experiment.


The last magnet going in.

Inside those magnets they have got to assemble the rest of the detector and other bit bits and pieces


The professor was working inside it, testing lots of cables and things during the assembly.


What can’t be seen clearly is the very large quantities of cable required.


In the above image towards, the top right, a flash of an orange frame can just be seen. It was a UK is contribution. It was nice to see it as most of the UK equipment is hidden away.

The equipment has to be tested out.


For that piece just put into the detector, as shown above at the time, required about 30,000 connections to be made. Everything has to be cut to the right length in the tunnel, in the accelerator. They can’t have wire the right length or build before they might not get it exactly right. And they don’t want to have heaps of wire in the middle.

Two years were needed for testing connections and making sure everything was correctly wired up.

It’s harder to see the completed experiment when it is working because it is covered by “caps”.


The hadronic end-cap (HEC) caps off the calorimeters.

ATLAS is an incredibly precise device. Some parts are as small as 10 microns.


Layout of the ATLAS Inner Detector: it comprises the Transition Radiation Detector, the Semiconductor Tracker and the Pixel system.

The above image shows a part of the tracker, that was put in the orange frame mentioned earlier, which was one of a set of trackers that fitted around the experiment.

Two protons will come in and collide in the middle of the beampipe at ATLAS (or one of the other experiments) producing many new particles which will fly outwards and pass through multiple layers of detectors.


In the above image, a square/rectangle shape coloured blue which is called the SCT is made up of lots of rectangular pieces (see one of them in the image below). RAL built about 780 of them.


They are silicon detectors and they could have ended up as memory chips in a smartphone, but here they are used to detect particles.

Silicon is a semiconductor so it’s easy to knock electrons off their atoms. So when particles pass through the detector electrons are freed

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave the opposite. Its conducting properties may be altered in useful ways by introducing impurities (“doping”) into the crystal structure. When two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behaviour of charge carriers, which include electrons, ions and electron holes, at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called “metalloid staircase” on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

Each one is 12cm by 6cm consisting of 1536 readout strips, 12 chips (front and back reading at 40MHz.

1536 readout strips are put on the top and electrons drift along because of an electric field to the nearest wire. This wire’s position is noted and the position of the electron is worked out from that.

Collisions happen 40 million times a second.

As mentioned above there are 12 readout chips and 1536 readout strips.

Multiplying 1536 readout strips and 40MHz gives about 1 DVD quantity of data per second from this little detector.

So, 1 DVD of data per second is recorded. Built to within a few mm. Reports ‘hits’ by particles with a precision of 20mm and about 700 of the them is used to tile around the barrel collision point.

And there are hundreds of them used to make one ring. And are there are many, many other components in the ATLAS experiment.

The huge amount of data is a problem and the values of the data need to be known extremely precisely.


The data is used to work out what sort of particles are being produced.

The above left image is a cutaway of the CMS experiment.

The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors built on the Large Hadron Collider (LHC) at CERN in Switzerland and France. The goal of CMS experiment is to investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter.

CMS is 21 metres long, 15 m in diameter, and weighs about 14,000 tonnes. Over 4,000 people, representing 206 scientific institutes and 47 countries, form the CMS collaboration who built and now operate the detector. It is located in a cavern at Cessy in France, just across the border from Geneva. In July 2012, along with ATLAS, CMS tentatively discovered the Higgs boson. By March 2013 its existence was confirmed.

ATLAS and CMS both have a similar layered structure,

In the above right image, you can see the particles interact in different ways with the different layers of the detector.

Particle physicists reconstruct the long-lived particles in the detector. Basically, there are only a few types: photons, electrons, muons, protons, pions and neutrons.

The particle can be identified by the interaction in the detector.

The momentum is measured by bending the particle path in a magnetic field

First of all, there’s a tracker that measures where the particles are going, but it only “sees” the charged particles (electrons, muons, protons and pions) and not the uncharged ones (photons and neutrons).

Then there’s two layers of calorimeter which stop most of the particles, except the muon which can pass right through to the back where there is another set of tracking detectors called the muon chambers because every other particle has been filtered out. Oh, except for the neutrinos. We can’t see neutrinos at all. They just disappear through the detector.

The equipment can also tell electrons and photons from protons and neutrons by how they deposit their energy.

So, the particles can be sorted out. The momentum and energy for electrons muons and photons can be measured rather well. Measurements are really precise,

The other important thing about electrons, muons and photons is there aren’t any inside the proton.

If you smashed two protons together you get lots of stuff made of quarks coming out such as pions, protons and neutrons.

But only rarely will they make an electron or a photon. If they appear it suggests something interesting gas happened.

The LHC timeline

The LHC started running in 2008 and very soon after stopped because of a major electrical fault.

Professor Brian Cox: “I once caused the Hadron Collider to be shut down, after I spilled yoghurt in it.” Season 4 Episode 9 of “Would I lie to you”

Spoiler: he lied.

The LHC ran for the first time between 2010 and 2012 at half the design energy to protect the magnets.

25fb-1 of data was generated, which was enough to discover the Higgs particle.

The first official shutdown occurred in 2013/2014 to improve thousands of connections.

The LHC ran for the second time between 2015 and 2018 at 13 TeV (almost the design energy) generating 139fb-1 of data. It made ten times more Higgs particles than it did in run one.

At the time of writing (2020, what a wonderful year) the LHC is “enjoying” shutdown number two. It was supposed to start running again in 2021 but due to Covid-19 will probably not start running until 2022.

When the LHC is running, what is seen?



The above plot has got far too much information on it. The x axis is showing different particle types that might be being produced. The first entry is just the just proton-proton collisions. But then there might be Ws’ Zs, or ttbar pairs or even Higgs.

The y axis is the cross-section. This is a measure of how often a particle is produced.

So, proton-proton collisions have a cross section of 1011 and the Higgs have a cross section of about 50.

For every 109 (1 billion) collisions a Higgs Boson is made.

On this plot is a grey line for theory predictions of each particle.

Then there are blobs for the measurements that have been made and they line up very well with the predictions, even down to the data processing for rarer events like Higgs.

It’s a triumph, that particle physicists can predict and measure things so precisely, but it’s also illustrates the difficulty in finding the Higgs boson. Only one collision in a billion creates one and nearly all the collisions just make hadrons i.e., pions protons and neutrons.

Search for new heavy particles decaying into dielectron final states, full Run 2 dataset.

Event display of the dielectron candidate with the highest invariant mass in the 2015-18 data taking period with mee = 4.06 TeV. The properties of the leading electron are ET = 2.01 TeV, η = 0.47 and φ = -0.78. The properties of the subleading electron are ET = 1.92 TeV, η = -0.03 and φ = 2.37. The analysis ATLAS-CONF-2019-001clip_image001 searched for new heavy particles decaying into dilepton final states. This event display is from this analysis but was not included in the CONF note.

The image below is showing a collision that is not quite typical collision, but it’s one event that was recorded. Two beams passed through each other. Looking at the main picture there are lots of new tracks coming out of activation point (coloured blue). Two electrons (coloured yellow) deposit energy.

The “side” picture (bottom part of the image) looks the same, but it you enlarge the picture (right hand side of the image) you get a view of the side of it, at a length of about 5cm. There are several different sources for the tracks that are appearing and each of those sources are where a pair of protons have collided.


Bunches of protons collide 40 million times a second and proton-proton collisions happen each time.

The other 20 or 30 collisions, that are going on, can be separated out a bit, but getting back to the measurements in the calorimeter and just a little distance from the collision point these collisions can’t be told apart.

So, this particular event has two electrons which give very large high energy deposits in the electromagnetic calorimeter and the red lines are just tags to indicate something is happening here (the production of the two electrons, indicated by yellow in the green region).


There is a rather distinctive signature which shows why it is useful to look for electrons because they can be picked out from all the other tracks, which are not leaving similar deposits

The same would be true of muons and photons. They stand out by leaving distinctive tracks.

Electrons, photons and muons stand out at the LHC. Many measurements rely on them.

How are Higgs produced at the LHC?



Higgs interacts with mass. Quarks in a proton are light. So typically, W, Z or t are made. Higgs can be produced from them

There are various different ways that colliding protons together could make Higgs bosons.

The bits inside the protons are actually rather light. The up and the down quark inside the proton don’t produce Higgs bosons or interact with Higgs boson’s very much at all. So typically, what’s happening in these events is that two bits of the proton, with the gluons binding them together or two of the quarks are creating top quarks, W or Z particles and those are creating the Higgs. So, it’s a kind of two-step production process.

Higgs decay modes used

Particle physicists also have to worry about the Higgs decaying


Higgs decay in lots of different ways, which is actually a good thing. It gives particle physicists lots to measure and lots to understand about it. It’s supposed to interact with all particles and it should decay into every particle at some level.

Some of these product particles are too rare to measure.

In the pie chart above the dark blue section is indicating the likelihood of a Higgs particle decaying into two b quarks. It is the most common but not distinctive.

But quarks at the LHC are pretty difficult to pick out and identify. It’s not an easy thing to see.

The pale blue section indicates the likelihood of the Higgs boson decaying in to two W quarks. This is a bit cleaner but the W’s themselves decay and the products are quarks.

So, particle physicists are looking for Ws decaying into leptons, i.e., electron neutrino or muon neutrino pair. This is actually quite a good method, but the neutrinos are very difficult to spot so the event can’t be fully reconstructed. A couple of leptons can be spotted but the total energy isn’t known.

The green section represents the theoretical decay of the Higgs boson to gluons. The gluons are just impossible. There are too many gluons produced in collisions at the LHC so it would be difficult to decide if they came from the decay of the Higgs.

Then there are Tau bosons which are very complicated in their decays as well. But it turns out that Higgs decays to Z bosons, and the Z bosons decay into electrons or muons, although this is incredibly rare at 0.2%. There are many Z decay modes.


But it is the golden mode which is a very distinctive signature for electrons or muons. Also, the Higgs decaying into two photons, which is rare and distinctive at 0.2%

Below is an example of an event with two muons in it.

After being produced, the Z boson lives for t » 10-24 s, and decays into different pairs of stable particles, e.g., Z→muon+muon


Most particles are stopped at the green calorimeter. The two muons reach the outside (they are coloured yellow in the images above). They are very penetrating. Muons are heavier versions of leptons than electrons. Both muons come from the same collision vertex (although there are other collisions). Are they related? Particle physicists don’t really know what goes on at that collision point, but it is ok to ask the question.

What would the mass of some particle have to be if it broke up to make those two muons? You can calculate that from the muon data.

Make a plot of the masses of pairs of muons, and do it many, many times. Remember a billion collisions are being created a second. So, there are a lot of collisions to look at.

Distributions of those masses are made. Usually, a random number is obtained.

The energy of accelerator, of course is 14,000 GeV in total. So, thousands are not expected but hundreds and occasionally more can be obtained but tens or ones are very likely.


Find collisions with two muons; Calculate the mass of the pair; enter all the mass in a histogram; Look for a bump; Each bump is a particle decaying.

The interesting feature of the above graph is that there are spikes sitting on top, and each of those corresponds to particle.


So, the ψy particle breaks up into muons quite frequently. And so, every time a ψy is made it indicates a pair of muons and 3.1 is recorded.

This bump is built up at 3.1 so if we hadn’t known all these particles existed before we switch on the LHC particle physicists would just need to look at this one plot and say,” wow, look at all these things we found”. Each one of them is a decaying particle.

Hunting for the Higgs again

By looking for photons decaying. Photon decay is reasonably frequent

Event display of a diphoton event candidate where both photon candidates are unconverted. The event number is 56662314 and it was recorded during run 203779 at sqrt(s) = 8 TeV. The leading photon has eT = 62.2 GeV and eta = 0.39. The subleading photon has eT = 55.5 GeV and eta = 1.18. The measured diphoton mass is 126.9 GeV. The pT and pTt of the diphoton are 9.3 GeV and 6.5 GeV, respectively. Only reconstructed tracks with pT > 1 GeV, hits in the pixel and SCT layers and TRT hits with a high threshold are shown (ATLAS-CONF-2012-091clip_image001[4]).

This is not a common decay

2 per 1000 Higgs, but can be measured well.

Photon is neutral so no track, but there is a cluster of energy in the electromagnetic calorimeter.

Photons are light, so lots of light comes out of the collisions.



Photons give these blobs of energy and in the calorimeter without a track pointing at them because they’re not charged. They’re neutral so there’s nothing pointing at this measurable energy, but of course photons are light and there’s quite a lot of times when you get a high energy collision and you see light coming out of a collision. That’s not surprising. Although it is surprising to get light coming out in just two lumps.

But there are a lot of events.

The discovery of the Higgs


Again, that distribution of the mass of a pair of photons was made. The above plots are from the ATLAS and CMS experiments, where the “bumps” indicate the moment of the Higgs discovery.

Both experiments saw significant peaks at 125 GeV. The weighted sum was clearer.

The particle physicists would not have been convinced by the above by itself. The bump is there but look at the error bars on the points.

Perhaps they’re just fluctuations.


The above shows quite a bit of “fluctuation” on the CMS data. It’s not much different from the value for the Higgs boson, but the fact that two experiments have it is pretty convincing. But fortunately, particle physicists didn’t just have that one experiment they also had the Higgs decaying into four leptons.

The image below shows the tracks involved with a Higgs decaying first into two Z bosons. These Z bosons then decay into four leptons.



The above shows two green electron tracks stopping the calorimeter giving up their energy and to “red” muons going straight through the detector. This is really rather unusual.

If you look at the distribution. Now,


Background shapes decently seen in both. ATLAS sees 20% too many high mass ZZ pairs. Peak is seen at 125GeV in both experiments. These are mostly Higgs.

There aren’t the hundred thousand events of the photon plots. There are maybe 100 or 200 events where four leptons are seen.

The red background in the plot above left is stuff that was known about already.


91GeV is the Z boson. 200GeV is two Z bosons

But at 125GeV there are 6 plus 7 = 13 events seen, when without the Higgs, there should only have been four. It’s not a huge fluctuation. There’s probably a bigger fluctuation there but it’s 125GeV in just the same places as in the photon channel. CMS was seeing much the same thing, 125 GeV (3 + 4 = 7) with seven events, which was expected.

In each case, it was enough to provide five-sigma evidence for the Higgs.

So, putting together the four electrons and two photons each experiment had had five sigma evidence which is the standard insisted upon for a discovery to be accepted.

The decay of the Higgs to two Ws was measured soon afterwards as well, but there is no sharp peak so it doesn’t look as convincing.


The discovery of the Higgs was made via leptons and photons. The plots gave well defined peaks. You can look at the plots and see something is there. The plots also allowed the mass to be measured.

The Higgs decaying to two W bosons which in turn decayed to give two leptons and two neutrinos came soon after. It is more common that the four leptons and two photons.

Presence of two leptons is distinctive but neutrinos can’t be measured, so energy is lost and measuring the mass is impossible. There is no sharp peak in the plot.

In 2012 there was a meeting where the discovery of the Higgs was announced.

This was quite a high pressure to time the Friday beforehand, we’d had a private meeting between experiments and


Fabiola Gianotti for ATLAS and Joe Incandela for CMS

As a result Peter Higgs and François Englert won the Nobel Prize in 2013. (bottom left)

Peter Ware Higgs CH FRS FRSE FInstP (born 29 May 1929) is a British theoretical physicist, Emeritus Professor in the University of Edinburgh, and Nobel Prize laureate for his work on the mass of subatomic particles.

image (Top right)

François, Baron Englert (born 6 November 1932) is a Belgian theoretical physicist and 2013 Nobel prize laureate.

So, what next?

The Higgs particle had been found. It pretty clearly decayed to photons and Z bosons.

Fairly soon particle physicists expect to have evidence of the Higgs decaying into W bosons.

But is it The Higgs?

Its mass needs to be measured. In the standard model this is an unknown.

If the standard model is right it should not have a spin. This needs to be checked.

Its interaction with other particles needs to be checked. Its coupling strength to other particles. Many theories are around to modify these.

Pair production to check self-interaction needs to be done.

Its width should be narrow

And lots of other things.

Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson.

The Brout-Englert-Higgs (BEH) mechanism is at the core of the Standard Model, the theory that describes the fundamental constituents of matter and their interactions.

The BEH mechanism also predicts that the Higgs field can interact with itself; in other words, a single (virtual) Higgs boson can decay into two Higgs bosons. Observing and measuring this self-interaction, or “Higgs self-coupling”, would be the ultimate validation of the theory of mass generation, while any deviation from Standard Model predictions would open a window on new physics.

Mass measurement today

CMS is in the lead


The above is the best measurement outcomes from the CMS experiment using two photons and four leptons.


Their peak for Higgs has got big bigger and bigger with time as more events were recorded. They’re actually measuring it with a precision of about 0.1%


Particle physicists will be measuring spin. It’s a little bit hard to see how to do this. The angular distributions of the decay products are connected to the spin of the particle. The angles distributions of the decay products will be looked at.

Many studies were made in the 2012-2014 run.

Particle physicists were looking for zero spin and they have been able to rule it out completely

Exclude spin 0, parity minus

Exclude spin 1 completely

Spin 2: a dozen models tested, all excluded

Spin 3 or higher are ruled out by theory principles


Spin 2 is more complicated. There are lots of different models that have spin 2.

Every model tested has been excluded. But it can’t be said that spin 2 has been completely ruled out. But most particle physicists have stopped worrying. It looks like spin zero, but there isn’t complete proof of that.

Production and decay modes

There are predictions. How often those four different production modes should happen.

Five decay modes have been seen. They are all occurring at the rates expected. As well as the original three, the tau tau and the bbb cases have been added.

With a lot of a lot of effort the bb was added just a couple years ago and particle physicists finally managed to confirm that the Higgs decayed into b quarks.

It all looks just the way the standard model predicts. The evidence is consistent with the plot below.


There are four different processes that can produce a Higgs particle: gluon-gluon fusion (ggF), VBF, W/Z associated production (W/ZH), and top quark-antiquark pair associated production (ttH). Of these, ggF contributes 88% of the production, VBF counts for 7%, and the other two together around 5%. The ggF process was the major contributor to the Higgs discovery in 2012.

So now four production modes are seen with a confidence of 5σ. These are


If two protons collide and in each one there is a gluon that has a lot of energy, these two energetic gluons can collide with each other. This is gluon fusion generally.

There can be so much energy in this gluon collision collision that they can actually create some top quarks (again via E = mc2). In fact, they can even create a top quark and anti-top quark together. Then these two top quarks will quickly annihilate with each other and have chance of producing a Higgs. So indirectly we’ve managed to produce a Higgs via with top quarks!

VBF is “via Weak Boson Fusion” or “vector boson fusion”



(a) SM Higgs boson production cross sections as a function of the centre-of-mass energy, √s, for pp. collisions. The VBF process is indicated here as qqH. The theoretical uncertainties are indicated as bands. (b) Branching ratios for the main decays of the SM Higgs boson near mH = 125GeV. The theoretical uncertainties are indicated as bands

The dominant Higgs-boson production mechanism, labeled pp. → H in Fig. a above (for masses up to ≈ 700 GeV) is gluon–gluon fusion.

The production of the Higgs boson in association with W and Z boson, labelled pp. → W or Z H or the production via the t-tbar fusion

The Higgs boson decays in one of several ways (decay modes) into known SM particles, the types depending on its mass. Hence a search had to be envisaged not only over a large range of masses but also many possible decay modes: into pairs of photons, Z bosons, W bosons, t leptons, and b quarks.

In the mass interval 110 < mH < 150 GeV, early detailed studies indicated that the two-photon decay would be the main channel likely to give a significant signal. Detailed studies of another mode, H → ZZ() → ℓℓℓℓ, where ℓ stands for a charged electron or a muon, dubbed the “golden” mode, suggested that it could be used to cleanly detect the Higgs boson over a wide range of masses starting around mH = 130 GeV. One or both of the Z bosons would be virtual for mH < 180 GeV, and the upper end of the detection range was indicated to be about mH < 600 GeV.

In the region 700 < mH < 1000 GeV the cross-section decreases so Higgs boson decays via W and Z decays, where the W and Z decays are to channels with higher branching fractions, have to be employed.

The five decay modes:


Do interactions scale with mass?


So, one of the predictions, is that the interaction should scale with mass. So, in the above plot the x axis is the mass of particles. The y axis is the interaction strength and there is a very nice straight line that all particles “appear” on. It is definitely evidence for the Higgs boson.

At the bottom end of the graph the muon has been added. It wasn’t on the original list of particles discovered, but there is some evidence of an interactive muon, but its not yet at the level of evidence of the standard of proof required. It clearly tracks the muon far less than it does with the tau and the other particles. Other than that, the muons are identical to the other particles.

So, the interactions with the five heaviest particles in the standard model have been found and work is now being done on the lighter ones

Everything looks. Looks exactly the way it should.

The five strongest have been see; The W/Z couplings have been measured to 6% so the findings are getting very precise. Evidence for the Higgs-muon interaction appeared in 2020. But the proof isn’t quite there yet.

So, what does it all mean? The key property of the Higgs boson is the field.

It’s not like an electromagnetic field. You switch off the source of the electromagnetic field and the electromagnetic field goes away.

The Higgs field is always there. There is a sort of “standard density” of Higgs bosons per volume. It’s another way of expressing the field. There is a couple of Higgs bosons inside every atom (inside every proton). There are very high densities predicted for the field.


There is this famous Mexican hat potential plot, which helps to explain what’s happening. The centre axis corresponds with zero density of Higgs bosons and rolling off in any direction is energetically favourable. So, it’s like a hill. If you’re at the top of the hill you can roll down and release some energy. Similarly, this is what happens to the Higgs boson. Particle physicists don’t know which way it rolls down and they call this spontaneous symmetry breaking. The universe chooses the direction it goes down.

Spontaneous symmetry breaking is a spontaneous process of symmetry breaking, by which a physical system in a symmetric state ends up in an asymmetric state.

When the Higgs boson is on the top of the peak it is symmetrical but it may spontaneously break this symmetry by rolling down the peak into the trough, a point of lowest energy. Afterward, the Higgs boson has come to a rest at some fixed point on the perimeter. The peak and the Higgs retain their individual symmetry, but the system does not.

So, what is found in the universe today is that the Higgs field is sitting at the expected density and by measuring the interaction with the W and Z bosons particle physicists are measuring the curvature of the potential at the bottom of the hat (the region around the minimum), checking that is comes out right. This has been measured with an error margin of 6% and is coming in at the expected value and seems to be consistent.

A little bit more detail


The Mexican hat curve with different axes, one is energy and the other is density. Ordinary matter obeys E = mc2 and energy rises with density. But the Higgs particle isn’t like that at all. It obeys the red curve where it starts at some high energy at zero density with the minimum energy at a standard density (“1”). Past “1” the energy rises with density but in the form of a steep curve. This is very unusual but it is what creates the field

The quantity of ordinary matter is fixed, if the mass is doubled so is the energy, but Higgs particles can be created or destroyed to move to the lowest point. Away from “1” particle physicists have to trust the theory .. or not. So what?

A way of looking at it is to put 10 Higgs particles in a box and close the lid. Then when the lid is opened there are only 3. The number of Higgs are not conserved in the same sense that electrons are conserved. The number of electrons in the box would always be the same. The number of Higgs just moves.

That is what Peter Higgs proposed and that is what seems to have been discovered. It fits beautifully with everything that has been seen, but it has some other implications. One of those is that we are all doomed.

The red curve drawn above applies to the Higgs at a low temperature, but if the Higgs temperature is increased the equations change (the equations describing the Higgs field depend on temperature). It’s a complicated interaction involving the top quark and some other things as well. But the equations adapt as the energy/heat rises (the Higgs field becomes unstable) and eventually a second minimum appears dropping down to infinity (increasing the temperature causes the Higgs field to drop to its true minimum). So, the Universe (as a vacuum) is sitting with the Higgs field at the density of “1” but should it get pushed up the second hill (the top being a meta-stable state), which would require a lot of energy (caused by the heating), it would roll down the other side and its density would increase, releasing energy, which would heat up the Higgs field around it, which would fall as well. In falling it will heat up neighbouring fields which will drop too.


If this were to happen anywhere in the Universe a ball in a super-dense would expand at the speed of light destroying everything it touches.


And this is really what the prediction is. It is the best estimate if the equations describing the Higgs are correct. This seems to be what will ultimately happen to the Universe.

Now, the expected lifetime of the Universe is very big indeed, (the smallest estimate is over 15 billion years from now) much bigger than the current age of the Universe (13.8 billion years)


The ultimate fate of an expanding universe depends on the matter density (WM) and the dark energy density (WA)

However, it doesn’t seem likely that the Higgs is going to cause the end of the Universe. There isn’t any proof of this but it would seem likely that if it were true the energy from the “Big Bang” would have set it off at the start of the Universe. So, it implies that the Higgs equations are missing something. Maybe this is how the Universe will end. Physicists just don’t know, yet.

The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of extremely high density and high temperature, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.

The Cosmic Microwave Background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe.


Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects where the effects of gravity are strong, such as neutron stars.

Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density or about 2.241 x 10−27 kg/m3. Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think that dark matter is abundant in the universe and that it has had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect or emit electromagnetic radiation, and is therefore difficult to detect.

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from supernovae measurements, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating.

The Big Bang should have created equal amounts of matter and antimatter in the early universe. But today, everything we see from the smallest life forms on Earth to the largest stellar objects is made almost entirely of matter. Comparatively, there is not much antimatter to be found. Something must have happened to tip the balance. One of the greatest challenges in physics is to figure out what happened to the antimatter, or why we see an asymmetry between matter and antimatter.

A neutrino is a fermion (an elementary particle with spin of 1/2) that interacts only via the weak subatomic force and gravity.

Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavour state is a linear combination of the three discrete mass eigenstates. Although only differences of squares of the three mass values are known as of 2016, experiments have shown that these masses are tiny in magnitude. From cosmological measurements, it has been calculated that the sum of the three neutrino masses must be less than one millionth that of the electron

An eigenstate is the measured state of some object possessing quantifiable characteristics such as position, momentum, etc. The state being measured and described must be observable (i.e., something such as position or momentum that can be experimentally measured either directly or indirectly), and must have a definite value, called an eigenvalue.

There are lots of things missing in the equations such as dark matter. When physicists know more the problems will go away. But the theory has been found to be correct so far. So maybe the Higgs will cause the end of the Universe.


The Higgs might relate to the beginning of the universe.

The assumption is that the Universe began as an empty space, a complete vacuum with a Higgs density at zero. Its energy slid down until the density reached “1”. But what if it followed the baryogenesis curve.

In physical cosmology, baryogenesis is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e., the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe.

The baryogenesis curve looks similar around the density of “1” where measurements are being made (in fact with the errors involved it would be very difficult to see the difference between the Higgs and the baryogenesis curve, which one was actually happening), however at near zero the baryogenesis curve has a little dip. So, when the Universe began it would have sat in a false minimum until, at some point, it managed to get over the little hill and roll down. It would be very like a bubble expanding).

But it wouldn’t keep rolling down forever. It would just roll down to the vacuum that exists now (density 1, energy 0), the Higgs density we’re now in.

So, this is possible if the Higgs field isn’t quite the way that Peter Higgs described it.

Rather like a “doom-bubble”. This would be called a first order phase transition. Like with boiling water you get bubbles forming. Here you get bubbles forming in the early universe.

Why is that interesting? Well, if the Universe just expands and the Higgs smoothly drops down into its current state. It leaves physicists with lots of questions.


If the universe goes through this complicated first order phase transition with bubbles, those bubbles/inhomogeneities associated with it could create the matter/antimatter asymmetry (the movement of the Higgs is a symmetry choice). We know the Universe is all matter but we don’t know where the antimatter is. This work might provide an explanation for it.

What this is to do with matter/antimatter might be to do with the choice of which direction the Higgs rolls down. That is a symmetry choice that is linked to this problem.

Perhaps the Higgs is responsible for why we have matter not antimatter. We don’t know.

It could also be this this very frothy bubbling change might also have produced some black holes. There is a lot of speculation at the moment that dark matter is actually primordial back holes produced in the early Universe.

Potentially gravitational wave detectors might be able to see the gravitational waves that are associated with all this happening so perhaps physicists can explain one or even two mysteries by a little bit of a modification to the Higgs potential.

How would physicists know? Well at the moment there isn’t a good enough measurement, but physicists would like to measure how Higgs particles interact with each other. That’s what makes this density look the way it does.

The LHC would need to produce 2 Higgs bosons. It’s going to be difficult because the LHC wasn’t designed for this. The process is much rarer than making one and the LHC might not be able to do it. Also, physicists would need to be able to recognise them. It’s going to be a real challenge.

The best Higgs modes are:

Higgs decaying to two photons (BR of 0.002) or the Higgs decaying to two Z bosons which in turn decays to four leptons (BR of 0.002)

In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay.

If two Higgs particles are produced, they would be expected to decay to four photons. Particle physicists only expect one by 2035 and that is not enough to measure.

More abundant modes are needed.

Two Higgs decaying to two photons and two b quarks, expect 300 events at best?

Two Higgs decaying to four b quarks, or two tau leptons and two b quarks will require higher energy physics.

So, the HL-LHC is being prepared in order to look for more events producing two Higgs bosons, which will still be difficult. It will produce a lot more information

The High Luminosity Large Hadron Collider (HL-LHC; formerly SLHC, Super Large Hadron Collider) is an upgrade to the Large Hadron Collider started in June 2018 that will boost the accelerator’s potential for new discoveries in physics, starting in 2027. The upgrade aims at increasing the luminosity of the machine by a factor of 10 (10 times more data), up to 1035 cm−2s−1, providing a better chance to see rare processes and improving statistically marginal measurements.

A 100km ring is being designed now which would be able to repeat what was done with electrons and positrons, but at a higher energy and allow Z and Higgs bosons to be made in a more precise way (cleaner precise measurements).

In the very long term make another proton-proton collide, possibly at 100 TeV, which will make a lot of Higgs boson pairs and allow particle physicists to study their potential properly.

So, there’s a lot more to discover about the Higgs, but physicists are starting plan the next few years and produce long term plans for the future.



Maths has guided particle physicists to the Higgs boson. They have followed the equations that Peter Higgs wrote down in 1964. The boson fits this 1964 model.

The Brout-Englert-Higgs field is real.

The equations brought the physicists to the present time and they predicted what was seen in 2012. But they also continue. Knowledge is advancing rapidly. Many production and decay modes are being seen and more is to come from the LHC.

Possible future colliders are now being designed.

Surprises should be expected

The Higgs boson warns us about the possible end of the Universe and may explain the matter-antimatter asymmetry that allows us to be around.

Theoretical physicists are hoping the Universe end is wrong and they are looking at the maths, because it can’t be complete. Dark matter is missing for instance. Perhaps the Higgs boson can explain it.

The Higgs boson is being studied in great detail and hopefully give more information about the beginning and end of the Universe.

Questions and answers

1) If the Higgs provides mass to everything else, what is the origin of the mass of the Higgs particle

Itself. It drags the Higgs field around. It is a Higgs self-interaction. It doesn’t explain every mass. It’s just an arbitrary number you just make up a number and put it into equations and a lot of physicists find that very unsatisfactory.

Just make up a number, put it in to the equation and out pops the mass. It’s kind of circular if you like, but it works.

2) The press seems to say that the Higgs particles act as a drag on everything moving. But objects keep moving, keep going at a constant speed in a vacuum. There is no drag at all. Does it take energy to get something moving?

Yes, this is this is difficult. I mean, first of all, I should point out any discussion of the Higgs boson particle physicists are using analogies, which aren’t always perfect.

A comparison between things that have similar features, often used to help explain a principle or idea.

A colleague of mine prefers a different analogy, he would say that the Higgs field doesn’t act as a drag on objects. Objects continue to move with speed of light, but they keep scattering off the Higgs field. So, they take a longer path than you might have expected to get from here to there.

But they’re still traveling at the speed of light, but bouncing around as they do which means they are taking longer and we see them as being slower and heavier.

That analogy doesn’t explain why the object continues in a straight line.

An example to help the understanding can come by discussing momentum (product of mass x velocity). You start with a particle with very little momentum. Once its going it’s dragging that bit of Higgs field along with it. And so, once it’s moving there’s no interaction that stops it.

Yes, the Higgs field is a very unusual field. It’s not like air which just drags and slows down an object. The object picks the field up and moves with it.

The analogy is not terribly useful.

Back in back in the 19th century people worried about how light could travel through a vacuum at all and something called the luminiferous ether term was invented as an explanation of how light could wave when there was nothing for it to wave in

Luminiferous aether or ether (“luminiferous”, meaning “light-bearing”) was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do. The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.

The Higgs field has some connections to the luminiferous ether. The physicists of the time might have been right in some sense as there is something there that you are moving through, but it’s completely co-moving with the object dragging the little piece of field with it.

3) Can the UK keep working at CERN after Brexit.

The answer is yes, actually, we’re, we’re very lucky in that respect, I mean Brexit is going to have a huge impact on science or it could do but the UKs treaty with CERN predates joining the EU, so there is no direct impact but there might be all sorts of complications.

I have spent 10 years working and living in the area and that’s certainly made much easier by the freedom of movement, but in fact the actual continuation of the UK as a member is not affected.

4) And how about your own research, will it not be affected by Brexit?

Well, it’s a related question. The hundred-kilometre accelerator, which I proposed is being considered by Europe as being the next big project but it’s also being seriously discussed in China.

It might even be the case that the UK decides it’s a Chinese project. If that particular project goes ahead it might be an interesting alternative to CERN. Whether it’s really something we would want to consider. I’m not sure, but politics certainly impacts on this. I talked about the closing of the 87-kilometre ring that the Americans were building. The reason given for closing was not really because it saved money. It’s that it gives the appearance of saving money. So, for civil engineering projects on this scale, it’s all politics and quite hard for mere mortals to understand

5) How do you stop heat coming in from the pipe supports into the super cooled ring?

With difficulty. It would be beautiful if you could just put them in a vacuum and have the magnets sitting there but they would just fall down. So, you have to have legs. I ought to know what they’re made of, and I don’t. I do know that three legs support the magnets inside the tunnel and inside the vacuum insulation. About half the heat comes in through there and half the heat is radiant heat from the fact that the outside pipe, the outside of the magnet wall is warm (room temperature). So, it’s radiating a bit of heat.

So, it’s a kind of balance. Once you’ve got half the heat blocked (half the heat is coming from one source and half coming from another). There’s a reduced advantage in making a huge effort to block either of them. So, it’s a balance but yes. Those feet, are real problem. They’re also a problem because if you take those magnets, which are 15 meters long, and you make them at room temperature, then cooling them down to minus 271 causes them to shrink several centimetres. But the outside bore of the pipe is at room temperature still. So, the inner bore is contracting several centimetres. The outer bore is not. So, out of the three feet the middle one is bolted down so the other two have to slide in and out as the equipment warms up and cools down. We don’t like warming up the magnets if we can avoid it.

6) I suppose you need supercomputers to analyse the results. Are there any citizen science projects at CERN?

Super computer is, maybe, slightly, the wrong word now. We used to use supercomputers at CERN. More and more of what we’re doing is on commodity processors such as might be found in your laptop.

We are using supercomputer facilities. You can borrow things occasionally and get very large resources, but the jobs we’re doing break into the unit of analysing one collision. If you can do that on a laptop, then you can buy yourself a lot of laptops. Right. So yes, it does lend itself to Citizen Science projects. There is a an “LHC at home” project that can you can join. It was originally used for simulating the movements of the protons around the ring. There is something like 15,000 magnets around the ring which have to be individually tuned to make sure the protons keep orbiting. It was a complicated task and it was something where Citizen Science did help in tuning the best settings of those magnets. But there are also projects where you can help with simulating events.

Actually, today it’s going more in the other direction. We are lending out some of our computing resources to Covid protein research. Which is analysing the response of two different two chemicals to together.

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