Particle Physics Masterclass 2015

The Large Hadron Collider

By Dr Kristian Harder


The status of particle physics


The standard model is an excellent description of the universe; many phenomena can be predicted with unprecedented precision, it has defied all attempts to find flaws in it for decades, however it has just two problems.

a) It is incomplete. It doesn’t explain gravity, it doesn’t explain dark matter or dark energy and it doesn’t explain mass, it doesn’t explain why there is more matter than anti-matter also and it doesn’t explain why there are three generations of matter.

b) It is “wrong”. There are mathematical inconsistencies at higher energies (i.e. the Standard Model is a low energy approximation). Some things just don’t feel right as there are so many free parameter and so many different mass scales.

The question of mass may be answered with the Higgs Particle. In our theory, forces are mediated by particles (the gauge bosons). The mathematics only works if they are massless. However the Z and W bosons are heavy.


Some particles are not massless and this could potentially be explained by the Higgs mechanism. Maybe the massive particles appear massive due the Higgs field.

In particle physics, the Higgs mechanism is essential to explain the generation mechanism of the property “mass” for gauge bosons. In the Standard Model, the three weak bosons gain mass through the Higgs mechanism by interacting with the Higgs field that permeates all space.

A field, in physics, is a region of space where certain things experience a force.

The Higgs Field is an energy field that exists everywhere in the universe. The field is accompanied by a fundamental particle called the Higgs Boson, which the field uses to continuously interact with other particles. As particles pass through the field they are “given” mass, much as an object passing through treacle (or molasses) will become slower.

So a Higgs field is a region of space where certain particles experience a force that enables them to have mass. There is an interaction between the particle and the field.

Mass itself is not generated by the Higgs field- the creation of matter or energy would conflict with the laws of conservation. However, mass is “imparted” to particles from the Higgs field, which contains the relative mass in the form of energy. Once the field has endowed a formerly massless particle the particle slows down because it has become heavier.

If the Higgs field did not exist, particles would not have the mass required to attract one another, and would float around freely at light speed.

The process of giving a particle mass is known as the Higgs Effect.


A computer generated image of a Higgs interaction


The Higgs boson or Higgs particle is an elementary particle in the Standard Model of particle physics. It allows scientists to explore the Higgs field – a fundamental field first suspected to exist in the 1960s that unlike the more familiar electromagnetic field (charges are not needed to create the field) cannot be “turned off”, but instead takes a non-zero constant value almost everywhere. The presence of this field, now believed to be confirmed, explains why some fundamental particles have mass even though the symmetries controlling their interactions should require them to be massless, and also answers several other long-standing puzzles in physics, such as the reason the weak force has a much shorter range than the electromagnetic force. The Higgs boson is named after Peter Higgs, one of six physicists who, in 1964, proposed the mechanism that suggested the existence of such a particle.


Peter Ware Higgs CH FRS FRSE (born 29 May 1929) is a British theoretical physicist, Nobel Prize laureate and emeritus professor at the University of Edinburgh

An analogy of the Higgs mechanism


Imagine a room full of physicists discussing quietly. This is like space filled with the Higgs field. When a famous physicist enters the room, people will cluster around him to talk to him. This creates resistance to his movement — he/she acquires mass, like a particle moving through the Higgs field! This represents a heavy particle interacting with the Higgs field.

The Higgs field is a background field permeating space, the Higgs boson is a field quantum (a quantum of this field!)

Construction of the Standard Model Lagrangian

Quantum field theory provides the mathematical framework for the Standard Model, in which a Lagrangian controls the dynamics and kinematics of the theory. Each kind of particle is described in terms of a dynamical field that pervades space-time. The construction of the Standard Model proceeds following the modern method of constructing most field theories: by first postulating a set of symmetries of the system, and then by writing down the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries.

The picture below right shows Professor John Ellis, a British theoretical physicist, wearing the formula on a T shirt. You can also get it on mugs, bags and other items, from CERN. It has a very nice little shop.


The Standard Model Lagrangian equation shows the theory of particle physics. The theory looks good and at the moment no measurements disagree with it however it doesn’t include gravity. Trying to include gravity only produces rubbish.

The first line of the equation describes the dynamics of all the force fields — the gauge bosons which carry the force; e.g., the photon, which is the massless particle behind the electromagnetic field.

The second line of the equation describes the matter fields. This accounts for fermions and anti-fermions and their coupling to bosonic fields; i.e., the electron.

The third and fourth lines of the equation represent the coupling of matter fields with the Higgs field and the dynamics of the Higgs field itself, respectively. The Higgs field not only accounts for the mass of both the gauge bosons and the matter fermions, but it also hides some other secrets of our universe.


Flip Tanedo writes that you can think the model is the origin of all the Feynman rules. He says that ach term on the right-hand side of the above equation actually encodes several Feynman rules. Roughly speaking, terms with an F or a D contain gauge fields (photon, W, Z, gluon), terms with a ψ include fermions, and terms with a ϕ include the Higgs boson. Some representative diagrams coming from each of the terms are depicted above.

However he feels there’s a bit of a problem with the design. It appears that there’s an extra term which isn’t included in the usual parametrization of the Standard Model. He feels that part of the formula shouldn’t be there.


Finding the Higgs

The theory was first proposed about sixty years ago but it took until 2012 for physicists to be able to produce the energy needed (E = mc^2). This was why the LHC was built.

The Higgs particle was expected to be identified by the products it decayed into; Higgs to photons or Higgs to Leptons.

On the 4th July 2012 a new massive particle in the mass region around 126 GeV was observed which physicists thought might be the Higgs Particle.


CMS Higgs proof on the left and ATLAS Higgs proof on the right

From its observed decay to two photons this particle, has intrinsic spin equal to 0 or 2 so it, must be a boson. There is also strong evidence that it decayed to the massive vector gauge bosons, the W and Z particles. At the time the only remaining undiscovered fundamental particle in the standard model, the Higgs boson, shared all of these properties, but initial evidence was insufficient to rule out other possibilities.

Since 2012 the properties of the Higgs particle has been investigated.

On the 17th March 2015 in Geneva, during the 50th session of “Rencontres de Moriond” in La Thuile Italy, ATLAS and CMS presented for the first time a combination of their results on the mass of the Higgs boson. The combined mass of the Higgs boson is mH = 125.09 ± 0.24 (0.21 stat. ± 0.11 syst.) GeV, which corresponds to a measurement precision of better than 0.2%.

The Higgs boson decays into various different particles. For this measurement, results on the two decay channels that best reveal the mass of the Higgs boson had been combined (Higgs boson decaying to two photons and to 4 leptons, leptons being electron or muon here). Each experiment had found a few hundred events in the Higgs to photons channel and a few tens in the Higgs to leptons channel, using the data collected at the LHC in 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, having examined about 4000 trillion proton-proton collisions. The two collaborations worked together and reviewed the analyses and their combination. Experts of the analyses and of the different parts of the detectors that play a major role in this measurement were closely involved.

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.

ATLAS (A Toroidal LHC ApparatuS) is one of the seven particle detector experiments (ALICE, ATLAS, CMS, TOTEM, LHCb, LHCf and MoEDAL) constructed 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. It might shed light on new theories of particle physics beyond the Standard Model.


The image above left shows the Higgs boson decaying into two W bosons, where one W decays into two jets (yellow cones) and the other one into a muon (red line) and a muon neutrino (yellow dashed line). The image above right shows the Higgs boson decaying into two Taus, each of which subsequently decay into either an electron (blue line) or a muon (red line).


The image above left shows the Higgs boson decaying into two Z bosons each of which subsequently decays into two muons (red lines). The image above right shows the Higgs boson decaying into two photons (yellow cones).

The Nobel Prize in Physics 2013 was awarded jointly to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.


There are still questions to be asked

Is there any difference between the Higgs particle’s properties and those predicted in the Standard Model?

Is it the only Higgs boson, or are there others?

Does it have any connection with the heaviest Top quark?

What gives its mass to this Higgs boson?

Is it truly an elementary particle, or is it made of some smaller constituents?

Is it a portal to some new physics beyond the Standard Model, such as dark matter?

The next run of the LHC, starting in the spring of 2015, will set about answering some of these questions. For example, its higher energy will enable the LHC experiments to probe more deeply for deviations from the Standard Model predictions, and to search for heavier Higgs bosons. It will be possible to measure directly this Higgs boson’s coupling to the top quark, and to box in its possible coupling to the muon. These measurements may reveal some substructure inside this Higgs boson, or provide some other evidence for physics beyond the Standard Model.

Other questions to be answered

How does gravity work?

Why are there more matter particles than we actually need?

Is string theory valid?

Are there extra dimensions besides the ones we are familiar with?

One possibility is that we don’t feel the full effect of gravity because part of it spreads to extra dimensions. Though it may sound like science fiction, if extra dimensions exist, they could explain why the universe is expanding faster than expected, and why gravity is weaker than the other forces of nature.

So why was the LHC built


Not to create black holes opening a stargate to swallow Earth or to allow Satan to come to Earth?


Not to create an antimatter bomb to destroy the Vatican or trying to be cool.

The LHC was built to

Find the Higgs Boson

Understand how forces behave at higher energy

Find new particles at higher energy

Specifically, look for Supersymmetry (double the number of particles, many theory problems solved)

Check whether quarks + leptons might be composite

Investigate black holes

Watch out for tiny extra spatial dimensions

What processes are involved

It could be said that the best way to understand something is to break it. When protons collide some of their kinetic energy gets turned into new particles (E = mc^2). With enough energy new high energy particles are produced.


J.J. Thomson discovered the electron in 1897

Sir Joseph John “J. J.” Thomson, OM, FRS (18 December 1856 – 30 August 1940) was an English physicist. He was elected as a fellow of the Royal Society of London and appointed to the Cavendish Professorship of Experimental Physics at the Cambridge University’s Cavendish Laboratory in 1884.

Bevatron 6.5 GeV proton accelerator (Berkley, USA) where the antiproton (1955) and antineutron (1956) were discovered

The Bevatron was a particle accelerator — specifically, a weak-focusing proton synchrotron — at Lawrence Berkeley National Laboratory, U.S.A., which began operating in 1954.


Edwin McMillan and Edward Lofgren on the shielding of the Bevatron. The shielding was only added later, after initial operations.

How does the LHC work?

The LHC is the biggest machine ever built


It has a circumference or 27km

It accelerates bunches of proton to 99.9999991% the speed of light, so they circulate 11 245 times/s

700 million proton-proton collisions per second at CMS/ATLAS


A recipe for building the LHC

1) Get a bottle of protons (use hydrogen and ionise it!) use them sparingly: one LHC fill has 2 beams × ≈ 3000 bunches × 1011 protons, i.e. about 1 nanogram, which should circulate ≈ one day


They are ionised by injecting them into a duoplasmatron where the electrons are ripped off.

2) keep your protons in vacuum pipes at all times so your protons don’t get disturbed too much and don’t interact with any dust particles.

LHC: 1/10,000,000,000,000th of atmospheric pressure! (better vacuum than space around the Intl. Space Station)


3) Accelerate your proton beams with electric fields LHC (RF cavities) to kinetic energies of a proton beam equal to the Eurostar at around 100 mph!

This is done by using a Linear accelerator to 50 MeV. Then by using a Proton Synchrotron Booster to 1.4 GeV

You cannot get the required energy from one pass through the accelerating cavities once. You will have to bend the beam around with magnetic fields and accelerate it repeatedly (taking ~20mins to get up to energy).


The Proton Synchrotron accelerates the protons to 25 GeV

The Super Proton Synchrotron accelerates the protons to 450 GeV

This then feeds the LHC in both directions


The strong magnets used require huge currents — only manageable with superconducting magnets!

There are about 9300 magnets used.


With 1232 large 15m long cryodipole

LHC is the largest fridge on the planet! 6000 tons kept at -271 degrees C corresponding to ≈ 150,000 household fridges at a temperature colder than the coldest regions of outer space!

The LHC takes protons from the SPS with 2808 bunches per beam and 1 x E11 protons per bunch

• Accelerates them up to (4) 7 TeV per beam

• Collides the beams at 4 places around the ring

• Then dumps the beams into large carbon/steel cylinders

• ~30 collisions every 50 ns

• Need huge numbers of collisions to analyse the rare ones. The beams are micrometre sized and need to hit head on so the beam position needs to be extremely precise.


LHC is sensitive to the location of the Moon!

– Through ground (25 cm) and water tides

– Varies the circumference by 1 mm


Collisions in 4 places are investigated by big detectors

ATLAS (general purpose) 7000 tons, 25m diameter, 46m length 2500 scientists & engineers


CMS (general purpose) 14500 tons, 15m diameter, 22m length 3000 scientists & engineers



LHCb (b physics) – 5600 tons, 13 m width, 21 m length 700 scientists & engineers

ALICE (heavy ion physics) 10000 tons, 16m diameter, 26m length 1000 scientists & engineers


General role of detectors

Protons (quarks and gluons) smash into each other at specific locations in LHC

Interesting massive particles are created

Interesting massive particles typically decay almost immediately

Not so interesting decay products fly in all directions


Intercept and analyse those with detectors,

Reconstruct the interesting part of the event mathematically using computers & brains

What detectors detect

Most interesting particles (charm, tau, bottom, top, Z boson and W boson) unfortunately decay very quickly a few mm further out from where the collision happened.

Also neutrinos are undetectable quarks and gluons never appear isolated, always give rise to whole bundle of protons, pi mesons, neutrons etc. (a jet)

Charged particle detection

Charged particles ionise material they pass through and if we put a small amount of material in their way and detect ionisation charge in there and localise where the ionisation happened then we can follow the path of charged particles almost without disturbing them!

The LHC typically use layers of silicon detectors to follow the path of the charged particles:

• quite thin (few hundred micrometre)

• high resolution (few micrometre)

• radiation hard (survive LHC collisions for years)

• mass production technology has cost benefits


Typically silicon is used for the job and the sensors are similar to digital camera sensors. The sensors are quite thin and high resolution and radiation is hard.


Another trick to derive information about charged particles is to immerse the tracking detector in a magnetic field → path of particle gets bent → We can calculate particle momentum from the curvature!

BUT: high energy collider → high energy particles → small curvature!

You need a large tracking detector (few metre flight distances) and high spatial resolution (few micrometres) to get a useful measurement!


Detecting more particles

Tracking detectors cover charged particles but not for neutrals:

• electrons

• muons

• protons

• charged pions

• …

But what about neutral particles?

• photons

• neutrons

• neutral pions

Use a complementary detector type: calorimeters!



Put a massive amount of material in the particle path (obviously after they passed the tracking detectors!). The particle loses energy due to material interaction creating showers of secondary particles.

Use absorbers to stop the particle → all kinetic energy transformed into shower energy use detectors to measure the shower energy (ionisation, light, number of charged secondary particles, …).

Derive energy of the particle from observed shower energy! (and get flight direction from location of the shower)

Calorimeter types

Two main classes of calorimeters:

Electromagnetic calorimeter: absorption via electromagnetic cascade of lightweight particles (electron, photon):


Hadronic calorimeter: nuclear interaction with absorber


Nuclear interaction with absorber material (for protons, neutrons, mesons etc)

But as we are dealing with high energy particles → need very thick material (few metres) of high density (iron, lead, even uranium)

Muons are almost undetectable and neutrinos can’t be detected, also there may be unknown particles.


– Are a special case: they are almost unstoppable

– You need specific muon detectors to find them

– E.g. CMS uses iron absorbers to stop any other particles from reaching the muon detectors, which work in a similar way to tracker systems (but with gas systems, for example)


– Can’t be detected

– These & any others we don’t know about and can’t detect have to be inferred from missing energy

– We expect there to be zero net momentum perpendicular to the beams

– Look for imbalances from all other particles à Hermetic detectors!

Missing momentum can be used to identify missing particles.

Particle identification

Observing particle energy and momentum is not enough!

We need to know particle type!

Solution: use tracker, electromagnetic calorimeter and hadronic calorimeter together


Unfortunately the muons go through

Muons are a special case:

Too heavy to develop electromagnetic showers

No nuclear interactions → no hadronic shower

Could be identified from feeble signals in calorimeters (not very reliable)

So use yet another detector type behind tracker & calorimeters:

Muon detectors:

Often like a coarse tracker to identify muon direction without help of main tracking detector

It can even have its own magnets to measure muon momentum independent of tracker!

Now muons are easy to identify:

Whatever gets through the calorimeters and is visible is probably a muon

We’ve seen tools to detect and identify almost all particles we can catch:

Important exeption: neutrinos!

Neutrinos cannot be detected also there might be unknown particles that do not interact with normal matter!

Is there any way we can tag their presence anyway?

Missing momentum

We cannot see neutrinos, but we can measure the momentum they carry away!

Before collision: zero momentum perpendicular to beams

After collision: must also be zero net momentum perpendicular to beams!

add up momenta of all visible particles

— is there an imbalance?

This might be momentum carried away by a neutrino!


Missing momentum identification requires our detectors to be “wrapped around” the interaction region! (“hermetic detector”)

Experiment: Compact Muon Soleniod

One of two general purpose detectors around the LHC

• Over 14 ktonnes; 21 m long and 15 m tall

• Like a big camera: records the results of protons (& HI) smashing into each other

• Filters interesting collisions from 100’s millions/s -> 100 thousand/s -> 100/s saved (triggers to save the data)


One of the LHC experiments: ATLAS

ATLAS works in a very similar way

– Although uses different technologies for some of the detectors


25m diameter, 46m length, ≈ 70 m^2 of silicon detectors 7000 tons

Reading data from ATLAS

100 million individual readout channels (pixels, cells, modules, …) signals need to be digitised for computer processing

There are lots of specialised electronics and 3000 km of cables



The silicon tracker was built at RAL.

Simulated example collision (“event”) in ATLAS:

Higgs particle decaying to ZZ decaying to e+ e− µ+µ−

Two strong electron signals and two straight muon tracks


A W decaying to µνµ and a Higgs decaying to two jets

one muon (w/o track!) two jets

Neutrino disappears, but in this case no obvious momentum imbalance (need to look at numbers)


Two jets

Clearly missing momentum

Exotic particle escaping the detector unnoticed!


These example events were very special ones. The vast majority of events looks like these:


Quarks just brushing past each other


• fast special electronics identifies interesting signals as part of the first stage of triggering

• selection in several stages of increasing complexity and precision

• Later stages of triggering uses clusters of computers

• Higher levels done by PC farms

• Computers then reconstruct the particles from the data recorded off the detectors

• ≈ 100 events per second stored permanently (filling ≈ 1 CD per second), distributed all over planet for computerized reconstruction and analysis

Control rooms running the detectors are staffed around the clock


CERN is an international organisation and analysis is done all over the world.

Data is distributed across the globe (e.g. via a node at RAL to the rest of the UK)

• These are then analysed by the physicists

• Data is turned into information/knowledge!


How data analysis works

Only the “boring” particles show up in the detector, e.g. the muons and electrons, but not the charm, top, bottom, tau, Z boson, W boson or Higgs that decayed into them!

Strange and gluons never appear isolated. Whole bundles of protons, neutrons, mesons etc. (jet) are produced.

Electron neutrino is undetectable.

How do the physicists reconstruct what happened before?

Simple example: Z →µ+ µ− (same concept works for e.g. Higgs)

Find events with two muons assume these muons come from a Z decay calculate mass of supposed Z from muon momenta do we get consistent results for the mass?

YES: we found the Z!

NO: these muons do not come from Zs


More information

Higgs announcement:

• LHC status:

• LHC:


• Youtube:


• Images of CERN facilities and the experiments are from cds:

Images of CERN facilities and the experiments are from cds:

• Pictures of the particles (plushies):

• Angels and Demons picture from:

• Einstein Art picture from flickr (wokka) via boredpanda

• Good idea: 

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