Tara Shears is professor of physics at the University of Liverpool and a researcher at the Large Hadron Collider at CERN, the European centre for particle physics. Having started her career in Switzerland, Tara was subsequently awarded a Royal Society Research Fellowship with the University of Liverpool in 2000. She joined the LHC-B experiment at the LHC in 2004, where she started a programme to test the limits of the standard model. She’s also interested in why there is so little antimatter in the universe.
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 Shears and my readers will forgive any mistakes and let me know what I got wrong.
What is particle physics? It is the study of the structure of the Universe at the smallest most detailed level.
Knowing the structure that makes up mundane things enables us to understand large scale things, i.e. the Universe.
The structure of the Universe is surprisingly simple. It is made up of 12 matter particles
Quarks compared to an atom is like comparing atoms to us.
Quarks are so small that they can’t be measured -size is limited to what we can measure.
Bosons provide the forces that hold things together
The W and Z bosons are responsible for the weak force. It regulates the rate at which stars burn energy. It affects every single quark and lepton. These bosons are exchanged between quarks and leptons.
The photon, g, is the boson responsible for electromagnetism. It acts on anything with a charge.
The strong force boson is called the gluon. It only acts on quarks. It produces a stable nucleus by stopping protons repelling each other.
What about gravity? We know it acts on anything with mass. It is very weak and can be ignored at the tiny level. It is not understood at the quantum level. Its exchange boson is the graviton. As far as we know, this has no observable effect in particle physics experiments and the graviton has not yet been discovered.
The Higgs Field
The Higgs field is the stuff that gives all other particles a mass. Every particle in our universe “swims” through this Higgs field. Through this interaction every particle gets its mass. Different particles interact with the Higgs field with different strengths; hence some particles are heavier (have a larger mass) than others. (Some particles have no mass. They don’t interact with the Higgs field; they don’t feel the field.)
The standard model equation
The Standard Model Lagrangian on a mug available in the CERN shop (Image: CERN)
A new, open-access paper, published in Physics Education, helps teachers to explain the Standard Model and the equation used to describe it.
The Standard Model of particle physics is one of the most successful theories about how our Universe works, and describes the fundamental interactions between elementary particles. It is encoded in a compact description, the so-called ‘Lagrangian’, which even fits on t-shirts and coffee mugs.
This particular formula draws a lot of attention and everyone who visits CERN will come across it at some point. For example, the CERN gift shop sells t-shirts and coffee mugs featuring this four-line version of the Lagrangian.
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.
Construction of the Standard Model Lagrangian
Technically, 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.
It is a predictive theory. It predicts the types of particles produced in experiments they behave. What forces they experience.
We can’t produce any measurements that disagree with the standard model
It isn’t the whole story. It can’t explain gravity and some other things seen in the Universe. So, this tells us the standard model is wrong. There must be a better theory.
So experimental work now is to find where the standard model breaks down.
What we don’t know
1. We know that anti-matter is real but why is it rare? Each fundamental particle should have a fundamental anti-particle.
0.25g of matter + 0.25g of anti-matter should result in an explosion equivalent of 1 5Ktonne of TNT.
At the time of the Big Bang the amounts of matter and antimatter was balanced. Annihilation would have been expected resulting in photons and after less than the second these photons would not have enough energy to produce matter.
One reason for the presence of matter now is that there could have been more matter produced at the time of the Big Bang.
The only accepted explanation is that matter must have slightly different behaviour to anti-matter. This is a mystery. We need to understand how we got from the Big Band to now.
2. The presence of dark matter is inferred by observation. The movement of stars and galaxies are not what calculations show.
Particle physicists would like to know the particle structure and what forces they experience.
Where does our theory break down?
The red section in the pie chart is mostly what we know along with some things we don’t know such as why gravity doesn’t, yet, fit into the standard model and why there is so little ant-matter.
The most famous set of experiments into particle physics takes place at CERN
The European Organization for Nuclear Research (known as CERN derived from the name Conseil européen pour la recherche nucléaire), is a European research organization that operates the largest particle physics laboratory in the world. Established in 1954, the organization is based in a northwest suburb of Geneva on the Franco–Swiss border and has 23 member states. Israel is the only non-European country granted full membership. CERN is an official United Nations Observer.
Many activities at CERN currently involve operating the Large Hadron Collider (LHC) and the experiments for it. The LHC represents a large-scale, worldwide scientific cooperation project.
The LHC tunnel is located 100 metres underground, in the region between the Geneva International Airport and the nearby Jura mountains. The majority of its length is on the French side of the border. It uses the 27 km circumference circular tunnel previously occupied by the Large Electron–Positron Collider (LEP), which was shut down in November 2000. CERN’s existing PS/SPS accelerator complexes are used to pre-accelerate protons and lead ions which are then injected into the LHC.
Eight experiments (CMS, ATLAS, LHCb, MoEDAL, TOTEM, LHCf, FASER and ALICE) are located along the collider; each of them studies particle collisions from a different aspect, and with different technologies.
The blue tubes contain the superconducting magnets. The electromagnets use a current of 11,080 amperes to produce the field, and a superconducting coil allows the high currents to flow without losing any energy to electrical resistance.
Thousands of “lattice magnets” on the LHC bend and tighten the charged particles’ trajectory. They are responsible for keeping the beams stable and precisely aligned.
Dipole magnets, one of the most complex parts of the LHC, are used to bend the paths of the particles.
When particles are bunched together, they are more likely to collide in greater numbers when they reach the LHC detectors. Quadrupoles help to keep the particles in a tight beam. They have four magnetic poles arranged symmetrically around the beam pipe to squeeze the beam either vertically or horizontally.
When the particle beams enter the detectors, insertion magnets take over. Particles must be squeezed closer together before they enter a detector so that they collide with particles coming from the opposite direction.
After the beams collide in the detector, enormous magnets aid the measurement of particles. For example, physicists look at how charged particles bend in the magnetic field to determine their identity. Charged particles are deflected by the magnetic field in the detector, and their momentum can be calculated from the amount of deflection.
After colliding, the particle beams are separated again by dipole magnets. Other magnets minimize the spread of the particles from the collisions. When it is time to dispose of the particles, they are deflected from the LHC along a straight line towards the beam dump. A “dilution” magnet reduces the beam intensity by a factor of 100,000 before the beam collides with a block of concrete and graphite composite for its final stop.
Insertion magnets are also responsible for beam cleaning, which ensures that stray particles do not come in contact with the LHC’s most sensitive components.
The accelerator complex at CERN is a succession of machines that accelerate particles to increasingly higher energies. Each machine boosts the energy of a beam of particles, before injecting the beam into the next machine in the sequence. In the Large Hadron Collider (LHC) – the last element in this chain – particle beams are accelerated up to the record energy of 6.5 TeV per beam.
The usual particles used are protons. The proton source is a simple bottle of hydrogen gas where an electric field strips the lone electron.
These protons are accelerated in stages before entering the two beam pipes in the LHC. The beam in one pipe circulates clockwise while the beam in the other pipe circulates anticlockwise. It takes 4 minutes and 20 seconds to fill each LHC ring, and 20 minutes for the protons to reach their maximum energy of 6.5 TeV. Beams circulate for many hours inside the LHC beam pipes under normal operating conditions. The two beams are brought into collision inside four detectors – ALICE, ATLAS, CMS and LHCb – where the total energy at the collision point is equal to 13 TeV. The proton beams collide 40 million times a second.
The collisions result in high temperatures which produce new high energy particles.
The 4 main detectors act as 3d digital cameras which show the distribution of energies which identify the particles present and infer what the original particles might have been. This is what is being done to work out what existed at the time of the Big Bang
The Higgs Boson was identified because of its decay into pairs of high energy g photons. Its behaviour was shown.
Results from all the experiments will show how robust our theories are.
The Large Hadron Collider beauty (LHCb) experiment specializes in investigating the slight differences between matter and antimatter by studying a type of particle called the “beauty quark”, or “b quark” (Although absent from the Universe today ‘beauty (b) quarks’ were common in the aftermath of the Big Bang, and are generated in their billions by the LHC, along with their antimatter counterparts, anti-beauty quarks.).
You would expect matter and anti-matter particles to be produced in equal amounts and to be completely identical except for their charge.
‘b’ and ‘anti-b’ quarks are unstable and short-lived, decaying rapidly into a range of other particles. Physicists believe that by comparing these decays, they may be able to gain useful clues as to why nature prefers matter over antimatter.
Look at the mass of kaons produced. Look at the electrical charges.
The above image shows two graphs. They show the resultant signals for the decays B matter and B anti-matter and show that they are different (the colours show background signals). The Y axes show that the signal peak is greater for matter.
We’ve known since the 1960s that matter and anti-matter behave differently. It was first seen with strange quarks. The standard model can’t explain this. Some patching up needs to be done.
Just one parameter to describe the difference.
Comparing the prediction with the actual measurement we can estimate a value for this difference. This makes the theory consistent with what we’ve seen.
The idea is to repeat the process and extract another estimate. Compare the estimates.
Results for the CKMfitter global fit of the CKM parameters as of Summer 2018. In the standard model quarks can change flavour by emission of a W+/W- boson. So must also change charge (e.g. up to down or vice versa). The probability for. such a transition is governed by elements of a 3 x 3 unitary CKM matrix. It contains the only source of CP violation in the standard model/the quark sector. Unitarity imposes several conditions:
Gives rise to “unitary” triangles and the areas of these triangles dictates the total amount of CP violation in the quark sector.
The bands show estimates but they are not precise enough for a point. The different colours represent different types of measurements.
All estimates correspond to a non-zero point which are put back into the theory enabling predictions to match what is seen in experiments.
Shows the difference between matter and anti-matter.
Difference is far too small to explain the amount of anti-matter as at the time of the Big Bang it should have been equal to the amount of matter. Now it would only correspond to 9 galaxies worth.
We can’t find answers through quarks
In the Standard Model of particle physics, the Cabibbo–Kobayashi–Maskawa matrix, CKM matrix, quark mixing matrix, or KM matrix is a unitary matrix which contains information on the strength of the flavour-changing weak interaction. Technically, it specifies the mismatch of quantum states of quarks when they propagate freely and when they take part in the weak interactions. It is important in the understanding of CP violation.
In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry (parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry) while its spatial coordinates are inverted (“mirror” or P symmetry).
It plays an important role both in the attempts of cosmology to explain the dominance of matter over antimatter in the present universe, and in the study of weak interactions in particle physics.
Then March 2019
New observations were unveiled.
Anti-matter charm particles behaved differently to matter charm particles
The results are consistent with the B quark picture.
A whole new laboratory. For LHCb – watch the news
It was thought that the LHC would find it before the Higgs particle
Supersymmetry extends the standard model and predicts more fundamental particles.
In particle physics, supersymmetry (SUSY) is a conjectured relationship between two basic classes of elementary particles: bosons, which have an integer-valued spin, and fermions, which have a half-integer spin. A type of spacetime symmetry, supersymmetry is a possible candidate for undiscovered particle physics, and seen by some physicists as an elegant solution to many current problems in particle physics if confirmed correct, which could resolve various areas where current theories are believed to be incomplete. A supersymmetrical extension to the Standard Model could resolve major hierarchy problems within gauge theory, by guaranteeing that quadratic divergences of all orders will cancel out in perturbation theory.
In supersymmetry, each particle from one group would have an associated particle in the other, which is known as its superpartner, the spin of which differs by a half-integer. These superpartners would be new and undiscovered particles. For example, there would be a particle called a “selectron” (superpartner electron), a bosonic partner of the electron. In the simplest supersymmetry theories, with perfectly “unbroken” supersymmetry, each pair of superpartners would share the same mass and internal quantum numbers besides spin. Since we expect to find these “superpartners” using present-day equipment, if supersymmetry exists then it consists of a spontaneously broken symmetry allowing superpartners to differ in mass. Spontaneously broken supersymmetry could solve many mysterious problems in particle physics including the hierarchy problem.
There is no experimental evidence at this time that supersymmetry is correct or whether or not other extensions to current models might be more accurate. In part this is because it is only since around 2010 that particle accelerators specifically designed to study physics beyond the Standard Model have become operational, and because it is not yet known where exactly to look nor the energies required for a successful search.
The main reasons for supersymmetry being supported by some physicists is that the current theories are known to be incomplete and their limitations are well established, and supersymmetry could be an attractive solution to some of the major concerns.
The lightest of these super symmetrical particles is considered a good candidate for dark matter. Massive to explain the gravitational effects seen but completely invisible.
The data has been looked for
Mass reach of the ATLAS searches for Supersymmetry. A representative selection of the available search results is shown. Results are quoted for the nominal cross section in both a region of near-maximal mass reach and a demonstrative alternative scenario, in order to display the range in model space of search sensitivity. Some limits depend on additional assumptions on the mass of the intermediate states, as described in the references provided in the plot. In some cases, these additional dependencies are indicated by darker bands showing different model parameters.
Each row in the table corresponds to a new super symmetrical particle looked for. The blue bands are the masses of that particle.
Data was analysed but these super-symmetrical particles were not found. The theory hasn’t been found to be true – yet.
Who says dark matter is super-symmetrical?
Some things can be tested at the LHC. 5 papers a week are published telling something new about incremental searches. Motivated by theory
Just as a picture can be worth a thousand words, so the rarest processes at the Large Hadron Collider (LHC) can sometimes have the most to tell us. By isolating and counting decays of B+ mesons to a kaon and two leptons, the LHCb experiment has tested a key assumption of the Standard Model – lepton universality, the idea that electrons, muons and tau leptons should behave in the same way, and be produced equally often in weak decays. In a presentation given at this week’s Large Hadron Collider Physics conference, LHCb results reveal the first hints of a difference.
Lepton universality is the idea that all three types of charged lepton particles—electrons, muons and taus—interact in the same way with other particles. As a result, the different lepton types should be created equally often in particle transformations, or “decays,” once differences in their mass are accounted for. However, some measurements of particle decays made by the LHCb team and other groups over the past few years have indicated a possible difference in their behaviour. Taken separately, these measurements are not statistically significant enough to claim a breaking of lepton universality and hence a crack in the Standard Model, but it is intriguing that hints of a difference have been popping up in different particle decays and experiments.
If the ratio of the number of decays containing muons, to those containing electrons, is measured instead of the individual rates, many theoretical uncertainties cancel to allow a precise probe of universality. The Belle and Babar experiments have previously measured this ratio, with limited precision, and found it consistent with the Standard Model. With its precise silicon tracker and particle identification systems, and access to large datasets, LHCb is well placed to explore the B+ meson system further.
LHCb has now analysed their entire dataset of proton-proton collisions from the LHC and finds that B+ mesons decay to muons about 25% less often than they decay to electrons. As these decays only occur a couple of times in every 10 million B+ decays the measurement is still dominated by statistical error, even if it is the world’s most precise determination. The observed difference has a significance of 2.6 standard deviations, corresponding to a chance of one in a hundred that it is due to a statistical fluctuation. More data from the forthcoming high energy LHC run is needed to confirm if this tantalising result is indeed the first sign of something new in the universe.
Recent data (several years later) was complementary
B meson was seen to decay to a K* meson and a pair of muons compared to K* meson and a pair of electrons as a function of energy.
Same level of significance though.
The LHCb update is frustrating. New black point is consistent with the old one but with less discrepancy. Muons occur 15% less often but overall, still 1 in 100 significance.
Want more data.
The more you try to understand the less you understand.
Only 5% of the possible data has been collected.
Physicists at looking at the next generation of machines for higher energies.
More videos by Professor Shears