Particle physics – Beyond the standard model

The decay of the b quark and the new physics

Dr Harry Cliff


Dr Harry Cliff is a particle physicist based at the Cavendish Laboratory at the University of Cambridge who works on the Large Hadron Collider, the world’s largest particle accelerator at CERN, near Geneva. Dr Cliff’s research involves searching for signs of new physics by detailed studies of bottom quark decays recorded by the LHCb experiment. He was also, until recently, a curator at the Science Museum, London, where he put together the critically acclaimed Collider exhibition and the 2018 exhibition, The Sun – Living With Our Star. Dr Cliff is an enthusiastic communicator, giving frequent public lectures as well as occasional TV and radio interviews.

In the talk Dr Cliff discussed the search for new physics using the decays of bottom quarks at the LHCb experiment. He explored a set of intriguing anomalies that have emerged in this area over the past few years, which “could” be the first signs of physics beyond the standard model.

My notes from the lecture (if they don’t make sense then it is entirely my fault)

Seeking truth in beauty. Searching for new physics at the LHCb experiment.

Decay of the b quark

The bottom quark or b quark, also known as the beauty quark, is a third-generation quark with a charge of −1/3e.

All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exceptionally low rates of transition to lower-mass quarks. The bottom quark is also notable because it is a product in almost all top quark decays, and is a frequent decay product of the Higgs boson.

The bottom quark was first described theoretically in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP violation. The name “bottom” was introduced in 1975 by Haim Harari. bottom left

image above right

In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation 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). (below left) (below right)


The bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation.

On its discovery, there were efforts to name the bottom quark “beauty”, but “bottom” became the predominant usage, by analogy of “top” and “bottom” to “up” and “down”.

The bottom quark’s “bare” mass is around 4.18 GeV/c2 – a bit more than four times the mass of a proton, and many orders of magnitude larger than common “light” quarks.

Although it almost-exclusively transitions from or to a top quark, the bottom quark can decay into either an up quark or charm quark via the weak interaction. CKM matrix elements Vub and Vcb specify the rates, where both these decays are suppressed, making lifetimes of most bottom particles (~10−12 s) somewhat higher than those of charmed particles (~10−13 s), but lower than those of strange particles (from ~10−10 to ~10−8 s).

ATLAS and CMS identified the Higgs boson


The discovery of the Higgs boson completed the 20th century version of particle physics – The Standard Model

There could be more particles than those seen in the standard model because of dark matter and dark energy

The standard model only accounts for 5% of the universe

The standard model has 12 known matter particles and since 1932 we know there are 12 anti-matter particles,

Collisions produce particles and antiparticles. Why is there more matter particles than antimatter particles – asymmetry.

The manifestation of the Higgs field gives particles mass. There are 2 natural values of the Higgs field.

The Higgs field is a field of energy that is thought to exist in every region of the universe. The field is accompanied by a fundamental particle known as the Higgs boson, which is used by the field to continuously interact with other particles, such as the electron. Particles that interact with the field are “given” mass and, in a similar fashion to an object passing through a treacle (or molasses), will become slower as they pass through it. The result of a particle “gaining” mass from the field is the prevention of its ability to travel at the speed of light.

What would be the effect if the Higgs had different specific values?

If the energy was large e.g. Planck energy, the effect would be bad. The Universe would be one large black hole.

If the energy was zero the effect wouldn’t be good either. Atoms wouldn’t exist. No matter, no structure.

The actual value is 246GeV. If the value were to be slightly changed the universe would become inhospitable. Nature seems to be tinkered to give the desired Higgs energy. Could the cause of this be supersymmetry?

In particle physics, supersymmetry (SUSY) is a principle that proposes a 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 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 would 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 (sparticle), 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.

Each fermion and boson would get a sparticle. Superpartners cancel out the need for fine-tuning and could account for dark matter.

The LHC is a 27km circumference particle accelerator that uses superconducting magnets to bend the paths of charged particles, normally protons, causing them to collide. It is a matter factory converting the kinetic energy of the particles into matter that doesn’t normally exist. So far it has not found supersymmetry.

The LHCb (Large Hadron Collider beauty) experiment is one of seven particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). Such studies can help to explain the matter-antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections, exotic hadron spectroscopy, charm physics and electroweak physics in the forward region. The LHCb collaboration, who built, operate and analyse data from the experiment, is composed of approximately 1260 people from 74 scientific institutes, representing 16 countries. As of 2017, the spokesperson for the collaboration is Giovanni Passaleva. The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment shares the same cavern.


Direct and indirect investigations

Direct method: If two particles of equal energy E1 and E2 collide the maximum mass of the resultant particles would be 2E/c2.

Indirect method: Looking at what happens as particles decay. The standard model enables predictions to be made. If these aren’t fulfilled then we could have new physics.

The bottom quark can decay into either an up quark or charm quark via the weak interaction.

In particle physics, a B-factory, or sometimes a beauty factory, is a particle collider experiment designed to produce and detect a large number of B mesons so that their properties and behaviour can be measured with small statistical uncertainty. Tauons and D mesons are also copiously produced at B-factories.

The processes were standard model predictions indicating new physics. Detection of the new particles infers higher energy quantum fields.

In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics and is used to construct physical models of subatomic particles (in particle physics) and quasiparticles (in condensed matter physics).

The bottom quark has a negative charge (-1e/3) and is the second heaviest particle. It is a third-generation quark.

Strong interaction with high energy particles.

Bottom particles have a long life (~10−12 s) in comparison to charm particles (~10−13 s) but lower than those of strange particles (from ~10−10 to ~10−8 s).





The fact that the two b-hadrons are predominantly produced in the same forward cone (produced along the beamline) is exploited in the layout of the LHCb detector. The LHCb detector is a single-arm forward spectrometer with a polar angular coverage from 10 to 300 milliradians (mrad) in the horizontal and 250 mrad in the vertical plane. The asymmetry between the horizontal and vertical plane is determined by a large dipole magnet with the main field component in the vertical direction.

The vertex detector (VELO) is built around the proton interaction region. It is used to measure the particle trajectories close to the interaction point in order to precisely separate primary and secondary vertices.

The detector operates at 7 millimetres from the LHC beam. This implies an enormous flux of particles; The VELO has been designed to withstand integrated fluences of more than 1014 p/cm2 per year for a period of about three years. The detector operates in a vacuum and is cooled to approximately −25 °C using a biphase CO2 system. The data of the VELO detector are amplified and read out by the Beetle ASIC.


Six key measurements have been identified involving B mesons.

The core physics programme for the first high energy LHC running in 2010–2012 include:

  • Measuring the branching ratio of the rare Bs → μ+ μ decay.
  • Measuring the forward-backward asymmetry of the muon pair in the flavour-changing neutral current Bd → K* μ+ μ decay. Such a flavour changing neutral current cannot occur at tree-level in the Standard Model of Particle Physics, and only occurs through box and loop Feynman diagrams; properties of the decay can be strongly modified by new Physics.
  • Measuring the CP-violating phase in the decay Bs → J/ψ φ, caused by interference between the decays with and without Bsoscillations. This phase is one of the CP observables with the smallest theoretical uncertainty in the Standard Model and can be significantly modified by new Physics.
  • Measuring properties of radiative B decays, i.e. B meson decays with photons in the final states. Specifically, these are again flavour-changing neutral current decays.
  • Tree-level determination of the unitarity triangle angle γ.
  • Charmless charged two-body B decays.


A beam of protons enters the LHCb detector on the left, creating a B0s particle, which decays into two muons (purple tracks crossing the whole detector). (Image: LHCb/CERN)

The Standard Model of particle physics predicts that the B0S particle, which is made of a bottom antiquark bound to a strange quark, should decay into a pair of muons (μμ) about 3 times in every billion (109) decays. LHCb’s measurement, from an analysis of data from 2011 and part of that from 2012, gives a value of (3.2+1.5-1.2) x 10-9.  LHCb spokesperson Pierluigi Campana told the CERN Bulletin that the value is “in very good agreement with the prediction.”

Particle physicists describe the certainty of a result on a scale that goes up to 5 sigma. One sigma could be a random statistical fluctuation in the data, 3 sigma counts as evidence, but only a full 5-sigma result is a discovery. The significance of the LHCb measurement is 3.5 sigma and therefore is classified as the first evidence for the B0s →μμ decay.

In theoretical physics, flavour-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical expressions that change the flavour of a fermion current without altering its electric charge.


W+ boson can change one quark into another quark. This is the only way to change quarks.


Above right: The strengths of the weak interactions between the six quarks. The “intensities” of the lines are determined by the elements of the CKM matrix.

You cannot go sideways


Probability is very low



How likely are these decays?


L = lepton

B+ Bo Bs

Data doesn’t lie with prediction

Not evidence for new physics but?

Lepton universality

In particle physics, a lepton is an elementary particle of half-integer spin (spin ​½) that does not undergo strong interactions. Two main classes of leptons exist, charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.

There are six types of leptons, known as flavours, grouped in three generations. The first-generation leptons, also called electronic leptons, comprise the electron (e) and the electron neutrino (ne); the second is the muonic leptons, comprising the muon (μ−) and the muon neutrino (nμ); and the third is the tauonic leptons, comprising the tau (t) and the tau neutrino (nt). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state.

The couplings of the leptons to the gauge bosons are flavour-independent (i.e., the interactions between leptons and gauge bosons are the same for all leptons). This property is called lepton universality and has been tested in measurements of the tau and muon lifetimes and of Z boson partial decay widths, particularly at the Stanford Linear Collider (SLC) and Large Electron-Positron Collider (LEP) experiments.

A fundamental assumption of the Standard Model is that the interactions of these elementary particles are the same despite their different masses and lifetimes. That’s lepton universality. In other words, each of the different charged leptons are identical except for their mass. Precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, but recent studies of the higher-mass tau lepton have produced observations that challenge the theory.



A new review of results from three experiments points to the strong possibility that lepton universality—and perhaps ultimately the Standard Model itself—may have to be revised.

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. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge bosons—usually as virtual particles.

SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Department of Energy Office of Science and located in Menlo Park, California.

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 LHC, which re-used the LEP tunnel. To date, LEP is the most powerful accelerator of leptons ever built.

Recent tests of lepton universality in B meson decays, performed by the LHCb, BaBar and Belle experiments, have shown consistent deviations from the Standard Model predictions. However, the combined statistical and systematic significance is not yet high enough to claim an observation of new physics.

The three experiments, which measured the relative ratios of B meson decays, posted remarkably similar results.

The probability of a b meson decaying via the weak force into an electron and electron neutrino appears to be the same for the decay of a b meson into a muon and muon neutrino.

The rates for some decays involving the heavy lepton tau, relative to those involving the light leptons—electrons or muons—were higher than the Standard Model predictions.

The experiments collect the decays and compared. If there are not equal amounts it might mean new physics.

The results are not considered sufficient to establish a violation of lepton universality. To overturn this long-held physics precept would require a significance of at least five standard deviations.

The fact that all three experiments observed a higher-than-expected tau decay rate while operating in different environments is noteworthy. A confirmation of these results would point to new particles or interactions and could have profound implications for the understanding of particle physics.

Standard model shows a balance between the number of electrons and muons produced when the b mesons decay.


1.0000 +/- 0.0001

B+ measurement in 2014 indicated

0.75 +/-0.10


3s evidence

In 2017 measurement indicated



And in 2019


The flavour problem: Why are there three generations?

New physics?


Is there a fifth force?

In physics, there are four conventionally accepted fundamental forces or interactions that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The characteristics of this fifth force depend on the theory being advanced. Many postulate a force roughly the strength of gravity (i.e. it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimetre to cosmological scales. Another proposal is a new weak force, mediated by W′ and Z′ bosons.

In particle physics, W′ and Z′ bosons (or W-prime and Z-prime bosons) refer to hypothetical gauge bosons that arise from extensions of the electroweak symmetry of the Standard Model. They are named in analogy with the Standard Model W and Z bosons. Do they actually exist?

Leptoquarks are hypothetical particles that would carry information between a generation of quarks and a generation of leptons, thus allowing quarks and leptons to interact. Generally speaking, the quark generation and the lepton generation do not need to be identical. Leptoquarks are colour-triplet bosons that carry both lepton and baryon numbers. They are encountered in various extensions of the Standard Model, such as technicolour theories or GUTs based on Pati–Salam model, SU(5) or E6, etc. Their quantum numbers, like spin, (fractional) electric charge and weak isospin, vary among theories. Do they exist?

Bottom quarks produced more than the top quark

The Future Circular Collider (FCC) is a conceptual study that aims to develop designs for a post-LHC particle accelerator with an energy significantly above that of previous circular colliders (SPS, Tevatron, LHC). After injection at 3.3 TeV, each beam would have a total energy of 560 MJ. At collision energy of 100 TeV this increases to 16.7 GJ. These total energy values exceed the present LHC by nearly a factor of 30.

The FCC study explores the feasibility of different particle collider scenarios with the aim of significantly increasing the energy and luminosity compared to existing colliders. It aims to complement existing technical designs for linear electron/positron colliders (ILC and CLIC).

The study has an emphasis on proton/proton (hadron) and electron/positron (lepton) colliders while a hadron/lepton scenario is also examined. The study explores the potential of hadron and lepton circular colliders, performing an analysis of infrastructure and operation concepts and considering the technology research and development programmes that are required to build and operate a future circular collider. A conceptual design report was published in early 2019, in time for the next update of the European Strategy for Particle Physics.

It will have a 100km long circumference.

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