# The rare Bs0 →µ+µ− decay

The Standard Model of Particle Physics describes the fundamental particles and their interactions via the strong, electromagnetic and weak forces, providing precise predictions for measurable quantities that can be tested experimentally.  Here’s the latest!!  It’s hot!!!  It’s exciting!!!  At least, if you’re a particle physicist…

Writing in Nature, physicists working on the CMS (Compact Muon Solenoid) and LHCb (Large Hadron Collider “beauty”) experiments at CERN – the Conseil Européen pour la Recherche Nucléaire – announced the discovery of a rare decay of the strange B-meson, as well as further information regarding an even rarer decay of the B0-meson.

In both cases, the decays produce two oppositely charged muons.

# Quarks and Mesons

In particle physics, mesons are hadronic sub-atomic particles composed of one quark and one antiquark, bound together by the strong interaction.  Because mesons are composed of sub-particles, they have a physical size, with a diameter roughly one femto-metre (10-15 metre), which is about 23 the size of a proton or neutron.  Mesons appear in Nature as short-lived products of very high-energy interactions between particles made of quarks.

## Quarks

Quarks are elementary particles and fundamental constituents of matter.  They have various intrinsic properties, including electric charge, mass, colour charge and spin.  They are the only elementary particles in the Standard Model of Particle Physics to experience all four fundamental interactions, known as fundamental forces (i.e. electromagnetism, gravitation, strong interaction and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge.

There are six “flavours” of quarks:

up, down, strange, charm, top, bottom.

Up and down quarks have the lowest masses of all quarks.  The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state.

For that reason, up and down quarks are generally stable and the most common quarks in the Universe, whereas the strange, charm, bottom, and top quarks may only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators).

For every flavour of quark, there is a corresponding type of anti-particle – an antiquark, that differs from the quark in that some of its properties have equal magnitude but opposite sign.

Unlike alpha-particles, mesons are not produced by radioactive decay.  In cosmic ray interactions, such particles are ordinary protons and neutrons.  All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond.  Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos.  Uncharged mesons may decay to photons.

Mesons are also produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles.

# The B-meson

Now, B mesons are mesons composed of a bottom antiquark and either an up (B+), down (B0), strange (B0s) or charm quark (B+c) – the combination of a bottom antiquark and a top quark being assumed impossible because of the top quark’s exceedingly short lifetime.

#### The Standard Model predicts that the $B^0 _s \longrightarrow \mu^+ \mu^-$$B^0 _s \longrightarrow \mu^+ \mu^-$, and $B^0 \longrightarrow \mu^+ \mu^-$$B^0 \longrightarrow \mu^+ \mu^-$ decays are very rare, with about four of the former occurring for every billion B0s mesons produced, and one of the latter occurring for every ten billion B0 mesons.

The probabilities, or branching fractions, of the strange B meson B0s and the B0 meson decaying into two oppositely charged muons (μ+ and μ) are especially interesting because of their sensitivity to theories that extend the Standard Model.

A difference in the observed branching fractions with respect to the predictions of the standard model would provide a direction in which the Standard model should be extended.

The Standard Model of particle physics also predicts that both processes are very unlikely, and this means that both decays should be very sensitive to the existence of physics beyond the Standard Model.  In other words, if the measured decay rates differ from those predicted by the Standard Model, this could provide important clues about some of physics most exotic mysteries, such as dark matter and the scarcity of antimatter in our Universe.

Indeed, a deviation could even be an important milestone on the long journey towards a Theory of Everything that reconciles the Standard Model with the General Theory of Relativity.  But before the Large Hadron Collider (LHC) at CERN started operating, no evidence for either decay mode had been found.  Upper limits on the branching fractions were an order of magnitude above the Standard Model predictions.

# CMS and LHCb Collaboration

The CMS Collaboration & LHCb Collaboration (2015) have performed a joint analysis of the data from proton-proton collisions collected in 2011 at a centre-of-mass energy of 7 TeV (tera-electronvolts), and in 2012 at 8 TeV.

$1 TeV = 10^{12} eV$.

The particle physicists report the first observation of the $B^0 _s \longrightarrow \mu^+ \mu^-$ decay, with a statistical significance exceeding six standard deviations – the best measurement so far of its branching fraction.

They obtained evidence for the $B^0 _s \longrightarrow \mu^+ \mu^-$ decay with a statistical significance of three standard deviations.  Both measurements are statistically compatible with Standard Model predictions and allow stringent constraints to be placed on theories beyond it.

The LHC experiments resumed taking data in March 2015, recording proton-proton collisions at a centre-of-mass energy of 13 TeV (tera-electronvolts), which will approximately double the production rates of B0s and B0 mesons and lead to further improvements in the precision of these crucial tests of the Standard Model.

But the Standard Model holds, and the combined CMS/LHCb decay rate for the strange B-meson is just as predicted.

The B0 decay rate also appears to fall in line with the SM.  However, it is not yet deemed a “discovery” because the statistical significance of the measurement is only about 3σ – well short of the required 5σ.

Dr Vakhtang Kartvelishvili who works on the ATLAS experiment at CERN, pointed out in a paper called “Particle physics discovery raises hope for a theory of everythingthat the measured B0 decay rate is about four times larger than predicted by the Standard Model – while still being statistically compatible.  The title reflects the hope in the particle physics community that important clues about physics beyond the Standard Model could soon be forthcoming at the LHC.

So there are two possible scenarios:

• If the Universe is kind to particle physicists, this discrepancy will endure as more data are collected on the decay in the upcoming runs of the LHC.
• If the Universe is very unkind, you may get what some critics have called the “nightmare scenario of particle physics” in which physicists build increasingly energetic accelerators, yet never reach energies high enough to realise physics beyond the Standard Model.

With collisions at 13 TeV at the LHC, we may not have to wait for long to see if a breakthrough is forthcoming…

An animation of how the strange B-meson decay is detected by the CMS appears in the video below: