by Daria Zieminska from Nature (2015) doi:10.1038/nature14520
The extremely rare decays of particles known as neutral B mesons have been observed at CERN’s Large Hadron Collider. The result may be a glimpse of physics beyond that of the standard model of particle physics.
For more than three decades, physicists have been looking for the decay of the ‘strange B meson’ particle into a pair of muons, the heavy cousins of electrons. The process is incredibly rare, and harder to find than the famous Higgs particle, the discovery of which at the Large Hadron Collider at CERN, near Geneva, Switzerland, was celebrated worldwide in 2012. The standard model of elementary particle physics(1) makes an exact prediction of the number of particle-decay events researchers should observe in an experiment. Anything more than the predicted value means potential trouble for the standard model. In a paper published on Nature‘s website, researchers working on the CMS and LHCb collaborations(2) at the Large Hadron Collider describe a joint analysis of data from proton collisions that set the decay rate of the strange B meson at about three in one billion — in agreement with the standard-model prediction. However, they find that the decay rate of another type of neutral B meson, the ‘non-strange’ B meson, is at odds with the expectation from the standard model.
The standard model is at a crossroads. It has been very successful in describing elementary particles and their interactions, but such particles comprise only 4% of the known Universe. The theory does not provide a candidate for the dark matter that binds galaxies together and makes up one-quarter of the cosmos. Nor does it accommodate dark energy, the remaining, unknown component of the Universe that is causing it to expand at an accelerated rate. It also does not explain the preponderance of matter over antimatter. Lastly, it makes a worrisome warning that the Universe is probably unstable, ready to collapse in a ‘big crunch’.
Many models have been proposed to solve some of these problems. One of the most compelling ideas for unknown physics beyond that of the standard model is supersymmetry(3), affectionately called SUSY. Supersymmetry states that, for every known particle, there is a twin ‘superparticle’ of much higher mass. These superparticles could in principle be produced in colliders. They should quickly decay to lighter superparticles and ordinary particles, except for the lightest superparticle, which should be stable — and that is SUSY’s candidate particle for dark matter.
Physicists have been searching for SUSY superparticles for years, so far with no success. In the absence of direct observations, they watch for discrepancies of measurements of particle properties from standard-model predictions. The decay of neutral B mesons to muons (Fig. 1) is a sensitive test of the standard model because the model predicts the decay rate with good precision. B mesons are made up of one quark and a ‘bottom’ antiquark, the antimatter partner to the quark; quarks are the elementary building blocks of protons and neutrons, and come in six flavours (up, down, strange, charm, top and bottom). There are two kinds of neutral B meson, which have no charge. One type, the strange B meson (Bs0), contains a bottom antiquark and a strange quark. The other, the non-strange B meson (B0), has a bottom antiquark paired with a down quark. The decay of the neutral B mesons to a pair of muons would mean that the bottom antiquark and its quark partner annihilate, and that the energy released in the process is given to the muons.
But the standard model forbids the annihilation of quarks of different flavours, so it predicts the decay of the neutral B mesons into muons through an intermediate process that involves the exchange of a top quark between the quarks and the emission of two W bosons (elementary particles that mediate the weak nuclear force). The decay of the strange B meson is expected to occur by this process in about four parts in one billion, and that of the non-strange B meson in about one part in ten billion. However, if yet-unknown SUSY superparticles are exchanged between the quarks in addition to the top-quark exchange, these decay rates will be greatly enhanced relative to the standard-model rate.
The decay rate of the strange B meson observed by the CMS and LHCb collaborations confirms the standard-model prediction. That is good news for the standard model, but not such good news for physics beyond it. However, the decay of the non-strange B meson, which the authors also observed, albeit with a lower statistical significance than obtained for the strange B meson, exceeded the standard-model expectation by almost fourfold — something to watch in the years to come.
CMS and LHCb are two of seven particle detectors at the Large Hadron Collider. Their designs follow different concepts. CMS is a large cylinder (21.6 metres long and 14.6 metres in diameter) in which two counter-propagating beams of protons collide and give rise to neutral B mesons, among many other particles. LHCb is specifically designed to study B mesons, which tend to stay close to the line of the beam pipe. Unlike the CMS detector, which surrounds the proton collision point, the LHCb detector is a stack of instruments stretching for 20 metres along the beam pipe on one side of the collision point. But the two teams adopted a similar strategy to analyse their data. Both groups selected particle events that involved two oppositely charged muons travelling from a common point, which is displaced by a few hundred micrometres from the point at which the protons collide. The events associated with the decay of a neutral B meson are a small fraction of initial candidates. The rest are random pairs of muons originating from other, more common processes.
To separate the signal of the neutral B meson from background events, the teams each built a ‘decision tree’ — a sequence of binary splits of data into signal-like and background-like parts. The system ‘learns’ to distinguish between signal and background by ‘training’ on a simulated sample of the signal and on a sample of real data representing background events. For the selected signal-like events, the researchers deduced the mass of the parent particles using the momenta and directions of travel of the two muons. They then compared the spectrum of the deduced masses with that predicted for a sum of two bell-shaped curves corresponding to the two kinds of neutral B meson, strange and non-strange, and a smooth background.
The two collaborations had previously performed this type of analysis, and each reported their results in separate publications(4, 5). But it was only the combination of data from the two experiments that allowed the researchers to observe with high statistical significance the decay of the strange B meson. In the process, the researchers identified, and corrected, issues with the previous analyses. In particular, they isolated and subtracted a background from the decay of a particle called a bottom Lambda baryon that mimics the signal of a neutral B meson.
Studies of B-meson decays will continue in the coming years. The Large Hadron Collider has just restarted after a two-year break for upgrades, and will soon accelerate proton beams to an energy of 13 teraelectronvolts (TeV), increased from the 8-TeV level reached before the upgrades. The proton beams will also be more tightly focused and will collide at a higher rate than that achieved so far. Both experiments will collect a large number of rare events, and should eventually find which path away from the standard model nature has chosen.
(1) The Standard Model
(2) CMS Collaboration & LHCb Collaboration. Nature (2015).
(3) Supersymmetry
(4) Chatrchyan, S. et al. (CMS Collaboration) Phys. Rev. Lett. 111, 101804 (2013).
(5) Aaij, R. et al. (LHCb Collaboration) Phys. Rev. Lett. 111, 101805 (2013).