This article is a result of a combined analysis done by Compact Muon Solenoid (CMS) and Large Hadron Collider beauty (LHCb) collaborations at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The newly setup National Centre for Particle Physics (NCPP) at UM has been a member of the CMS collaboration since October 2013. The CMS detector is one of four gigantic and multipurpose detectors built for experimentation at the LHC and the goal of CMS experiments is to investigate a wide range of physics.
A major milestone
The CMS and LHCb experiments reveal a new rare particle decay. In an article published recently in Nature, the CMS and LHCb collaborations describe the first observation of the very rare decay of the B0s particle into two muon particles. This is the first time that CMS and LHCb have analysed their data together. This exciting result is a major milestone in a search involving many experiments over almost three decades. It has important implications in the search for new particles and phenomena beyond the Standard Model, when the LHC physics run restarts in subsequent weeks.
Developed in the early 1970s, the Standard Model is currently the best description of the subatomic world. It has successfully explained almost all experimental results in particle physics and precisely predicted a wide variety of phenomena. However, it doesn’t answer some important aspects such as dark matter or the matter-antimatter asymmetry. To solve some of these enigmas, LHC experiments are trying to find hints of “new” physics. There are two complementary strategies to probe the physics “beyond” the Standard Model, which are both employed by the experiments at the LHC. The direct one, which looks for new particles predicted by theoretical models that go beyond the Standard Model, such as supersymmetry, and the indirect one, which challenges the Standard Model on its predictions for very rare decays. Any discrepancy between the experimental results on these very rare processes and the Standard Model’s predictions would point to signs of new physics. This is the strategy adopted by the CMS and LHCb experiments in studying the rare decays of the B0s and B0 particles into two muons.
The analysis is based on data taken at the Large Hadron Collider (LHC) in 2011 and 2012. These data also contain early hints of a similar, but even more rare decay into two muons of the B0, a cousin of the B0s. The B0s and B0 are mesons, in other words, non-elementary unstable subatomic particles composed of a quark and an antiquark, bound together by the strong interaction. Such particles are produced only in high-energy collisions – at particle accelerators, or in nature, for example in cosmic-ray interactions. The two collaborations first released their individual results for B0s meson decay in July 2013. While the results were in excellent agreement, both fell just below the 5 sigma statistical precision historically needed to claim an observation. The combined analysis easily exceeds this requirement, reaching 6.2 sigma.
This combined analysis shows that the probability for an B0s meson to decay into two muons, and the probability for a B0 meson to decay into two muons, are consistent with the Standard Model's predictions. So at least, for now, these rare decays have not revealed any hints of new physics. However, the data to be gathered in future runs of the LHC will increase the precision of the B0s measurement and will determine whether the possible hints of the related decay of the B0 are confirmed. These results will be crucial for disentangling any signs of new phenomena from Standard Model effects and will advance the hunt for new physics.
