Observation of the rare Bs0 →μ+μ− decay from the combined analysis of CMS and LHCb data

Abstract

Combined analysis of proton-proton collision data from the Large Hadron Collider at CERN by the CMS and LHCb collaborations leads to the observation of the extremely rare decay of the strange B meson into muons; the result is compatible with the standard model of particle physics, and does not show any signs of new physics, such as supersymmetry. When searching for physics beyond the standard model of particle physics, one promising route is to consider processes at very high energies that can be produced in particle colliders. Here, the CMS and LHCb collaborations working at the Large Hadron Collider at CERN, the largest particle accelerator in the world, report the observation of the extremely rare decay of a B meson into muons. In this decay, discrepancies from the standard model predictions might point towards supersymmetry which is thought to be a plausible candidate for a theory beyond the standard model. The combined results from the CMS and LHCb collaborations, however, confirm the standard model and do not show any signs of supersymmetry. The restart of the Large Hadron Collider this spring with higher operation energies will increase the production rate of B mesons and might bring new surprises and constraints for theories beyond the standard model of particle physics. The standard model of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The probabilities, or branching fractions, of the strange B meson ( ) 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. The standard model predicts that the and decays are very rare, with about four of the former occurring for every billion mesons produced, and one of the latter occurring for every ten billion B0 mesons1. 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. Before the Large Hadron Collider (LHC) at CERN2 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. The CMS (Compact Muon Solenoid) and LHCb (Large Hadron Collider beauty) collaborations have performed a joint analysis of the data from proton–proton collisions that they collected in 2011 at a centre-of-mass energy of seven teraelectronvolts and in 2012 at eight teraelectronvolts. Here we report the first observation of the µ+µ− decay, with a statistical significance exceeding six standard deviations, and the best measurement so far of its branching fraction. Furthermore, we obtained evidence for the µ+µ− 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 the standard model. The LHC experiments will resume taking data in 2015, recording proton–proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of and B0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.