This measurement yielded a lifetime of about 1.7 picoseconds, consistent with the SM. The interest in examining the B s ⟶ μ +μ - lifetime is that, just as for the decay rate, new physics might alter its value from the SM expectation. Moreover, CMS managed to measure the effective lifetime of the B s meson, using the several dozen B s ⟶ μ +μ - decay events that were observed. The peaks corresponding to the B s and B 0 mesons are shown by hatched histograms, while dashed lines represent the various background components. There was no sign of the B 0 ⟶ μ +μ - decay yet, but a stringent limit of less than 0.36 ppb was set on its rate.įigure 2: The reconstructed mass spectrum of the dimuon pair. The branching fraction was found to be consistent with the SM prediction, and thus no evidence for new physics was observed. This new result allowed observation of the elusive B s ⟶ μ +μ - decay with a significance of 5.6 standard deviations. The first observation based on the LHC Run 1 data was presented in 2013, in a joint publication ( Nature 522 (2015) 68) by the CMS and LHCb Collaborations.Įarlier this month, the CMS Collaboration updated its 2013 analysis and added the 2016 LHC Run 2 data (collected in 13 TeV proton-proton collisions, rather than at 7 and 8 TeV of Run 1).
It took over a quarter-century of extensive effort to establish the B s ⟶ μ +μ - decay. In watching a billion or more B s mesons disintegrate, one would, therefore, observe only a mere handful of these events! The B 0 meson, a lighter cousin of the B s meson, made from a down quark and a beauty antiquark, is expected to decay to a μ +μ - pair even more rarely, about once every ten billion decays. In the SM, the rate of this decay process is predicted to be only about 3.6 parts per billion (ppb). The two muons come not from the collision point (yellow dot) but form a displaced vertex of the B s meson decay. The tracks other than the muon ones have been removed for clarity. The inset zooms in on the innermost CMS detector region. The two curved red lines correspond to the two muons from the decay. The sensitivity to new physics rivals and might even exceed that of direct searches for new particles.įigure 1: An event display of a B s ⟶ μ +μ - candidate in Run 2 data. Why are searches for this rare decay so important? It is because the rate might be enhanced significantly by the presence of new particles, and this decay thereby offers excellent sensitivity to physics beyond the standard model (SM). The B s meson is a bound state of a strange quark and a beauty antiquark – as such it possesses both beauty and strangeness! For many years, searches for an extremely rare decay of the B s meson, to a μ +μ - pair, remained a holy grail of particle physics.
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