First evidence for the decay Bs -> mu+ mu-

TL;DR

This paper reports the first evidence for the rare decay Bs -> mu+ mu- with a 3.5 sigma significance, measuring its branching fraction consistent with the Standard Model, based on LHCb data from 2011-2012.

Contribution

First experimental evidence for the decay Bs -> mu+ mu- with a measured branching fraction aligning with Standard Model predictions.

Findings

Evidence for Bs -> mu+ mu- with 3.5 sigma significance

Measured branching fraction BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9

Upper limit for B0 -> mu+ mu- set at 9.4 x 10^-10

Abstract

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.