Measurement of properties of B$^0_\mathrm{s}\to\mu^+\mu^-$ decays and search for B$^0\to\mu^+\mu^-$ with the CMS experiment

TL;DR

This paper reports the measurement of the B_s to mu+mu- decay properties, including its branching fraction and lifetime, and searches for the B0 to mu+mu- decay using CMS data from 2011 to 2016, confirming Standard Model predictions.

Contribution

First measurement of B_s to mu+mu- branching fraction and lifetime, and the most stringent limit on B0 to mu+mu- decay with CMS data.

Findings

B_s to mu+mu- decay observed with 5.6 sigma significance.

Branching fraction of B_s to mu+mu- measured as (2.9 ± 0.7 (exp) ± 0.2 (frag)) × 10^{-9}.

No significant excess for B0 to mu+mu-, upper limit set at 3.6 × 10^{-10}.

Abstract

Results are reported for the B$^0_\mathrm{s}\to\mu^+\mu^-$ branching fraction and effective lifetime and from a search for the decay B$^0\to\mu^+\mu^-$. The analysis uses a data sample of proton-proton collisions accumulated by the CMS experiment in 2011, 2012, and 2016, with center-of-mass energies (integrated luminosities) of 7 TeV (5 fb$^{-1}$), 8 TeV (20 fb$^{-1}$), and 13 TeV (36 fb$^{-1}$). The branching fractions are determined by measuring event yields relative to B$^+\to$ J/$\psi$K$^+$ decays (with J/$\psi\to\mu^+\mu^-$), which results in the reduction of many of the systematic uncertainties. The decay B$^0_\mathrm{s}\to\mu^+\mu^-$ is observed with a significance of 5.6 standard deviations. The branching fraction is measured to be $\mathcal{B}$(B$^0_\mathrm{s}\to\mu^+\mu^-$) = [2.9 $\pm$ 0.7 (exp) $\pm$ 0.2 (frag)] $\times$ 10$^{-9}$, where the first uncertainty combines the experimental statistical and systematic contributions, and the second is due to the uncertainty in the ratio of the B$^0_\mathrm{s}$ and the B$^+$ fragmentation functions. No significant excess is observed for the decay B$^0\to\mu^+\mu^-$, and an upper limit of $\mathcal{B}$(B$^0\to\mu^+\mu^-$) < 3.6 $\times$ 10$^{-10}$ is obtained at 95% confidence level. The B$^0_\mathrm{s}\to\mu^+\mu^-$ effective lifetime is measured to be $\tau_{\mu^+\mu^-}$ = 1.70 $^{+0.61}_{-0.44}$ ps. These results are consistent with standard model predictions.