Measurement of properties of Bs0 → μ⁺μ ⁻ decays and search for B⁰ → μ⁺μ ⁻ with the CMS experiment

Results are reported for the Bs0→ μ⁺μ⁻ branching fraction and effective lifetime and from a search for the decay B⁰→ μ⁺μ⁻. 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⁺→ J/ψK⁺ decays (with J/ψ → μ⁺μ⁻), which results in the reduction of many of the systematic uncertainties. The decay Bs0→ μ⁺μ⁻ is observed with a significance of 5.6 standard deviations. The branching fraction is measured to be ℬ(Bs0→μ+μ−)=[2.9±0.7(exp)±0.2(frag)]×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 Bs0 and the B⁺ fragmentation functions. No significant excess is observed for the decay B⁰→ μ⁺μ⁻, and an upper limit of ℬ(B⁰ → μ⁺μ⁻) < 3.6 × 10⁻¹⁰ is obtained at 95% confidence level. The Bs0→ μ+μ− effective lifetime is measured to be τμ+μ−=1.70−0.44+0.61 ps. These results are consistent with standard model predictions. [Figure not available: see fulltext.].