Analysis of Neutral B-Meson Decays into Two Muons

Branching fraction and effective lifetime measurements of the rare decay B0s → μþμ− and searches for the decays B0 → μþμ− and B0s → μþμ−γ are reported using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a luminosity of 9 fb−1. The branching fraction BðB0 → μþμ−Þ 1⁄4 ð3.09þ0.46þ0.15Þ × 10−9 and the effective lifetime τðB0 → μþμ−Þ 1⁄4 s −0.43−0.11 s 2.07 0.29 0.03 ps are measured, where the first uncertainty is statistical and the second systematic. No significant signal for B0 → μþμ− and B0s → μþμ−γ decays is found and upper limits BðB0 → μþμ−Þ < 2.6 × 10−10 and BðB0s → μþμ−γÞ < 2.0 × 10−9 at the 95% C.L. are determined, where the latter is limited to the range mμμ > 4.9 GeV=c2. The results are in agreement with the standard model expectations.