Measurement of the B0 s → μ + μ − Branching Fraction and Effective Lifetime and Search for B0 → μ + μ − Decays

A search for the rare decays B0 s → μþμ− and B0 → μþμ− is performed at the LHCb experiment using data collected in pp collisions corresponding to a total integrated luminosity of 4.4 fb−1. An excess of B0 s → μþμ− decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be BðB0 s → μþμ−Þ¼ð3.0 0.6þ0.3 −0.2 Þ × 10−9, where the first uncertainty is statistical and the second systematic. The first measurement of the B0 s → μþμ− effective lifetime, τðB0 s → μþμ−Þ ¼ 2.04 0.44 0.05 ps, is reported. No significant excess of B0 → μþμ− decays is found, and a 95% confidence level upper limit, BðB0→ μþμ−Þ < 3.4 × 10−10, is determined. All results are in agreement with the standard model expectations. D