  Results on $B \to \mu^+ \mu^-$ decays with the CMS experiment are reported,
using 61 fb$^{-1}$ of data recorded during LHC Run 1 and 2016. With an improved
muon identification algorithm and refined unbinned maximum likelihood fitting
methods, the decay $B_s^0 \to \mu^+ \mu^-$ is observed with a significance of
5.6 standard deviations. Its branching fraction is measured to be BF($B^0_s \to
\mu^+ \mu^-$) = [2.9+/-0.7(exp)+/-0.2(frag)]$\times 10^{-9}$, where the first
error is the combined statistical and systematic uncertainty and the second
error quantifies the uncertainty of the $B^0_s$ and $B^+$ fragmentation
probability ratio. The $B^0_s \to \mu^+ \mu^-$ effective lifetime is
$\tau_{\mu^+ \mu^-} =1.70^{+0.61}_{-0.44}$ps. No evidence for the decay $B^0
\to \mu^+ \mu^-$ is found and an upper limit of BF($B^0 \to \mu^+ \mu^-$) <
3.6$\times 10^{-10}$ (at 95% confidence level) is determined. All results are
consistent with the standard model of particle physics.
