Measurement of the B${}_{\mathrm{S}}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay properties and search for the B${}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Measurements are presented of the B${}_{\mathrm{S}}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ branching fraction and effective lifetime, as well as results of a search for the B${}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay in proton-proton collisions at $\sqrt{s}$ = 13 TeV at the LHC. The analysis is based on data collected with the CMS detector in 2016-2018 corresponding to an integrated luminosity of 140 fb${}^{-1}$. The branching fraction of the B${}_{\mathrm{S}}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay and the effective B${}_{\mathrm{S}}^0$ meson lifetime are the most precise single measurements to date. No evidence for the B${}^0$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay has been found. All results are found to be consistent with the standard model predictions and previous measurements.