Measurement of the B${}_{\mathrm{s}}^0$ $\to$ J/$\psi$K${}_{\mathrm{S}}^0$ effective lifetime from proton-proton collisions at $\sqrt{s}$ = 13 TeV

The effective lifetime of the B${}_{\mathrm{s}}^0$ meson in the decay B${}_{\mathrm{s}}^0$ $\to$ J/$\psi$K${}_{\mathrm{S}}^0$ is measured using data collected during 2016-2018 with the CMS detector in $\sqrt{s}$ = 13 TeV proton-proton collisions at the LHC, corresponding to an integrated luminosity of 140 fb${}^{-1}$. The effective lifetime is determined by performing a two-dimensional unbinned maximum likelihood fit to the B${}_{\mathrm{s}}^0$ meson invariant mass and proper decay time distributions. The resulting value of 1.59 $\pm$ 0.07 (stat) $\pm$ 0.03 (syst) ps is the most precise measurement to date and is in good agreement with the expected value.