The efective lifetime of the B 0 s meson in the decay B 0 s → J/ψK 0 S is measured using data collected during 2016–2018 with the CMS detector in √ s = 13 TeV proton-proton collisions at the LHC, corresponding to an integrated luminosity of 140 fb−1 . The efective lifetime is determined by performing a two-dimensional unbinned maximum likelihood ft to the B 0 s meson invariant mass and proper decay time distributions. The resulting value of 1.59 ± 0.07(stat) ± 0.03(syst) ps is the most precise measurement to date and is in good agreement with the expected value.
