The decay-time-dependent CP asymmetry in Bs0→J/ψK+K−
decays is measured using proton-proton collision data, corresponding to an
integrated luminosity of 1.9fb−1, collected with the LHCb
detector at a centre-of-mass energy of 13TeV in 2015 and 2016.
Using a sample of approximately 117\,000 signal decays with an invariant K+K− mass in the vicinity of the ϕ(1020) resonance, the CP-violating
phase ϕs is measured, along with the difference in decay widths of the
light and heavy mass eigenstates of the Bs0-Bˉs0 system,
ΔΓs. The difference of the average Bs0 and B0 meson
decay widths, Γs−Γd, is determined using in addition a sample of
B0→J/ψK+π− decays. The values obtained are ϕs=−0.083±0.041±0.006rad, ΔΓs=0.077±0.008±0.003ps−1 and Γs−Γd=−0.0041±0.0024±0.0015ps−1, where the first uncertainty is statistical and the
second systematic. These are the most precise single measurements of these
quantities to date and are consistent with expectations based on the Standard
Model and with a previous LHCb analysis of this decay using data recorded at
centre-of-mass energies 7 and 8 TeV. Finally, the results are combined with
recent results from Bs0→J/ψπ+π− decays obtained using
the same dataset as this analysis, and with previous independent LHCb results