The C P-violating weak phase φs and the decay width difference s between the light and heavy B0
s
mass eigenstates are measured with the CMS detector at the LHC in a sample of 48 500 reconstructed
B0
s → J/ψ φ(1020) → μ+μ− K+K− events. The measurement is based on a data sample corresponding
to an integrated luminosity of 96.4 fb−1, collected in proton-proton collisions at √s = 13 TeV in
2017–2018. To extract the values of φs and s, a time-dependent and flavor-tagged angular analysis
of the μ+μ−K+K− final state is performed. The analysis employs a dedicated tagging trigger and a
novel opposite-side muon flavor tagger based on machine learning techniques. The measurement yields
φs = −11±50 (stat)±10 (syst) mrad and s = 0.114±0.014 (stat)±0.007 (syst) ps−1, in agreement with
the standard model predictions. When combined with the previous CMS measurement at √s = 8 TeV,
the following values are obtained: φs = −21 ± 44 (stat) ± 10 (syst) mrad, s = 0.1032 ± 0.0095 (stat) ±
0.0048 (syst) ps−1, a significant improvement over the 8 TeV result.