Measurement of the CP-violating phase ϕ_s in the B_s⁰→J/ψϕ(1020)→μ⁺μ⁻K⁺K⁻ channel in proton-proton collisions at √s=13 TeV

The CP -violating weak phase ϕ_s and the decay width difference ΔΓ_s between the light and heavy B_s⁰ mass eigenstates are measured with the CMS detector at the LHC in a sample of 48500 reconstructed B_s⁰→J/ψϕ(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 ϕ_s and ΔΓ_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 ΔΓ_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, ΔΓ_s = 0.1032 ± 0.0095 (stat) ± 0.0048 (syst) ps^-1, a significant improvement over the 8 TeV result.