Measurement of the C P-violating phase φs in the B0 s → J/ψφ(1020) → μ+μ− K+K− channel in proton-proton collisions at √s = 13 TeV

Abstract

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.