Updated measurement of CP violation and polarisation in Bs0→J/ψK¯∗8920 decays

A time-integrated angular analysis of the decay Bs0→J/ψK¯∗8920, with J/ψ → μ⁺μ⁻ and K¯∗8920→K−π+, is presented. The analysis employs a sample of proton-proton collision data collected by the LHCb experiment during 2015–2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb⁻¹. A simultaneous maximum-likelihood fit is performed to the angular distributions in bins of the K⁻π⁺ mass. This fit yields measurements of the CP-averaged polarisation fractions and CP asymmetries for the P-wave component of the K⁻π⁺ system. The longitudinal and parallel polarisation fractions are determined to be f₀ = 0.534 ± 0.012 ± 0.009 and f_|| = 0.211 ± 0.014 ± 0.005, respectively, where the first uncertainty is statistical and the second is systematic. The CP asymmetries are measured with 3–7% precision and are found to be consistent with zero. These measurements, along with an updated determination of the branching fraction relative to the B⁰ → J/ψK^*0 decay, are combined with previous LHCb results, providing the most precise values for these observables to date.