Measurement of $CP$ violation and mixing in $B_s^0 \to J/\psi\phi$ in ATLAS

An overview of a measurement of time-dependent $CP$ asymmetry parameters in $B_s^0 \to J/\psi(\mu^+\mu^-)\phi(K^+K^-)$ decays using 80.5 fb$^{-1}$ of integrated luminosity collected with the ATLAS detector from 13 TeV $pp$ collisions at the LHC is presented. A manifestation of the $CP$ violation is an interference between direct and mixing-mediated $B_s^0$ decays, producing a common final state. In the Standard Model, it is characterized by a phase shift $\phi_s$ related to the CKM matrix. In the case of $B_s^0 \to J/\psi\phi$, this shift is predicted to be small, $\phi_s = -0.0363^{+0.0016}_{-0.0015}$ rad. New physics can enhance $\phi_s$ whilst satisfying all existing constraints. Results presented in this talk are compatible with those obtained from 19.2 fb$^{-1}$ of 7 TeV and 8 TeV data as well as with the Standard Model predictions and other LHC measurements.