Measurement of the weak mixing phase $\phi_s$ through time-dependent $CP$ violation in $B_s^0 \to J/\psi\phi$ decay in ATLAS

An updated 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. In the Standard Model, $CP$ violation arises due to a single complex phase in the CKM quark mixing matrix. Precise measurements of the CKM parameters therefore constrain the Standard Model, and may reveal new physics effects. Results presented in this talk are compatible with those obtained from 19.2 fb$^{-1}$ of 7 TeV and 8 TeV ATLAS data as well as with the Standard Model predictions and other LHC measurements. The measurement of the weak mixing phase $\phi_s$ improves on the precision of the previous ATLAS result by a factor of two.