Sumario: | The ATLAS experiment has performed accurate measurements of mixing, CP violation, and the lifetime properties in the neutral B mesons using the 2015-17 data. This talk will focus on the latest results from the ATLAS collaboration on the CP violation in the $B_s^0 \to J/\psi \phi $ decays. The Standard Model predicts the CP-violating mixing phase, $\phi_s$, to be very small and its SM value is very well constrained. The decay width difference $\Delta \Gamma_s$ is not expected to be sensitive to New Physics and its value is also very well predicted. The effective lifetime of $B_s^0$ is given by the inverse value of the average decay width $\Gamma_s$. Latest measurements of $phi_s$, $\Delta \Gamma_s$, $\Gamma_s$ and several other parameters describing the $B_s^0 \to J/\psi \phi$ decays will be reported.
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