Measurement of the CP-violating phase $\phi_s$ in the $B_s \to J/\psi\phi$ decays at CMS

In this talk we present a measurement of the time-dependent CP-violating phase $\phi_s$ in the decays of $B_s \to J/\psi \phi$. The phase $\phi_s$ is the key parameter for the CP-violation of the $B_s$ and $\overline{B_s}$ system. An angular and proper decay time analysis is applied to the $B_s\to J/\psi \phi$ events. Using a data sample collected by the CMS experiment, the $B_s$ signal candidates are reconstructed and are used to extract the phase $\phi_s$. The theoretical prediction of the $\phi_s$ angle is particularly robust, thus any deviation from the prediction can be a smoking gun signal of new physics.