Mixing and CP Violation in the B System

The determination of the CP-violating phase $\phi_s$ in $B^0_s \to J/\psi\phi$ decays is one of the key goals of the LHCb experiment. Its value is predicted to be very small in the Standard Model but can be significantly enhanced in many models of new physics. We present the world’s best measurement of $\phi_s$ and the first observation of a non-zero $\Delta\Gamma_s$ based on 1.0 fb$^{−1}$ of data collected at LHCb during 2011. We also present a measurement of $\phi_s$ using the predominantly CP-odd $B^0_s \to J/\psi\pi\pi$ decay mode. Additionally we present results on other $B_s^0$ decay modes, relevant for potential future measurements of $\phi_s$, such as $B_s^0 \to \phi\phi$ and $B_s^0 \to J/\psi K^{*0}$ decays.