LHCb: Search for CP violation in $B_0 \to J/\psi K_s^0$ decays with first LHCb data

We report a measurement of the well-established CP violation in $B_0 \to J/\psi K_s^0$ decays. We perform a time-dependent analysis of the decays reconstructed in 35 pb $^{-1}$ of LHCb data that was taken in 2010. We measure the CP asymmetry parameter $S_{ J/\psi K_s^0}$ , which is connected to the CKM angle $\beta$ through $S_{ J/\psi K_s^0}$ = sin2$\beta$, neglecting CP violation in $B^0$-$ \overline{B^0}$ mixing and decay. We find $S_{ J/\psi K_s^0} $ = 0.53$^{+0.28}_{-0.29}$(stat) $\pm$ 0.05(syst).