Measurement of CP violation in $B^0\to \psi (\to {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-})K_S^0(\to {\pi }^{+}{\pi }^{-})$ decays

A measurement of time-dependent CP violation in the decays of $B^0$ and ${\bar{B}}^0$ mesons to the final states $J\mathrm{/}\psi (\to {\mu }^{+}{\mu }^{-})K_S^0$, $\psi (2S)(\to {\mu }^{+}{\mu }^{-})K_S^0$ and $J\mathrm{/}\psi (\to e^{+}{e}^{-})K_S^0$ with $K_S^0\to {\pi }^{+}{\pi }^{-}$ is presented. The data correspond to an integrated luminosity of 6 fb${}^{-1}$ collected at a center-of-mass energy of $\sqrt{s}=13$ TeV with the LHCb detector. The CP-violation parameters are measured to be \begin{align*} S_{ψK^0_S} &= 0.717 \pm 0.013 ($\text{stat}$) \pm 0.008 ($\text{syst}$), \ C_{ψK^0_S} &= 0.008 \pm 0.012 ($\text{stat}$) \pm 0.003 ($\text{syst}$). \end{align*} This measurement of $S_{\psi K_S^0}$ represents the most precise single measurement of the CKM angle $\beta$ to date and is more precise than the current world average. In addition, measurements of the CP-violation parameters of the individual channels are reported and a combination with the LHCb Run 1 measurements is performed.