Measurement of CP violation in $B^0\to\psi(\to\ell^+\ell^-)K^0_S(\to\pi^+\pi^-)$ decays

TL;DR

This paper reports a highly precise measurement of CP violation parameters in B meson decays, significantly improving the accuracy of the CKM angle beta determination using LHCb data.

Contribution

The study provides the most precise single measurement of the CKM angle beta to date, surpassing previous world averages with new data from LHCb at 13 TeV.

Findings

Measured CP-violation parameter S_{ψK^0_S} = 0.717 ± 0.013 (stat) ± 0.008 (syst)

Measured CP-violation parameter C_{ψK^0_S} = 0.008 ± 0.012 (stat) ± 0.003 (syst)

Combined results with previous LHCb data for improved precision.

Abstract

A measurement of time-dependent CP violation in the decays of $B^0$ and $\overline{B}^0$ mesons to the final states $J/\psi(\to\mu^+\mu^-)K^0_S$, $\psi(2S)(\to\mu^+\mu^-)K^0_S$ and $J/\psi(\to e^+e^-)K^0_S$ with $K^0_S\to\pi^+\pi^-$ 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_{\psi K^0_S} &= 0.717 \pm 0.013 (\text{stat}) \pm 0.008 (\text{syst}), \\ C_{\psi K^0_S} &= 0.008 \pm 0.012 (\text{stat}) \pm 0.003 (\text{syst}). \end{align*} This measurement of $S_{\psi K^0_S}$ 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.