Measurement of decay-time-dependent $\mathit{\text{CP}}$ violation in $B^0\to J\mathrm{/}\psi K_S^0$ decays using 2019-2021 Belle II data

We report a measurement of the mixing-induced and direct $\mathit{\text{CP}}$ violation parameters $S_{\text{CP}}$ and $A_{\text{CP}}$ from $B^0\to J\mathrm{/}\psi K_S^0$ decays reconstructed by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The data, collected at the center-of-mass energy of the $\mathrm{{\rm Y}}(4S)$ resonance, correspond to $190{\text{fb}}^{-1}$ of integrated luminosity. We measure $S_{\text{CP}}=0.720\pm 0.062\pm 0.016$ and $A_{\text{CP}}=0.094\pm 0.044_{-0.017}^{+0.042}$, where the first uncertainties are statistical and the second systematic. In the Standard Model, $S_{\text{CP}}$ equals $\sin (2{\phi }_1)$ to a good approximation.