Observation of time-dependent $CP$ violation and measurement of the branching fraction of $B^0\to J\mathrm{/}\psi {\pi }^0$ decays

We present a measurement of the branching fraction and time-dependent charge-parity ($CP$) decay-rate asymmetries in $B^0\to J\mathrm{/}\psi {\pi }^0$ decays. The data sample was collected with the Belle~II detector at the SuperKEKB asymmetric $e^{+}{e}^{-}$ collider in 2019-2022 and contains $(387\pm 6)\times {10}^6$ $B\bar{B}$ meson pairs from $\mathrm{{\rm Y}}(4S)$ decays. We reconstruct $392\pm 24$ signal decays and fit the $CP$ parameters from the distribution of the proper-decay-time difference of the two $B$ mesons. We measure the branching fraction to be $B(B^0\to J\mathrm{/}\psi {\pi }^0)=(2.00\pm 0.12\pm 0.09)\times {10}^{-5}$ and the direct and mixing-induced $CP$ asymmetries to be $C_{CP}=0.13\pm 0.12\pm 0.03$ and $S_{CP}=-0.88\pm 0.17\pm 0.03$, respectively, where the first uncertainties are statistical and the second are systematic. We observe mixing-induced $CP$ violation with a significance of $5.0$ standard deviations for the first time in this mode.