We measure the branching fraction and time-dependent $CP$-violating asymmetry for $B^0\to J/\psi {\pi }^0$ decays using a data sample of $711{\mathrm{fb}}^{-1}$ collected on the $\mathrm{\Upsilon }(4S)$ resonance by the Belle experiment running at the KEKB $e^{+}{e}^{-}$ collider. The branching fraction is measured to be $\mathcal{B}(B^0\to J/\psi {\pi }^0)=[1.62\pm 0.11(\mathrm{stat})\pm 0.06(\mathrm{syst})]\times {10}^{-5}$, which is the most precise measurement to date. The measured $CP$ asymmetry parameters are $\mathcal{S}=-0.59\pm 0.19(\mathrm{stat})\pm 0.03(\mathrm{syst})$ and $\mathcal{A}=-0.15\pm 0.14(\mathrm{stat})\genfrac{}{}{0ex}{}{+0.04}{-0.03}(\mathrm{syst})$. The mixing-induced $CP$ asymmetry ($\mathcal{S}$) differs from the case of no $CP$ violation by 3.0 standard deviations, and the direct $CP$ asymmetry ($\mathcal{A}$) is consistent with zero.

• Received 29 September 2018

DOI:https://doi.org/10.1103/PhysRevD.98.112008

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Published by the American Physical Society