The $C\!P$ asymmetry and branching fraction of the CKM-suppressed decay $B^+\!\to J\mskip -3mu/\mskip -2mu\psi\,\pi^+$ are precisely measured relative to the favoured decay $B^+\!\to J\mskip -3mu/\mskip -2mu\psi\,K^+$, using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4~\mathrm{ fb}^{ -1}$ recorded at center-of-mass energy of $13~\mathrm{ TeV}$ during 2016--2018. The results of the $C\!P$ asymmetry difference and branching fraction ratio are \begin{ align*} \Delta\mathcal{ A}^{ C\!P} &\equiv \mathcal{ A}^{ C\!P}(B^+ \to J\mskip -3mu/\mskip -2mu\psi\,\pi^+) - \mathcal{ A}^{ C\!P}(B^+ \to J\mskip -3mu/\mskip -2mu\psi\,K^+) = (1.29 \pm 0.49 \pm 0.08) \times 10^{ -2}, \end{ align*} \begin{ equation*} \mathcal{ R}_{ \pi/K} \equiv \frac{ \mathcal{ B}(B^+ \!\to J\mskip -3mu/\mskip -2mu\psi\,\pi^+)}{ \mathcal{ B}(B^+ \!\to J\mskip -3mu/\mskip -2mu\psi\,K^+)} = (3.852 \pm 0.022 \pm 0.018) \times 10^{ -2}. \end{ equation*} where the first uncertainties are statistical and the second systematic. A combination with previous LHCb results based on data collected at $7$ and $8~\mathrm{ TeV}$ in 2011 and 2012 yields $\Delta\mathcal{ A}^{ C\!P} = (1.42 \pm 0.43 \pm 0.08) \times 10^{ -2}$ and $\mathcal{ R}_{ \pi/K} = (3.846 \pm 0.018 \pm 0.018) \times 10^{ -2}$. The combined $\Delta\mathcal{ A}^{ C\!P}$ value deviates from zero by 3.2 standard deviations, providing the first evidence for direct $C\!P$ violation in the amplitudes of beauty decays to charmonium final states.
Categories: hep-ex