First evidence for direct CP violation in beauty to charmonium decays

The $C\text{\hspace{0.17em}⁣}P$ asymmetry and branching fraction of the CKM-suppressed decay $B^{+}\text{\hspace{0.17em}⁣}\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi {\pi }^{+}$ are precisely measured relative to the favoured decay $B^{+}\text{\hspace{0.17em}⁣}\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi K^{+}$, using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4{\mathrm{f}\mathrm{b}}^{-1}$ recorded at center-of-mass energy of $13\mathrm{T}\mathrm{e}\mathrm{V}$ during 2016–2018. The results of the $C\text{\hspace{0.17em}⁣}P$ asymmetry difference and branching fraction ratio are \begin{align*} Δ$\mathcal{A}$^{C!P} &\equiv $\mathcal{A}$^{C!P}(B^+ \to J\mskip -3mu/\mskip -2muψ,π^+) - $\mathcal{A}$^{C!P}(B^+ \to J\mskip -3mu/\mskip -2muψ,K^+) = (1.29 \pm 0.49 \pm 0.08) \times 10^{-2}, \end{align*} \begin{equation*} $\mathcal{R}$_{π/K} \equiv \frac{$\mathcal{B}$(B^+ !\to J\mskip -3mu/\mskip -2muψ,π^+)}{$\mathcal{B}$(B^+ !\to J\mskip -3mu/\mskip -2muψ,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 $8\mathrm{T}\mathrm{e}\mathrm{V}$ in 2011 and 2012 yields $\mathrm{\Delta }{\mathcal{A}}^{C\text{\hspace{0.17em}⁣}P}=(1.42\pm 0.43\pm 0.08)\times {10}^{-2}$ and ${\mathcal{R}}_{\pi \mathrm{/}K}=(3.846\pm 0.018\pm 0.018)\times {10}^{-2}$. The combined $\mathrm{\Delta }{\mathcal{A}}^{C\text{\hspace{0.17em}⁣}P}$ value deviates from zero by 3.2 standard deviations, providing the first evidence for direct $C\text{\hspace{0.17em}⁣}P$ violation in the amplitudes of beauty decays to charmonium final states.