First evidence for direct CP violation in beauty to charmonium decays

Abstract

The $CP$ asymmetry and branching fraction of the CKM-suppressed decay $B^{+}\to J/\psi {\pi }^{+}$ are precisely measured relative to the favoured decay $B^{+}\to J/\psi K^{+}$, 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 $CP$ 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 $8\mathrm{TeV}$ in 2011 and 2012 yields $\Delta {\mathcal{A}}^{CP}=(1.42\pm 0.43\pm 0.08)\times 10^{-2}$ and ${\mathcal{R}}_{\pi /K}=(3.846\pm 0.018\pm 0.018)\times 10^{-2}$. The combined $\Delta {\mathcal{A}}^{CP}$ value deviates from zero by 3.2 standard deviations, providing the first evidence for direct $CP$ violation in the amplitudes of beauty decays to charmonium final states.

arXiv comment18 pages, 2 figures, no conference or journal information All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/1623/ (LHCb public pages)

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