First evidence for direct CP violation in beauty to charmonium decays

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 $5.4~\text{fb}^{-1}$ recorded at a center-of-mass energy of $13~\text{TeV}$ during 2016–2018.

The results of the CP asymmetry difference and branching fraction ratio are

$\Delta A_{CP} \equiv A_{CP}(B^+ \to J/\psi \pi^+) – A_{CP}(B^+ \to J/\psi K^+) = (1.29 \pm 0.49 \pm 0.08) \times 10^{-2},$

$R_{\pi/K} \equiv \frac{\mathcal{B}(B^+ \to J/\psi \pi^+)}{\mathcal{B}(B^+ \to J/\psi K^+)} = (3.852 \pm 0.022 \pm 0.018) \times 10^{-2},$

where the first uncertainties are statistical and the second systematic.

A combination with previous LHCb results based on data collected at $7~\text{TeV}$ and $8~\text{TeV}$ in 2011 and 2012 yields

$\Delta A_{CP} = (1.42 \pm 0.43 \pm 0.08) \times 10^{-2} \quad \text{and} \quad R_{\pi/K} = (3.846 \pm 0.018 \pm 0.018) \times 10^{-2}.$

The combined $\Delta A_{CP}$ 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.