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.