## Abstract

The ratio of branching fractions and the difference in CP asymmetries of the decays $B^+ \to J/\psi\pi^+$ and $B^+ \to J/\psi K^+$ are measured using a data sample of pp collisions collected by the LHCb experiment, corresponding to an integrated luminosity of 3 fb$^{−1}$ at centre-of-mass energies of 7 and 8 TeV. The results are

$\frac{\mathcal{B} (B^+ \to J/\psi\pi^+)} {\mathcal{B} (B^+ \to J/psi K^+)} = (3.83 \pm 0.03 \pm 0.03) \times 10^{−2}$,

$\mathcal{A}^{CP} (B^+ \to J/\psi\pi^+) − \mathcal{A}^{CP} (B^+ \to J/\psi K^+) = (1.82 \pm 0.86 \pm 0.14) \times 10^{−2}$,

where the first uncertainties are statistical and the second are systematic. Combining this result with a recent LHCb measurement of $\mathcal{A}^{CP} (B^+ \to J/\psi K^+)$ provides the most precise estimate to date of CP violation in the decay $B^+ \to J/\psi\pi^+$,

$\mathcal{A}^{CP}(B^+ \to J/\psi\pi^+) = (1.91 \pm 0.89 \pm 0.16) \times 10^{−2}$.