Measurement of the Ratio of Branching Fractions and Difference in CP Asymmetries of the Decays B+ → J/ψ π+ and B+ → J/ψ K+

The ratio of branching fractions and the difference in CP asymmetries of the decays B+ → J/ψ π+ and B+ → J/ψ 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 B(B+ → J/ψ π+) / B(B+ → J/ψ K+) = (3.83 ± 0.03 ± 0.03) × 10^−2, A^CP(B+ → J/ψ π+) − A^CP(B+ → J/ψ K+) = (1.82 ± 0.86 ± 0.14) × 10^−2, where the first uncertainties are statistical and the second are systematic. Combining this result with a recent LHCb measurement of A^CP(B+ → J/ψ K+) provides the most precise estimate to date of CP violation in the decay B+ → J/ψ π+, A^CP(B+ → J/ψ π+) = (1.91 ± 0.89 ± 0.16) × 10^−2.