Measurement of the ratio of branching fractions B(B±-->J/ psi pi ±)/B(B±-->J/ psi K±)

We report a measurement of the ratio of branching fractions of the decays B-+/- -> J/psi pi(+/-) and B-+/- -> J/psi K-+/- using the CDF II detector at the Fermilab Tevatron Collider. The signal from the Cabbibo-suppressed B-+/- -> J/psi pi(+/-) decay is separated from B-+/- -> J/psi K-+/- using the B-+/- -> J/psi K-+/- invariant mass distribution and the kinematical differences of the hadron track in the two decay modes. From a sample of 220 pb(-1) of p (p) over bar collisions at root s = 1.96 TeV, we observe 91 +/- 15 B-+/- -> J/psi pi(+/-) events together with 1883 +/- 34 B-+/- -> J/psi K-+/- events. The ratio of branching fractions is found to be B(B-+/- -> J/psi pi(+/-))/B(B-+/- -> J/psi K-+/-) = (4.86 +/- 0.82(stat) +/- 0.15(syst))%.