Measurement of the Ratio of Branching Fractions Br(B+ -> JPsi pi+)/Br(B+ ->JPsi K+)

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