Measurement of the ratio of branching fractions B(B±→J/ ψπ±)/B(B±→J/ψK±)

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