Measurement of the Branching Fraction B(B+u→J/ψπ+) and Search for B+c→J/ψπ+

We report on a measurement of the branching fraction of the Cabibbo-suppressed decay B+u→J/ψπ+, where J/ψ→μ+μ-. The data were collected by the Collider Detector at Fermilab during 1992 endash 1995 and correspond to an integrated luminosity of 110 pb-1 in bar pp collisions at √s=1.8 TeV. A signal of 28-9+10 events is observed and we determine the ratio of branching fractions B(B+u→J/ψπ+)/B(B+u→J/ψK+) to be %. Using the world average value for B(B+u→J/ψK+), we calculate the branching fraction B(B+u→J/ψπ+) to be (5.0-1.9+2.1)x10-5. We also search for the decay B+c→J/ψπ+ and report a 95% confidence level limit on σ(B+c)B(B+c→J/ψπ+)/ σ(B+u)B(B+u→J/ψK+) as a function of the B+c lifetime. copyright 1996 The American Physical Society