Study of B→X(3872)K, with X(3872)→J/ψπ+π-

We present measurements of the decays B+→X(3872)K+ and B0→X(3872)K0 with X(3872)→J/ψπ+π-. The data sample used, collected with the BABAR detector at the PEP-II e+e- asymmetric-energy storage ring, corresponds to 455×106BB̅ pairs. Branching fraction measurements of B(B+→X(3872)K+)×B(X(3872)→J/ψπ+π-)=(8.4±1.5±0.7)×10-6 and B(B0→X(3872)K0)×B(X(3872)→J/ψπ+π-)=(3.5±1.9±0.4)×10-6 are obtained. We set an upper limit on the natural width of the X(3872) of Γ<3.3  MeV/c2 at the 90% confidence level.