Improved Measurement of B+→ρ+ρ0 and Determination of the Quark-Mixing Phase Angle α

We present improved measurements of the branching fraction B, the longitudinal polarization fraction f(L), and the direct CP asymmetry A(CP) in the B meson decay channel B+->rho(+)rho(0). The data sample was collected with the BABAR detector at SLAC. The results are B(B+->rho(+)rho(0))=(23.7 +/- 1.4 +/- 1.4)x10(-6), f(L)=0.950 +/- 0.015 +/- 0.006, and A(CP)=-0.054 +/- 0.055 +/- 0.010, where the uncertainties are statistical and systematic, respectively. Based on these results, we perform an isospin analysis and determine the Cabibbo-Kobayashi-Maskawa phase angle alpha=arg(-VtdVtb*/VudVub*) to be (92.4(-6.5)(+6.0))degrees.