Study of the Decay B0(B¯0)→ρ+ρ-, and Constraints on the Cabibbo-Kobayashi-Maskawa Angle α

Using a data sample of 89×106 Υ(4S)→BB̅ decays collected with the BABAR detector at the PEP-II asymmetric B Factory at SLAC, we measure the B0(B̅ 0)→ρ+ρ- branching fraction as [30±4(stat)±5(syst)]×10-6 and a longitudinal polarization fraction of fL=0.99±0.03(stat)-0.03+0.04(syst). We measure the time-dependent-asymmetry parameters of the longitudinally polarized component of this decay as CL=-0.17±0.27(stat)±0.14(syst) and SL=-0.42±0.42(stat)±0.14(syst). We exclude values of α between 19° and 71° (90% C.L.).