Study of the decay B0(B0bar)-->rho+rho-, and constraints on the CKM angle alpha

Using a data sample of 89 million Y(4s)-->B0B0bar decays collected with the BaBar detector at the PEP-II asymmetric B Factory at SLAC, we measure the B0(B0bar)-->rho+rho- branching fraction as $(30\pm 4 stat \pm 5 syst) \times 10^{-6}$ and a longitudinal polarization fraction of $f_L = 0.99 \pm 0.03 stat ^{+0.04}_{-0.03} syst$. We measure the time-dependent-asymmetry parameters of the longitudinally polarized component of this decay as $C_L = -0.17\pm 0.27 stat \pm 0.14 syst$ and ${S_L} = -0.42 \pm 0.42 stat \pm 0.14 syst$. We present constraints on the CKM angle $\alpha$.