Measurement of the CKM angle alpha at the BABAR detector using B meson decays to rho final states

This thesis contains the results of an analysis of B 0 → rho+rho- decays collected with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. From a fitted signal yield of 617 +/- 52 events, the longitudinal polarization fraction, fL, of the decay is measured to be 0.978 +/- 0.014(stat) +0.021-0.029 (syst). The nearly fully longitudinal dominance of the B 0 → rho+rho- decay allows for a measurement of the time dependent CP parameters SL and CL, where the first parameter is sensitive to mixing induced CP violation and the second one to direct CP violation. From this signal yield, the values are found to be SL = -0.33 +/- 0.24(stat) +0.08-0.14 (syst) and CL = -0.03 +/- 0.18(stat) +/- 0.09(syst). The CKM angle alpha is then determined from an isospin analysis that uses these results along with the branching fractions and polarizations of the decays B0 → rho0rho 0 and B+/- → rho+/-rho 0. Selecting the solution compatible with the Standard Model, one obtains alpha = 100° +/- 13°.