Measuring α in B(t)→ρ±π∓ -: art. no. 074031

Defining a most economical parametrization of time-dependent B-->rho(+/-)pi(-/+) decays, including a measurable phase alpha(eff) which equals the weak phase alpha in the limit of vanishing penguin amplitudes, we propose two ways for determining alpha in this processes. We explain the limitation of one method, assuming only that two relevant tree amplitudes factorize and that their relative strong phase delta(t) is negligible. The other method, based on broken flavor SU(3), permits a determination of alpha in B-0-->rho(+/-)pi(-/+) in an overconstrained system using also rate measurements of B-0,B-+-->K(*)pi and B-0,B-+-->rhoK. Current data are shown to restrict two ratios of penguin and tree amplitudes r(+/-) to a narrow range around 0.2 and to imply an upper bound |alpha(eff)-alpha|<15degrees. Assuming that delta(t) is much smaller than 90degrees, we find alpha=(93+/-16)degrees and (102+/-20)degrees using BABAR and BELLE results for B(t)-->rho(+/-)pi(-/+). Avoiding this assumption for completeness, we demonstrate the reduction of discrete ambiguities in alpha with increased statistics and show that SU(3) breaking effects are effectively second order in r(+/-).