Bound on cos alpha from exclusive weak radiative B decays

We present a bound on the weak phase a from isospin-breaking effects in wea k radiative decays, which requires the CP-averaged branching ratios for the weak radiative decays B+/- --> rho(+/-)gamma, B-0 --> rho(0)/omega gamma, B --> K*gamma and the photon energy spectrum in B --> gamma lv(l). We caref ully identify all sources of isospin-violating effects, which could possibl y mask information about the CKM parameters. They are introduced by diagram s with photon bremsstrahlung off the spectator quark and diagrams with anni hilation penguin topologies. The former can be eliminated by combining B -- > rho gamma and B --> K*gamma data, whereas the latter effects are OZI-supp ressed and can be controlled by measuring also the weak radiative modes B-s --> rho(omega)gamma. The resulting bound excludes values of alpha around 9 0 degrees, provided that the combined ratio B(B+/- --> rho(+/-)gamma)/B(B-0 --> rho(0)/omega gamma)xB(B-0 --> K*(0)gamma)/B(B+/- --> K*(+/-)gamma) is found to be different from 1. (C) 2000 Elsevier Science B.V. All rights res erved.