CP asymmetry in B-0(t)-> K-S pi(0)gamma in the standard model

The time-dependent CP asymmetry in exclusive B-0(t) -> K-*0 gamma decays has been proposed as a probe of new physics in B decays. Recently, this method was extended to radiative decays into multibody hadronic final states such as B-0(t) -> K-S pi(0)gamma and B-0(t) -> pi(+)pi(-)gamma. The CP asymmetry in these decays vanishes to the extent that the photon is completely polarized. In the standard model, the photon emitted in b -> s gamma has high left-handed polarization, but right-handed contamination enters already at leading order in Lambda/m(b) even for vanishing light quark masses. We compute here the magnitude of this effect and the time-dependent CP asymmetry parameter S-KS pi(0)gamma. We find that the standard model can easily accommodate values of S as large as 10%, but a precise value cannot be obtained at present because of strong interactions uncertainties.