CP asymmetry in B0(t)→KSπ0γ in the standard model

The time-dependent CP asymmetry in exclusive B0(t)→K*0γ 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 B0(t)→KSπ0γ and B0(t)→π+π-γ. 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→sγ has high left-handed polarization, but right-handed contamination enters already at leading order in Λ/mb even for vanishing light quark masses. We compute here the magnitude of this effect and the time-dependent CP asymmetry parameter SKSπ0γ. 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.