Time-dependent CP asymmetry in B→K∗γ as a (quasi)null test of the Standard Model

We calculate the dominant Standard Model contributions to the time-dependent CP asymmetry in B0→K∗0γ, which is O(1/mb) in QCD factorisation. We find that, including all relevant hadronic effects, in particular from soft gluons, the asymmetry S is very small, S=−0.022±0.012±0.01−0.01+0, and smaller than suggested recently from dimensional arguments in a 1/mb expansion. Our result implies that any significant deviation of the asymmetry from zero, and in particular a confirmation of the current experimental central value, SHFAG=−0.28±0.26, would constitute a clean signal for new physics.