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

Abstract We calculate the dominant Standard Model contributions to the time-dependent CP asymmetry in B 0 → K ∗ 0 γ , which is O ( 1 / m b ) 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 / m b expansion. Our result implies that any significant deviation of the asymmetry from zero, and in particular a confirmation of the current experimental central value, S HFAG = − 0.28 ± 0.26 , would constitute a clean signal for new physics.