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Probing New Physics with b → sγ decays 1

In the Standard Model, the photon emitted in b → sγ decays is predicted to be left-handed polarized. We discuss the types of New Physics which can produce a deviation from this prediction, focusing on the Minimal Supersymmetric Standard Model. A new method is proposed for testing these predictions, which makes use of angular correlations in exclusive B → K res (→ Kππ)γ decays. Rare radiative b → sγ decays have been extensively investigated both as a probe of the flavor structure of the Standard Model and for their sensitivity to any new physics beyond the SM. The present experimental average of the inclusive rate Br(B → X s γ) = (3.22 ± 0.40) × 10 −4 [1] agrees well with the Standard Model prediction. To next-to-leading order in perturbation theory one obtains Br(B → X s γ) = (3.54 ± 0.49)×10 −4 (corresponding to a pole mass ratio m c /m b = 0.25±0.06) [2]. In addition to the rather well predicted inclusive branching ratio, there is a unique feature of this process within the SM which drew only moderate theoretical attention and which has not yet been tested. Namely, the emitted photons are left-handed in radiative B − and B 0 decays and are right-handed in B + and B 0 decays. In the SM the photon in b → sγ is predominantly left-handed, since only left chiral quarks couple to the W into loops. This prediction holds in the SM to within a few percent, up to corrections of order Λ/m b , for exclusive and inclusive decays. On the other hand, in certain extensions of the Standard Model, an appreciable right-handed component can be induced in b → sγ decays. While measurements of the inclusive radiative decay rate agree with SM calculations, no evidence exists so far for the helicity of the photons in these decays.