Theoretical uncertainties and phenomenological aspects of B → X_Sγ decay

We analyze uncertainties in the theoretical prediction for the inclusive branching ratio BR[B → X_sγ]. We find that the dominant uncertainty in the leading order expression comes from its μ-dependence. We discuss a next-to-leading order calculation of B → X_sγ in general terms and check to what extent the μ-dependence can be reduced in such a calculation. We present constraints on the Standard and Two-Higgs-Doublet Model parameters coming from the measurement of b → sγ decay. The current theoretical uncertainties do not allow one to definitively restrict the Standard Model parameters much beyond the limits coming from other experiments. The bounds on the Two-Higgs-Doublet Model remain very strong, though significantly weaker than the ones present in the recent literature. In the Two-Higgs-Doublet Model case, the b → sγ, Z → bb and b → cτν_τ processes are enough to give the most restrictive bounds in the M_H ±-tan β plane.