NEXT-TO-LEADING QCD CORRECTIONS TO B-]X-S-GAMMA - STANDARD MODEL AND 2-HIGGS DOUBLET MODEL

We present the QCD corrections to the matching conditions of the Delta B = 1 magnetic and chrome-magnetic operators in the Standard Model an d in two-Higgs doubler models. We use an off-shell matching procedure which allows us to perform the computation using Taylor series in the external momenta, instead of asymptotic expansions. In the Standard Mo del case, we confirm previous results derived on-shell and we obtain B R(B --> X(s)gamma) = (3.62 +/- 0.33) x 10(-4). In the case of the usua l two-Higgs doublet model, we show that going from the leading to the next-to-leading order result improves the CLEO bound on the charged-Hi ggs mass from 260 GeV to 380 GeV. This limit is very sensitive to the definition of errors and we carefully discuss the theoretical uncertai nties. Finally, in the case of the two-Higgs doublet model in which bo th up- and down-type quarks couple to the same Higgs field, the theore tical prediction for BR(B --> X(s)gamma) can be reduced by at most 20% with respect to the Standard Model value. (C) 1998 Elsevier Science B .V.