b→sγ and supersymmetry with large tan β

We discuss the constraints on the supersymmetric parameter space from the decay mode b --> s gamma for large values of tan beta. We improve the theoretical prediction Tol the decay rate by summing very large radiative corrections to all orders in perturbation theory. This extends the validity of the perturbative calculation to the large tan beta regime. This resummation of terms of order alpha (n)(s) tan(n+1) beta is based on a recently proposed effective Lagrangian for the Yukawa interaction of bottom quarks. Moreover, we identify an additional source of tan beta -enhanced terms, which are of order alpha (s) tan beta and involve the charged Higgs boson, and analyse their behaviour in higher orders of perturbation theory. After correcting the current expressions for this rare decay branching ratio at next-to-leading order, we obtain that, contrary to recent claims, the measured branching ratio of b --> s gamma constrains the supersymmetric parameter space in a relevant way, even if tan beta is large. (C) 2001 Published by Elsevier Science B.V.