Large tanβ SUSY QCD Corrections to B → X_sγ

The charged-Higgs-boson contributions to the Wilson coefficients C ₇ and C₈, relevant for the decay B → X _sγ, are discussed in supersymmetric models at large tanβ. These contributions receive two-loop O(α, tanβ) corrections by squark-gluino subloops, which are possibly large and nondecoupling in the limit of heavy superpartners. In previous studies, the relevant two-loop Feynman integrals were approximated by using an effective two-Higgs-doublet lagrangian. However, this approximation is theoretically justified only when the typical supersymmetric scale M_SUSY is sufficiently larger than the electroweak scale m_weak ∼ (m_w, m_t) and the mass of the chargcd-Higgs boson m_H±. Here, we evaluate these two-loop integrals exactly and compare the results with the existing, approximated ones. We then examine the validity of this approximation beyond the region where it has been derived, i.e. for m_H ≳ MSUSY and/or MSUSY ∼ m_weak.