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Among the O(alpha_s tan beta) contributions to the Wilson coefficients C_7and C_8, relevant for the decay bar{B} -> X_s gamma, those induced by two-loopdiagrams with charged-Higgs-boson exchange and squark-gluino corrections arecalculated in supersymmetric models at large tan beta. The calculation of thecorresponding Feynman integrals is exact, unlike in previous studies that arevalid when the typical supersymmetric scale M_SUSY is sufficiently larger thanthe electroweak scale m_weak (\sim m_W, m_t) and the mass of the charged Higgsboson m_{H^\pm}. Therefore, the results presented here can be used for anyvalue of the various supersymmetric masses. These results are compared withthose of an approximate calculation, already existing in the literature, thatis at the zeroth order in the expansion parameter (m_weak^2,m_H^2)/M_SUSY^2,and with the results of two new approximate calculations in which the first andsecond order in the same expansion parameter are retained, respectively. Thiscomparison allows us to assess whether the results of these three approximatecalculations can be extended beyond the range of validity for which they werederived, i.e., whether they can be used for m_H^2 \gtap M_SUSY^2 and/orM_SUSY^2 \sim m_weak^2. It is found that the zeroth-order approximation workswell even for m_H \gtap M_SUSY, provided M_SUSY^2 >> m_weak^2. The inclusion ofthe higher-order terms improves the zeroth-order approximation for m_H^2 <<M_SUSY^2, but it worsens it for m_H \gtap M_SUSY$.Comment: 14 page

Beyond leading-order corrections to B¯→Xsγ at large tan β: The charged-Higgs-boson contribution