QCD corrections to (B)over-bar→Xd,sν(ν)over-bar, (B)over-bard,s→l+l-, K→πν(ν)over-bar and KL→μ+μ- in the MSSM

We compute for the first time QCD corrections to the rare decays (B) over bar --> X(d,s)nu(nu) over bar, (B) over bar (d,s) --> l(+)l(-), K --> pinu(nu) over bar and K-L --> mu(+)mu(-), where l = e or mu, in the context of a supersymmetric extension of the Standard Model (SM) with minimal flavour violation and new operators, in addition to those present in the SM. Assuming that the gluino is heavy, we consider an effective theory which consists of charged and neutral Higgs particles, charginos and squarks. We evaluate the QCD corrections to box and Z(0)-penguin diagrams with top-quark, charged Higgs boson, chargino and squark exchanges, as well as to neutral Higgs boson penguin diagrams. We provide a compendium of analytic formulae for the Wilson coefficients, which are valid for arbitrary values of tan beta (the ratio of the vacuum expectation values of the two Higgs fields) except for the case of the neutral Higgsboson contributions. These contributions have been obtained at large tan, which may compensate for the inevitable suppression by the masses of the light leptons in decays based on the b --> s(d)l(+)l(-) transition. We investigate the dependence of the various branching ratios on the renormalization scale mu, which is the main theoretical uncertainty in the short-distance calculation. We find that the mu dependence of the branching ratios is considerably reduced once the QCD corrections are taken into account. The contributions of new operators are found to be dominant at large tan beta in B-d,B-s --> mu(+)mu(-) while they are subleading in (B) over bar --> X(d,s)nu(nu) over bar and completely negligible in kaon decays. (C) 2002 Elsevier Science B.V. All rights reserved.