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Anticipating the important role of tree level FCNC processes in the indirectsearch for new physics at distance scales as short as 10^-19-10^-21 m, wepresent complete NLO QCD corrections to tree level Delta F=2 processes mediatedby heavy colourless gauge bosons and scalars. Such contributions can be presentat the fundamental level when GIM mechanism is absent as in numerous Z' models,gauged flavour models with new heavy neutral gauge bosons and Left-Rightsymmetric models with heavy neutral scalars. They can also be generated at oneloop in models having GIM at the fundamental level and MFV of which Two-HiggsDoublet models with and without SUSY are the best known examples. In modelscontaining vectorial heavy fermions that mix with the standard chiral quarksand models in which Z and SM neutral Higgs H mix with new heavy gauge bosonsand scalars also tree-level Z and SM neutral Higgs contributions to Delta F=2processes are possible. In all these extensions new local operators aregenerated having Wilson coefficients that are generally much stronger affectedby RG QCD effects than it is the case of the SM operators. The new aspect ofour work is the calculation of O(alpha_s) corrections to matching conditionsfor the Wilson coefficients of the contributing operators in the NDR-MS schemethat can be used in all models listed above. This allows to reduce certainunphysical scale and renormalization scheme dependences in the existing NLOcalculations. We show explicitly how our results can be combined with theanalytic formulae for the P_i^a QCD factors that include both hadronic matrixelements of contributing operators and RG evolution from high energy to lowenergy scales. For the masses of heavy gauge bosons and scalars O(1)TeV theremaining unphysical scale dependences for the mixing amplitudes are reducedtypically from 10-25%, depending on the operator considered, down to 1-2%.

Complete NLO QCD Corrections for Tree Level Delta F = 2 FCNC Processes