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Mass Dependent $α_S$ Evolution and the Light Gluino Existence

There is an intriguing discrepancy between α_s(M_Z) values measured directly at the CERN $Z_0$-factory and low-energy (at few GeV) measurements transformed to $Q=M_{Z_0}$ by a massless QCD α_s(Q) evolution relation. There exists an attempt to reconcile this discrepancy by introducing a light gluino $\gl$ in the MSSM. We study in detail the influence of heavy thresholds on α_s(Q) evolution. First, we consruct the "exact" explicit solution to the mass-dependent two-loop RG equation for the running α_s(Q). This solution describes heavy thresholds smoothly. Second, we use this solution to recalculate anew α_s(M_Z) values corresponding to "low-energy" input data. Our analysis demonstrates that using {\it mass-dependent RG procedure} generally produces corrections of two types: Asymptotic correction due to effective shift of threshold position; Local threshold correction only for the case when input experiment lies in the close vicinity of heavy particle threshold: $Q_{expt} \simeq M_h $. Both effects result in the effective shift of the \asmz values of the order of $10^{-3}$. However, the second one could be enhanced when the gluino mass is close to a heavy quark mass. For such a case the sum effect could be important for the discussion of the light gluino existence as it further changes the $\gl$ mass.