and $C_{-}$, we have taken into account the threshold effects, connected to the heavy quark masses.

In expressions (\[5\]) and (\[6\]), the scale $\mu$ has been taken approximately equal to $m_c$. In the Pauli interference term, we suggest that the scale can be determined on the basis of the agreement of the experimentally known difference between the lifetimes of $\Lambda_c$, $\Xi_c^{+}$ and $\Xi_c^{0}$ with the theoretical predictions in the framework described above[^6]. In any case, the choice of the normalization scale leads to uncertainties in the final results. Theoretical accuracy can be improved by the calculation of next-order corrections in the powers of QCD coupling constant.

The coefficients of leading terms, represented by operators $\bar bb$ and $\bar
cc$, are equivalent to the widths fot the decays of free quarks and are known in the next-to-leading logarithmic approximation of QCD [@12; @13; @14; @15; @16], including the mass corrections in the final state with the charmed quark and $\tau$-lepton [@16] in the decays of $b$-quark and with the strange quark mass for the decays of $c$-quark. In the numerical estimates, we include these corrections and mass effects, but we neglect the decay modes suppressed by the Cabibbo angle, and also 