Scale-setting, flavor dependence, and chiral symmetry restoration

We determine the flavor dependence of the renormalization-group-invariant running interaction through judicious use of both unquenched Dyson-Schwinger equation and lattice results for QCD's gauge-sector two-point functions. An important step is the introduction of a physical scale setting procedure that enables a realistic expression of the effect of different numbers of active quark flavours on the interaction. Using this running interaction in concert with a well constrained class of dressed-gluon-quark vertices, we estimate the critical number of active lighter-quarks above which dynamical chiral symmetry breaking becomes impossible: n(f)(cr) approximate to 9; and hence in whose neighborhood QCD is plausibly a conformal theory.