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: $n_f^{\mathrm{cr}}\approx 9$; and hence in whose neighborhood QCD is plausibly a conformal theory.

• Received 11 April 2017

DOI:https://doi.org/10.1103/PhysRevD.95.114009