Low-energy theorem revisited and OPE in massless QCD

A bstract We revisit a low-energy theorem (LET) of NSVZ type in SU( N ) QCD with N f massless quarks derived in [1] by implementing it in dimensional regularization. The LET relates n -point correlators in the l.h.s. to n + 1-point correlators with the extra insertion of Tr F 2 at zero momentum in the r.h.s. We demonstrate that, for 2-point correlators of an operator O in the l.h.s., the LET implies that, in general, the integrated 3-point correlator in the r.h.s. needs in perturbation theory an infinite additive renormalization in addition to the multiplicative one. We relate the above counterterm to a corresponding divergent contact term in a certain coefficient of the OPE of Tr F 2 with O in the momentum representation, thus extending to any operator O an independent argument that first appeared for O = Tr F 2 in [2]. Finally, we demonstrate that in the asymptotically free phase of QCD the aforementioned counterterm in the LET is actually finite nonperturbatively after resummation to all perturbative orders. We also briefly recall the implications of the LET in the gauge-invariant framework of dimensional regularization for the perturbative and nonperturbative renormalization in large- N QCD. The implications of the LET inside and above the conformal window of SU( N ) QCD with N f massless quarks will appear in a forthcoming paper.