© 2019 The Author(s) We show that a choice of Pauli-Villars regulators allows the cancellation of all the conformal and chiral anomalies in an effective field theory from Z3 compactification of the heterotic string with two Wilson lines and an anomalous U(1).

We consider an enlarged color sector which solves the strong CP problem via new massless fermions. The spontaneous breaking of a unified color group into QCD and another confining group provides a source of naturally large axion mass ma due to small size instantons. This extra source of axion mass respects automatically the alignment of the vacuum, ensuring a low-energy CP-conserving vacuum. The mechanism does not appeal to a Z2 “mirror” copy of the SM, nor does it require any fine-tuning of the axion-related couplings at the unification scale. There is no very light axion and uncharacteristically the lighter spectrum contains instead sterile fermions. The axion scale fa can be naturally brought down to a few TeV, with an exotic spectrum of colored pseudoscalars lighter than this scale, observable at colliders exclusively via strong interactions. The { ma, fa} parameter space which allows a solution of the strong CP problem is thus enlarged well beyond that of invisible axion models.