Color Unifed Dynamical Axion

TL;DR

This paper proposes a unified color sector model that naturally generates a large axion mass without fine-tuning, predicts new collider-accessible particles, and broadens the viable parameter space for solving the strong CP problem.

Contribution

It introduces a novel mechanism where spontaneous breaking of a unified color group yields a large axion mass and a low-energy CP-conserving vacuum without fine-tuning or mirror symmetries.

Findings

Large axion mass generated by small instantons.

Absence of light axion, presence of sterile fermions.

Potential collider signatures of colored pseudoscalars.

Abstract

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 $m_a$ 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 $Z_2$ 'mirror' copy of the SM, nor does it require any fine-tuning of the axion-related couplings at the unification scale. There is no light axion and uncharacteristically the lighter spectrum contains instead sterile fermions. The axion scale $f_a$ 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 $\{m_a, f_a\}$ parameter space which allows a solution of the strong CP problem is thus enlarged well beyond that of invisible axion models.