Speaker
Description
Small instantons can significantly modify the axion potential in KSVZ models containing vector-like quarks (VQs). Using naive dimensional analysis, we analyse models with a single VQ, multiple identical copies, and sets of distinct VQs, while requiring the gauge couplings to remain perturbative up to the Planck scale under two-loop running. Their differing fermion zero-mode structures require suitable combinations of mass insertions and scalar–Yukawa contractions for saturation. With sufficiently many VQs, the instanton-size integral can become dominated by the smallest instantons, rendering the induced potential sensitive to ultraviolet physics and potentially comparable to, or larger than, the ordinary QCD contribution. With the aligned QCD and small-instanton potentials, this enhances the physical axion mass at fixed (f_a), while (g_{a\gamma\gamma}) remains controlled by (f_a) and the anomaly ratio (E/N). The standard QCD relation among (m_a), (f_a), and (g_{a\gamma\gamma}) is therefore modified, opening new regions of axion parameter space.