A Lighter QCD-Axion from Small Instantons

Abstract

We propose models where the QCD axion mass can be much smaller than the standard expectation. We show that this is possible in models where both QCD and the Standard Model flavor structure get modified in the ultraviolet (UV). In these models, both the axion mass and the up-quark Yukawa can receive sizeable non-perturbative contributions from instantons in the UV. If both these effects are carefully taken into account, the QCD axion is not necessarily heavier than usual expectations and can, in fact, be much lighter. We show that if small instanton contributions account for a large fraction of the up-quark Yukawa, the QCD axion can be lighter than standard expectations whereas it is heavier otherwise. We propose two different models where this condition is realised: (1) one where the QCD gauge group emerges as a subgroup from a product of $SU(3)$ groups in the UV; and (2) a composite Higgs model that also implements partial compositeness. In both these models the QCD axion can lie not only in the region to the right of the QCD axion band but also far to its left. This is especially relevant for a range of proposed haloscope and helioscope experiments that will probe a region mostly to the left of the QCD band.