Observable dark radiation from cosmologically safe QCD axion

TL;DR

This paper introduces a QCD axion model with a global SU(3)_f symmetry that predicts observable dark radiation and exotic kaon decays, providing testable cosmological and collider signatures.

Contribution

It presents a novel QCD axion model embedding the PQ symmetry into a global SU(3)_f, avoiding the domain wall problem and predicting detectable dark radiation and rare kaon decays.

Findings

Predicts eight NG bosons contributing to dark radiation.

Future CMB observations can test the dark radiation predictions.

Exotic kaon decay rates are within reach of upcoming collider experiments.

Abstract

We propose a QCD axion model that avoids the cosmological domain wall problem, introducing a global SU(3)_f family symmetry to which we embed the unwanted PQ discrete symmetry. The spontaneous breaking of SU(3)_f and PQ symmetry predicts eight NG bosons as well as axion, all of which contribute to dark radiation in the Universe. The derivation from the standard model prediction of dark radiation can be observed by future observations of CMB fluctuations. Our model also predicts a sizable exotic kaon decay rate, which is marginally consistent with the present collider data and would be tested by future collider experiments.