The nano-hertz and milli-hertz stochastic gravitational waves in the minimal clockwork axion model

TL;DR

This paper proposes a minimal clockwork axion model with multiple domain walls, predicting stochastic gravitational wave signals detectable by current and future experiments, and explores its cosmological and astrophysical constraints.

Contribution

It introduces a novel minimal clockwork axion model with two domain walls, linking gravitational wave signals to specific cosmological phenomena and experimental prospects.

Findings

Nano-hertz GWs from domain wall annihilation detectable by Pulsar Timing Arrays.

Milli-hertz GWs from higher-dimensional operator bias detectable by LISA, Taiji, TianQin.

Model consistent with dark matter, BBN, CMB, and primordial black hole constraints.

Abstract

The clockwork framework can realize TeV-scale $U(1)_{PQ}$ symmetry breaking while generating a large axion decay constant \(f_a\). We propose a minimal clockwork axion model with three scalar fields, in which two domain walls (DWs) have non-zero tension. The DW associated with one of the fields is formed following the Peccei-Quinn (PQ) symmetry breaking and subsequently collapses due to the potential bias induced by the QCD instanton. The nano-hertz stochastic gravitational waves (GWs) generated from this DW annihilation can be probed by Pulsar Timing Arrays experiments. In addition, the DW related to the other field is annihilated by a bias potential originating from higher-dimensional operators, producing a significant GW signal with a peak frequency around \(9.41\times10^{-5}\) Hz, which can be detected by the LISA, Taiji, and TianQin experiments. Constraints on the model from SN1987, dark matter overproduction, Big Bang Nucleosynthesis, cosmic microwave background, and primordial black holes have been considered. The relic density of QCD axion dark matter can be explained through the misalignment mechanism.