Testing clockwork axion with gravitational waves

We investigate the gravitational waves (GWs) produced from the Peccei-Quinn (PQ) phase transition associated with the clockwork axion. The PQ phase transition can be first-order when the dimension-6 operator is included into the scalar potential. The GWs from the PQ phase transition at scale in the range of 10 3 -10 6 GeV are detectable for the BBO and ALIA interferometers. The LISA and Taiji interferometers can probe the GWs from the PQ scale f ≤ 10 4 GeV, while the GW signals from the scale f ≥ 10 5 GeV can be detected by the ground-based GW observatories ET and CE. We find that the parameter space κ m ∼ 0.06-0.001, κ l ∼ 0.04-0.001, and ε ∼ 0.1-0.01 at the scale f = 10 5 GeV and most of the parameter regions at the scale f = 10 6 GeV have been excluded by the LIGO O2 run. The LIGO O3 and design phases can further probe the remaining parameter space. We show that the GWs from the annihilation of domain walls with a PQ scale f ≃ 2 × 10 5 GeV can induce the stochastic signals with the right amplitude for the NANOGrav 12.5-year observations, but having a steeper spectral slope than the observations. The LIGO O3 run has the opportunity of detecting the GW signals from the first-order PQ phase transition around this scale.