Novel Physics with International Pulsar Timing Array: Axionlike Particles, Domain Walls and Cosmic Strings

After NANOGrav, the IPTA collaboration also reports a strong evidence of a stochastic gravitation wave background. This hint has very important implications for fundamental physics. With the recent IPTA data release two, we attempt to search signals of light new physics. and give new constraints on the audible axion, domain walls and cosmic strings models. We find that the best fit point corresponding to a decay constant $F\approx5\times10^{17}$ GeV and an axion mass $m_a\approx2\times10^{-13}$ eV from NANOGrav data is ruled out by IPTA at beyond $2\sigma$ confidence level. Fixing the coupling strength $\lambda=1$, we obtain a $2\sigma$ lower bound on the breaking scale of $Z_2$ symmetry $\eta>135$ TeV. Interestingly, we give a very strong restriction on the cosmic-string tension $\mathrm{log}_{10}\,G\mu=-8.93_{-0.06}^{+0.12}$ at $1\sigma$ confidence level. Employing the rule of Bayes factor, we find that IPTA data has a moderate, strong and inconclusive preference of an uncorrelated common power-law (CPL) model over audible axion, domain walls and cosmic strings, respectively. This means that it is hard to distinguish CPL from cosmic strings with current observations and more pulsar timing data with high precision are required to give new clues of underlying physics.