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.
