Cosmological simulation of axion-Higgs strings: Gravitational waves and dark matter

Axions have long been considered plausible candidates for dark matter. The axion dark matter emitted from cosmic strings after the Peccei-Quinn (PQ) symmetry breaking in the early Universe was extensively simulated. In this work, we study dark matter and gravitational waves through the lattice simulation of the axion-Higgs string. We gave the dark matter overproduction and the big bang nucleosynthesis bounds on the axion decay constant <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>f</a:mi><a:mi>a</a:mi></a:msub></a:math> and the axion mass <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>m</c:mi><c:mi>a</c:mi></c:msub></c:math> for axionlike particles, and found that the predicted gravitational wave spectra cannot be probed by the dataset of the current pulsar timing array experiments.