Thermal Metastable Strings in One-Scale Models and Gravitational Waves

Abstract

Metastable cosmic strings provide a cosmological interpretation of the nanohertz stochastic gravitational wave background reported by Pulsar Timing Array (PTA) experiments. We revisit this scenario in a minimal dark-sector gauge theory, in which a complex Higgs doublet breaks $\mathrm{SU}(2)\times \mathrm{U}(1)\to \mathrm{U}(1)$ at a single symmetry-breaking scale. This one-scale setup predicts metastable $Z$-strings whose endpoints are monopole-like defects, and whose zero-temperature decay rate is controlled by the gauge couplings and mass ratios. We show that, once the string-forming transition occurs in a thermal plasma, the dominant decay channel is not the zero-temperature monopole nucleation but thermally induced nucleation on the string worldsheet. We determine the nucleation temperature, $T_{\mathrm{nuc}}$, from the one-loop finite-temperature effective potential with daisy resummation, and use it to compute the string formation temperature throughout the model parameter space. Requiring both a viable first-order transition and a PTA-compatible gravitational wave signal selects a narrow region in the model parameter space, in the $({\sin }^2\hat{I}{¸}_w,{\sqrt{\hat{I}}}^2)$ plane, where $\hat{I}{¸}_w$ is the dark-sector weak mixing angle and ${\hat{I}}^2\equiv M_{\hat{I}}{¦}^2/M_Z^2$ is the squared Higgs-to-$Z$ mass ratio. Thermal effects modify the zero-temperature picture significantly, shifting the PTA-compatible region towards lower values of the dark fine-structure constant $\hat{I}{\pm }^{\prime }$ and larger values of the monopole-to-string-tension ratio $\hat{I}º$.

18 pages, 3 figures