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<p>Metastable cosmic strings provide a minimal and predictive origin for the stochastic gravitational-wave background reported by Pulsar Timing Array experiments. We analyse this possibility in electroweak-like dark sectors with a single-stage breaking SU(2) × U(1) → U(1) driven by one Higgs field. In the regime with dark sector Higgs mass below the <i>Z′</i> mass, and for sufficiently small <i>W′</i> mass, the resulting <i>Z</i>-string is classically stable but undergoes quantum decay via nucleation of monopole-antimonopole pairs along the string. We compute the corresponding semiclassical bounce action in a <i>thin-defect</i> approximation, treating both the string core and the monopole endpoints as localised defects whose sizes are small relative to their separation in the tunnelling configuration. This yields a decay rate per unit length that depends on the gauge couplings and the mass hierarchy. We delineate the parameter space in which single-scale dark-sector models reproduce the PTA signal, demonstrating the applicability of the thin-defect approximation throughout the phenomenologically favoured region and without invoking extended Higgs sectors or multi-stage symmetry breaking.</p>

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Metastable strings and gravitational waves in one-scale models

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月14日

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  • James Ingoldby,
  • Valentin V. Khoze,
  • Jessica Turner

摘要

Metastable cosmic strings provide a minimal and predictive origin for the stochastic gravitational-wave background reported by Pulsar Timing Array experiments. We analyse this possibility in electroweak-like dark sectors with a single-stage breaking SU(2) × U(1) → U(1) driven by one Higgs field. In the regime with dark sector Higgs mass below the Z′ mass, and for sufficiently small W′ mass, the resulting Z-string is classically stable but undergoes quantum decay via nucleation of monopole-antimonopole pairs along the string. We compute the corresponding semiclassical bounce action in a thin-defect approximation, treating both the string core and the monopole endpoints as localised defects whose sizes are small relative to their separation in the tunnelling configuration. This yields a decay rate per unit length that depends on the gauge couplings and the mass hierarchy. We delineate the parameter space in which single-scale dark-sector models reproduce the PTA signal, demonstrating the applicability of the thin-defect approximation throughout the phenomenologically favoured region and without invoking extended Higgs sectors or multi-stage symmetry breaking.