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Metastable cosmic strings and gravitational waves from flavor symmetry breaking

Stefan Antusch1,*, Kevin Hinze1,†, and Shaikh Saad2,‡

  • *Contact author: stefan.antusch@unibas.ch
  • †Contact author: kevin.hinze@unibas.ch
  • ‡Contact author: shaikh.saad@okstate.edu
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Phys. Rev. D 112, 035043 – Published 29 August, 2025

DOI: https://doi.org/10.1103/528x-qzs3

Abstract

Metastable cosmic strings (MSCSs) are among the best-fitting explanations of the 2023 pulsar timing array (PTA) signal for gravitational waves at nanohertz frequencies. We propose the novel possibility that a network of MSCSs generating this signal originates from the multistep spontaneous breaking of a gauged flavor symmetry. As a specific example, we construct a model of SU(2) flavor symmetry in the context of SU(5) grand unification, where the SU(2) acts exclusively on the first two generations of the matter 10-plet, such that it is “right for leptons” and allows for large lepton mixing. The model explains the mass hierarchies of the Standard Model fermions, and predicts the string scale of the MSCSs in a range compatible with the 2023 PTA signal. Cosmic inflation is associated with the latter step of (two-step) family symmetry breaking, and the phase transition ending inflation generates the cosmic string network.

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Phys. Rev. D 112, 035043– Published 29 August, 2025
  • Received 26 March 2025
  • Accepted 13 August 2025
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