Revisiting metastable cosmic string breaking

Metastable cosmic strings appear in models of new physics with a two-step symmetry breaking G → H → 1, where π₁(H) ≠ 0 and π₁(G) = 0. They decay via the monopole-antimonopole pair creation inside. Conventionally, the breaking rate has been estimated by an infinitely thin string approximation, which requires a large hierarchy between the symmetry breaking scales. In this paper, we reexamine it by taking into account the finite sizes of both the cosmic string and the monopole. We obtain a robust lower limit on the tunneling factor $e^{-S_B}$ \( {e}^{-{S}_B} \) even for regimes the conventional estimate is unreliable. In particular, it is relevant to the cosmic string interpretation of the gravitational wave signals recently reported by pulsar timing array experiments.