Cosmic Strings from Tribrid Inflation

TL;DR

This paper explores the formation of cosmic strings in Tribrid inflation models, analyzing conditions under which topological defects form and their potential implications for gravitational wave signals.

Contribution

It systematically investigates cosmic string formation in Tribrid inflation with U(1) symmetry, including cases with domain walls and metastable strings related to SO(10) breaking.

Findings

Cosmic strings can form in Tribrid inflation models with U(1) symmetry.

Domain walls are temporary and do not invalidate the models.

Metastable cosmic strings could explain PTA gravitational wave signals.

Abstract

Tribrid inflation is a class of supersymmetric inflation models where the scalar component of a matter superfield, or a $D$-flat direction of matter fields, drives inflation. Similar to Hybrid inflation, the end of inflation is reached when a "waterfall field", which was stabilized during inflation at a field value where the scalar potential features a large vacuum energy, starts rapidly rolling towards its minimum where a symmetry group $G$ is spontaneously broken. In contrast to standard supersymmetric Hybrid inflation, where the inflaton is a gauge singlet, in Tribrid inflation it can be a gauge non-singlet, which, via its vacuum expectation value, already breaks the gauge symmetry. This raises the question whether topological defects can still form after inflation in this class of models, and if so, which types of defects are generated. We investigate this question systematically in realisations of Tribrid inflation where $G = U(1)$ and we analyse under which conditions cosmic strings form. We find that in the considered cases where domain walls form, these are only temporary and do not invalidate the model realisations. We also discuss how our results can be used to analyse models of Tribrid inflation associated with the final step of $SO(10)$ breaking, where cosmic strings can be metastable and provide a promising explanation of the recent PTA results hinting at a stochastic gravitational wave background at nanohertz frequencies.