Tuning the violins: dark sector phase transition models for the PTA signal

TL;DR

This paper investigates dark sector first-order phase transition models as explanations for the nanohertz gravitational wave background observed by PTAs, analyzing three classes of models and their tuning requirements.

Contribution

It provides a comprehensive analysis of three dark sector phase transition models, assessing their ability to explain PTA signals and identifying conformal models as the most natural.

Findings

All three models can potentially explain the PTA signal.

Conformal models require less tuning and are more generic.

Future PTA and collider data will be key to testing these models.

Abstract

First-order phase transitions in a dark sector have been invoked as an intriguing possibility to explain the observed stochastic gravitational wave background at nanohertz frequencies. Here we perform a comprehensive study of the generic requirements for such a phase transition to explain the observed signal while being consistent with all relevant constraints. We consider three broad model classes for strong first-order transitions, realised by an Abelian dark Higgs boson, a two-step phase transition involving two scalar singlets, and a conformal scalar field with loop-induced symmetry breaking, respectively. We discuss the tuning that is required to successfully explain the Pulsar Timing Array (PTA) signal in each of these cases, and highlight the underlying physical mechanisms. We conclude that all three scenarios can in principle describe the data, but that conformal models stand out as the most generic, and least tuned, explanation. Future observations by the PTA collaborations and collider experiments will be crucial to test the viability of this hypothesis, and to further narrow in on the model parameters, if the PTA signal is indeed due to a strong first-order phase transition.