Sub-GeV dark matter and nano-Hertz gravitational waves from a classically conformal dark sector

TL;DR

This paper proposes a conformal dark sector model where a phase transition generates sub-GeV dark matter and explains nano-Hertz gravitational waves detected by pulsar timing arrays, with testable laboratory signatures.

Contribution

It introduces a classically conformal dark sector with a dark photon that links gravitational wave signals to dark matter production, constrained by laboratory and cosmological data.

Findings

Viable parameter space for dark matter and gravitational wave signals identified.

Dark Higgs bosons can remain in equilibrium or decay later, affecting experimental signatures.

Future beam-dump experiments can test the model's predictions.

Abstract

Strong first-order phase transitions in a dark sector offer a compelling explanation for the stochastic gravitational wave background in the nano-Hertz range recently detected by pulsar timing arrays (PTAs). We explore the possibility that such a phase transition at the same time gives mass to a stable fermion that accounts for the observed dark matter abundance and leads to testable effects in laboratory experiments. Concretely, we consider a classically conformal dark sector with a hidden $U(1)^\prime$ gauge symmetry that couples to the Standard Model via kinetic mixing. Since the PTA signal requires a phase transition in the MeV temperature range, spontaneous symmetry breaking gives rise to a sub-GeV dark matter candidate that couples to the Standard Model via a dark photon mediator and obtains its relic abundance via annihilations into electrons and dark Higgs bosons. Such a scenario is tightly constrained by laboratory searches for dark photons and cosmological constraints on the decays of dark Higgs bosons after the phase transition. We show that viable parameter regions can be found both for the case that the dark Higgs bosons remain in equilibrium with the Standard Model and that they decouple and only decay much later. In the latter case, the parameter regions preferred by the PTA signal and the dark matter relic abundance can be fully explored by future beam-dump experiments searching for missing energy.