Sterile neutrino dark matter in conformal Majoron models

TL;DR

This paper explores sterile neutrino dark matter within a conformal U(1)' extension of the Standard Model, demonstrating how freeze-in production and structure formation constraints shape viable parameter space, and proposing solutions to cosmological tensions.

Contribution

It introduces a novel sterile neutrino DM production mechanism via freeze-in in a conformal U(1)' model with detailed Boltzmann analysis and observational constraints.

Findings

Viable keV-scale sterile neutrino parameter space identified.

Late decays of a second sterile state can address the $S_8$ tension.

Potential explanation for 3.5 keV X-ray line and 220 PeV neutrino events.

Abstract

We study sterile neutrino dark matter (DM) in a classically conformal U(1)' extension of the Standard Model with three right-handed neutrinos and a Majoron-like singlet scalar that generate the observed pattern of active neutrino masses and mixing via the type-I seesaw mechanism. Working in the regime of strongly suppressed active-sterile mixing, we show that the observed DM abundance can be produced through freeze-in from feeble interactions mediated by the heavy Z' and the conformal scalar. We solve the Boltzmann equation for the nonthermal phase-space distribution and confront the scenario with Lyman-$\alpha$ data by computing the matter power spectrum. For keV-scale sterile neutrinos we identify the viable parameter space consistent with structure-formation and X-ray bounds, including regions compatible with a tentative 3.5 keV line. If a second sterile state is long-lived, late decays can realize a two-component setup that alleviates the $S_8$ tension. In a highly fine-tuned variant of the model, the 220 PeV KM3NeT event can also be explained by invoking the decay of a superheavy sterile neutrino.