Two-component dark matter from a flavor-dependent $U(1)$ gauge extension

TL;DR

This paper explores a flavor-dependent $U(1)_X$ gauge extension of the Standard Model, analyzing a new two-component dark matter scenario with one fermionic and one scalar particle, considering their thermal freeze-out and experimental constraints.

Contribution

It introduces a novel two-component dark matter model with mixed fermionic and scalar particles, expanding previous purely fermionic scenarios by relaxing scalar mass assumptions.

Findings

Identified parameter space compatible with relic density constraints.

Demonstrated viability of mixed fermion-scalar dark matter components.

Analyzed impact of scalar mixing on dark matter phenomenology.

Abstract

We revisit the dark matter phenomenology of a flavor-dependent $U(1)_X$ gauge extension of the Standard Model, where anomaly cancellation predicts the existence of exactly three fermion generations and requires the presence of three right-handed neutrinos. In Ref.~\cite{VanLoi:2023utt}, a strong hierarchy between the vacuum expectation values of two singlet scalars, $\La_2 \gg \La_1$, renders all $\mathbb{Z}_2$-odd scalar states heavy, resulting in a two-component dark matter scenario composed exclusively of fermions. In the present work, we relax this simplifying assumption and consider a more general mass spectrum. In particular, scalar mixing can naturally lead to a situation in which the lightest $\mathbb{Z}_2$-odd particle is a scalar rather than a fermion. As a consequence, the model admits a qualitatively new realization of two-component dark matter consisting of one fermionic and one scalar component, in addition to the purely fermionic scenario studied previously. We perform a dedicated phenomenological analysis of these two-component dark matter realizations, focusing on the coupled thermal freeze-out dynamics and the resulting relic abundance. Constraints from the observed relic density and current direct-detection limits are taken into account, and viable regions of parameter space are identified.