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

N. T. Duy, Duy H. Nguyen, Do Thi Ha, Duong Van Loi

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.

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

Abstract

We revisit the dark matter phenomenology of a flavor-dependent 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, , renders all -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 -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.
Paper Structure (9 sections, 10 equations, 4 figures, 3 tables)

This paper contains 9 sections, 10 equations, 4 figures, 3 tables.

Table of Contents

  1. Introduction
  2. The model
  3. Gauge anomaly cancellation and particle content
  4. Particle spectrum of the scalar and gauge sectors
  5. Residual discrete symmetry and scenarios for two-component dark matter
  6. Two-component dark matter phenomenology
  7. Two-fermion dark matter
  8. Fermion--scalar dark matter
  9. Conclusions

Figures (4)

  • Figure 1: Correlation between the masses of the two fermionic DM components, $m_{\nu_{1R}}$ and $m_{\nu_{3R}}$, that reproduce the observed total relic abundance, $\Omega_{\nu_{1R}}h^2 + \Omega_{\nu_{3R}}h^2 \simeq 0.12$Planck:2018vyg.
  • Figure 2: Spin-independent scattering cross sections of the two fermionic DM components, $\nu_{1R}$ and $\nu_{3R}$, as functions of their masses. The colored solid lines represent the current upper limits from direct-detection experiments, namely XENONnT XENON:2025vwd, LZ LZ:2022lsv, and PandaX-4T PandaX:2024qfu.
  • Figure 3: Correlation between the masses of the fermionic DM component $m_{\nu_{3R}}$ and the scalar component $m_{I_2}$ that reproduce the observed total relic abundance, $\Omega_{\nu_{3R}} h^2 + \Omega_{I_2} h^2 \simeq 0.12$Planck:2018vyg.
  • Figure 4: Spin-independent cross sections of the fermionic DM component $\nu_{3R}$ and the scalar component $I_2$ as functions of their masses. The colored solid lines represent the current upper limits from direct-detection experiments, namely XENONnT XENON:2025vwd, LZ LZ:2022lsv, and PandaX-4T PandaX:2024qfu.