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Light neutrinophilic WIMP in the $U(1)_{\rm B-L+xY}$ model

Tatsuya Aonashi, Shigeki Matsumoto, Yu Watanabe, Yuki Watanabe

TL;DR

The paper presents a neutrinophilic WIMP framework based on a light $Z'$ mediator from a gauged $U(1)_{\rm B-L+xY}$ symmetry, arranged so that DM annihilates predominantly to neutrinos and hadronic channels are closed in the sub-GeV regime. By tuning near the $Z'$ resonance, the model achieves viable thermal freeze-out while delivering potentially sizable DM self-interactions that address small-scale structure tensions, all within a broad but carefully constrained parameter space. Cosmological, astrophysical, and terrestrial constraints are systematically incorporated, with a particularly constrained MeV-scale window ($5.8\ \mathrm{MeV} \le m_\varphi \le 8.3\ \mathrm{MeV}$) that remains compatible with $N_{\rm eff}$, neutrino signals, and rare meson decays. The work highlights KamLAND and related neutrino experiments as promising avenues to probe this scenario, underscoring the potential of the neutrino portal to reveal light DM physics at the intensity frontier.

Abstract

Sub-GeV dark matter is an appealing thermal target because it can still be produced via the standard freeze-out mechanism; at such low masses, achieving freeze-out naturally points to the presence of a light mediator, which shifts the most promising discovery avenues from the energy frontier to the intensity frontier. Realizing this picture is nonetheless challenging, since CMB observations tightly constrain energy injection from dark-matter annihilation at recombination and therefore strongly disfavor simple $s$-wave annihilation into visible Standard-Model final states. In this work, we propose a concrete neutrinophilic framework for sub-GeV thermal dark matter (''light WIMPs'') based on an additional gauge symmetry $\mathrm{U}(1)_{\mathrm{B}-\mathrm{L}+x\mathrm{Y}}$; for an appropriate choice of $x$, the new gauge boson couples predominantly to dark matter and neutrinos while its couplings to charged leptons are suppressed, so that sub-GeV dark matter annihilates almost exclusively into neutrinos, with hadronic modes kinematically closed. We map the parameter space in which the observed relic abundance is reproduced via standard thermal freeze-out in a conventional cosmological history, and show that sizable regions remain viable after imposing current cosmological, indirect-detection, and terrestrial constraints; in part of the allowed parameter space, the dark matter also exhibits sufficiently large self-interactions to potentially alleviate small-scale structure tensions.

Light neutrinophilic WIMP in the $U(1)_{\rm B-L+xY}$ model

TL;DR

The paper presents a neutrinophilic WIMP framework based on a light mediator from a gauged symmetry, arranged so that DM annihilates predominantly to neutrinos and hadronic channels are closed in the sub-GeV regime. By tuning near the resonance, the model achieves viable thermal freeze-out while delivering potentially sizable DM self-interactions that address small-scale structure tensions, all within a broad but carefully constrained parameter space. Cosmological, astrophysical, and terrestrial constraints are systematically incorporated, with a particularly constrained MeV-scale window () that remains compatible with , neutrino signals, and rare meson decays. The work highlights KamLAND and related neutrino experiments as promising avenues to probe this scenario, underscoring the potential of the neutrino portal to reveal light DM physics at the intensity frontier.

Abstract

Sub-GeV dark matter is an appealing thermal target because it can still be produced via the standard freeze-out mechanism; at such low masses, achieving freeze-out naturally points to the presence of a light mediator, which shifts the most promising discovery avenues from the energy frontier to the intensity frontier. Realizing this picture is nonetheless challenging, since CMB observations tightly constrain energy injection from dark-matter annihilation at recombination and therefore strongly disfavor simple -wave annihilation into visible Standard-Model final states. In this work, we propose a concrete neutrinophilic framework for sub-GeV thermal dark matter (''light WIMPs'') based on an additional gauge symmetry ; for an appropriate choice of , the new gauge boson couples predominantly to dark matter and neutrinos while its couplings to charged leptons are suppressed, so that sub-GeV dark matter annihilates almost exclusively into neutrinos, with hadronic modes kinematically closed. We map the parameter space in which the observed relic abundance is reproduced via standard thermal freeze-out in a conventional cosmological history, and show that sizable regions remain viable after imposing current cosmological, indirect-detection, and terrestrial constraints; in part of the allowed parameter space, the dark matter also exhibits sufficiently large self-interactions to potentially alleviate small-scale structure tensions.
Paper Structure (12 sections, 34 equations, 3 figures, 2 tables)

This paper contains 12 sections, 34 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The neutrinophilic WIMP
  3. Phenomenology of the neutrinophilic WIMP
  4. Model parameters
  5. The self-scattering condition
  6. The relic abundance condition
  7. Constraints from cosmology
  8. Constraints from astrophysical observations
  9. Constraints from terrestrial experiments
  10. Quantitative analysis of the neutrinophilic WIMP
  11. Summary
  12. Kinematic mixing term

Figures (3)

  • Figure 1: The velocity dependence of the self-scattering cross-section (multiplied by the relative velocity between incident particles, $v$) predicted by the neutrinophilic WIMP is shown. The prediction using the parameter set $(m_\varphi, v_R, g_\varphi, \sigma_0/m_\varphi) = (7.7\,\mathrm{MeV}, 108\,\mathrm{km/s}, 2.4 \times 10^{-3}, 0.067\,\mathrm{cm}^2/\mathrm{g})$ is represented by a black solid line, while the one using the parameter set $(7.7\,\mathrm{MeV}, 65\,\mathrm{km/s}, 3.1 \times 10^{-3}, 0.067\,\mathrm{cm}^2/\mathrm{g})$ is represented by a red dashed line. The data obtained from kinematical observations are shown as black dots with error bars Kaplinghat:2015aga. See the main text for more details.
  • Figure 2: Left panel: The number density of the WIMP ($n_\varphi$), normalized by the entropy density ($s$), is shown as a function of the universe's temperature ($T$). The prediction using the parameter set $(m_\varphi, v_R, g_\varphi, \tilde{g}) = (7.7\,\text{MeV}, 108\,\text{km/s}, 2.4 \times 10^{-3}, 9.1 \times 10^{-11})$ is represented by a black solid line, while the one using $(7.7\,\text{MeV}, 65\,\text{km/s}, 3.1 \times 10^{-3}, 5.2 \times 10^{-11})$ is represented by a red dashed line. Both lines assume the initial condition $f_\varphi(p) = f_{\rm eq}(p)$ (i.e., $n_\varphi = n_{\rm eq}$) at $T = m_\varphi$ and satisfy the relic abundance condition, which is represented by the light-gray band. The comparison between the WIMP annihilation rate ($\Gamma_a$) and the expansion rate of the universe ($H$) as a function of $T$ is shown using the same parameter sets as in the left panel. See the main text for more details.
  • Figure 3: Regions of parameter space that satisfy the relic-abundance and self-scattering conditions, while being consistent with existing cosmological, astrophysical, and terrestrial constraints, shown in the $(m_\varphi,\langle\sigma v(\varphi\varphi^{*}\!\to\!\nu\bar{\nu})\rangle_{0})$ plane. We take the WIMP velocity dispersion to be $v_{0}\simeq 400\,\mathrm{km/s}$. Shaded regions indicate exclusions from indirect searches at neutrino observatories, together with the CMB bound from $N_{\rm eff}$ (implying $m_\varphi \ge 5.8\,\mathrm{MeV}$). See the main text for more details.