Electroweak right-handed neutrino portal dark matter

TL;DR

This work explores a unified framework where dark matter communicates with the Standard Model through electroweak-scale right-handed neutrinos in a Type-I seesaw setup. It analyzes three representative RHN realizations (RS, SC, SS) and uses Particle Swarm Optimization to fix seesaw parameters to neutrino data, then solves the full coupled Boltzmann equations for a minimal dark sector (a fermion χ and a complex scalar φ) interacting via RHN portals. A key finding is that dark-sector internal interactions can drastically alter freeze-in relic densities, with misestimates up to about 95% if internal dynamics are neglected, especially when the hidden sector attains its own thermal equilibrium. The study provides robust, testable benchmarks linking neutrino physics, heavy neutral lepton searches at colliders, and the cosmological DM abundance, highlighting multi-messenger probes as a powerful avenue to constrain or reveal this portal DM scenario.

Abstract

We study dark matter coupled to the standard model via electroweak scale right-handed neutrinos in a Type-I seesaw framework. We consider a minimal dark sector containing a fermion $χ$ and a complex scalar $φ$ whose only connection to the standard model is through renormalizable Yukawa interactions with right-handed Majorana neutrinos, thus realizing a neutrino portal after seesaw mixing. We discuss three representative realizations of electroweak right-handed neutrinos arising from the Type-I seesaw mechanism, spanning small, large, and ultraweak couplings to the standard model sector, so that the dark particles can either undergo secluded freeze-out or be produced via freeze-in. Instead of merely estimating the order of magnitude of the seesaw couplings, we use the Particle Swarm Optimization algorithm to obtain viable seesaw parameter sets consistent with neutrino data and other constraints, and then compute the coupled evolution of the dark particles and right-handed neutrinos, reproducing the observed dark matter relic abundance in representative benchmark scenarios. For the freeze-in case, we show that internal dark sector interactions can significantly modify the predicted relic density: treating each hidden particle as an independent freeze-in component and simply adding late decays can misestimate the final dark matter abundance by $30\%$, or even $95\%$, depending on the type of internal interactions, compared to a full solution of the coupled Boltzmann equations for all dark species, including the dark sector temperature. Electroweak right-handed neutrino portal dark matter thus provides a robust, testable framework that tightly connects neutrino physics, collider searches for heavy neutral leptons, and the cosmological dark matter relic density, offering a well-motivated benchmark for multi-messenger probes at the high energy frontier.

Figures (11)

• Figure 1: Exclusion limits at the 95% confidence level on the active-sterile mixing parameter $|U_{e\mathbb{N}}|^2$ are presented. The upper bounds are derived from various sources, including L3's search via $e^{+}e^{-} \to \mathbb{N}\nu$L3:2001zfe, ATLAS same-sign $WW$ scattering ATLAS:2023tkz, and several CMS analyses at $\sqrt{s} = 13~\mathrm{TeV}$: same-sign dilepton CMS:2018jxx, trilepton CMS:2018iafCMS:2024xdq, as well as global electroweak precision data (EWPD) constraints deBlas:2013gla.
• Figure 2: Exclusion limits at the 95% confidence level on the active sterile mixing parameter $|U_{\mu \mathbb{N}}|^2$ are presented. The upper bounds are obtained from several sources, including ATLAS same-sign $WW$ scattering ATLAS:2024rzi, CMS same-sign dilepton at $\sqrt{s} = 13~\mathrm{TeV}$CMS:2018jxx, CMS trilepton analysis CMS:2018iafCMS:2024xdq, CMS same-sign $WW$ scattering CMS:2022hvh, CMS same-sign dilepton at $\sqrt{s} = 8~\mathrm{TeV}$CMS:2015qur, and electroweak precision data (EWPD) deBlas:2013gla.
• Figure 3: Exclusion limits at the 95% confidence level on the active¨Csterile mixing parameter $|U_{\tau \mathbb{N}}|^2$ are presented. The upper bounds are derived from the CMS trilepton analysis at $\sqrt{s} = 13~\mathrm{TeV}$CMS:2024xdq and from electroweak precision data (EWPD) Cheung:2020buy.
• Figure 4: A schematic illustration of the dark sector evolution via freeze-out. At high temperatures, the dark particles $\chi$ and $\phi$ remain in thermal equilibrium with the SM plasma through the right-handed neutrino portal. The dominant freeze-out channels are $\chi\overline{\chi}, \phi\overline{\phi} \to \mathbb{N}\mathbb{N}$, and $\mathbb{N}$ subsequently decays into SM particles.
• Figure 5: An exhibition of the dark sector freeze-out evolution for Models $a,b$ (Benchmark-RS) and Models $c,e$ (Benchmark-SC). For each model, the figure shows the evolution of the comoving number densities of all dark particles $\chi,\phi$, and $\mathbb{N}_{1,2,3}$, where the dark fermion $\chi$ serves as the dark matter candidate. The red horizontal dashed line indicates the observed dark matter relic density with dark matter mass equal to $m_{\chi}$. For Benchmark-RS, the difference between two models is as follows. In Model $a$, $\phi$ freezes out via $\phi\overline{\phi}\to\mathbb{N}_{1,2,3}\mathbb{N}_{1,2,3}$ and annihilates into $\chi$ through $\phi\phi\to\overline{\chi\chi}$, $\phi\overline{\phi}\to\chi\overline{\chi}$. At later times, all remaining $\phi$ decay through $\phi\to\overline{\chi}\mathbb{V}_{1,2,3}$ before BBN. While in Model $b$, the mass hierarchy $m_{\phi}>(m_{\chi}+M_{1})$ allows the internal decay channel $\phi\to\overline{\chi}\mathbb{N}_{1}$. For Benchmark-SC, the portal neutrinos $\mathbb{N}_{1,2,3}$ remain tightly coupled to the thermal bath, leading to a delayed freeze-out. In Model $e$, $\mathbb{N}_{3}$ decays via $\mathbb{N}_{3}\to \chi\phi,\overline{\chi\phi}$, and the dominant freeze-out channel for dark particles is $\chi \phi \to L H$.