Electroweak right-handed neutrino portal dark matter

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.

💡 Research Summary

This paper presents a comprehensive study of dark matter (DM) that communicates with the Standard Model (SM) exclusively through electroweak‑scale right‑handed neutrinos (RHNs) embedded in a Type‑I seesaw framework. The authors introduce a minimal hidden sector consisting of a fermion χ and a complex scalar ϕ, stabilized by a discrete symmetry, and couple them to the RHNs via renormalizable Yukawa interactions. After electroweak symmetry breaking, the RHNs mix with the light neutrinos, providing a “neutrino portal” that links the hidden sector to the SM.

Three representative realizations of electroweak‑scale RHNs are explored:

1. Case‑RS (Regular Seesaw) – a conventional Type‑I seesaw with TeV‑scale Majorana masses and moderate Yukawa couplings.
2. Case‑SC (Structure Cancellation) – a special texture of the Yukawa matrix that yields sizable active‑RHN mixing while preserving the observed light‑neutrino masses.
3. Case‑SS (Split Seesaw) – a hierarchical spectrum where one RHN is ultra‑heavy and the remaining ones sit at the electroweak scale, leading to ultra‑weak couplings.

Instead of estimating the seesaw parameters by hand, the authors employ Particle Swarm Optimization (PSO) to scan the high‑dimensional parameter space. The PSO algorithm minimizes a χ²‑like cost function that incorporates neutrino oscillation data (mass‑squared differences, mixing angles, CP phase), non‑unitarity constraints, lepton‑number‑violating limits, and collider bounds. This yields concrete benchmark points for each case that satisfy all current experimental constraints.

The hidden sector dynamics are treated with a multi‑temperature formalism. When internal dark‑sector interactions (e.g., χχ↔ϕϕ, three‑point χχϕ couplings) are strong, χ and ϕ quickly thermalize among themselves, defining a hidden temperature T_h distinct from the visible temperature T_v. The evolution is then governed by a coupled set of Boltzmann equations for the number densities n_χ, n_ϕ, n_N and the temperature ratio η = T_v/T_h. If internal interactions are weak, each species behaves as an independent freeze‑in source and the standard single‑component approximation can be used.

The authors solve these equations for both freeze‑out and freeze‑in regimes across the three cases. In the freeze‑out scenario (Case‑RS and Case‑SC with sizable RHN‑SM couplings), χ and ϕ remain in thermal equilibrium with the SM plasma until the usual WIMP‑like decoupling, after which RHN decays and annihilations set the final relic density. In the freeze‑in scenario (all three cases with suppressed couplings), the hidden particles are never abundant enough to thermalize; they are slowly populated by RHN decays and scatterings such as SM + SM → χ + ϕ mediated by off‑shell RHNs.

A key result is that neglecting internal dark‑sector interactions leads to a substantial mis‑prediction of the relic abundance. Treating χ and ϕ as independent freeze‑in components and simply adding late RHN decays can underestimate or overestimate the final DM density by about 30 % when three‑point interactions are present, and by up to 95 % when such interactions are absent. This demonstrates that a full coupled Boltzmann treatment, including the hidden temperature evolution, is essential for accurate predictions.

Experimental constraints are discussed in detail. Collider searches for heavy neutral leptons (displaced vertices, lepton‑number‑violating signatures) bound the active‑RHN mixing angle θ, while precision electroweak and lepton‑flavor‑violation measurements constrain the non‑unitarity of the PMNS matrix. Cosmological limits from Big Bang Nucleosynthesis, the effective number of neutrino species (N_eff), and the Cosmic Microwave Background further restrict the hidden sector temperature and the number of relativistic degrees of freedom.

In conclusion, electroweak‑scale RHN portal dark matter provides a tightly knit framework that simultaneously addresses neutrino mass generation, dark matter phenomenology, and collider testability. The PSO‑derived benchmark models illustrate that viable parameter space exists where the observed DM relic density is reproduced, while remaining consistent with all current laboratory and cosmological bounds. Moreover, the analysis highlights the importance of internal dark‑sector dynamics in freeze‑in models, a point often overlooked in the literature. The authors argue that this setup serves as a robust, multi‑messenger benchmark for upcoming experiments at the energy frontier, offering clear targets for heavy neutral lepton searches, indirect dark matter probes, and precision neutrino measurements.