Thermal Dark Matter Through the Dirac Neutrino Portal

TL;DR

This paper explores a model where dark matter interacts with standard model neutrinos via a sterile neutrino mediator, analyzing its phenomenology, constraints, and potential signals in experiments and collider data.

Contribution

It introduces a simple thermal dark matter model with a Dirac fermion and scalar, mediated by a sterile neutrino, and assesses its experimental constraints and signatures.

Findings

Much of the cosmologically-motivated parameter space is already constrained.

Sterile neutrinos in the model have large active-sterile mixing angles.

New experimental probes include multi-body kaon decays and W boson production at the LHC.

Abstract

We study a simple model of thermal dark matter annihilating to standard model neutrinos via the neutrino portal. A (pseudo-)Dirac sterile neutrino serves as a mediator between the visible and the dark sectors, while an approximate lepton number symmetry allows for a large neutrino Yukawa coupling and, in turn, efficient dark matter annihilation. The dark sector consists of two particles, a Dirac fermion and complex scalar, charged under a symmetry that ensures the stability of the dark matter. A generic prediction of the model is a sterile neutrino with a large active-sterile mixing angle that decays primarily invisibly. We derive existing constraints and future projections from direct detection experiments, colliders, rare meson and tau decays, electroweak precision tests, and small scale structure observations. Along with these phenomenological tests, we investigate the consequences of perturbativity and scalar mass fine tuning on the model parameter space. A simple, conservative scheme to confront the various tests with the thermal relic target is outlined, and we demonstrate that much of the cosmologically-motivated parameter space is already constrained. We also identify new probes of this scenario such as multi-body kaon decays and Drell-Yan production of $W$ bosons at the LHC.