Effective theory of light Dirac neutrino portal dark matter with observable ${\Delta N_{\rm eff}}$

TL;DR

This paper explores a model where light Dirac neutrinos act as a portal for dark matter, leading to detectable effects on the effective number of relativistic species in the early universe, with implications for future CMB observations.

Contribution

It introduces a comprehensive effective field theory framework for Dirac neutrino portal dark matter, including a UV complete gauged B-L model, and analyzes its cosmological signatures.

Findings

Enhanced $N_{\rm eff}$ within reach of future CMB experiments.

Complementarity between dark matter detection and cosmological observations.

Viable UV complete model with Dirac neutrino portal dark matter.

Abstract

We study the possibility of light Dirac neutrino portal dark matter (DM) in an effective field theory setup. Dirac nature of light neutrino automatically includes its right chiral part $\nu_R$ which, in our setup, also acts like a portal between DM and the standard model (SM) particles. Considering a Dirac fermion singlet DM stabilized by an unbroken $Z_2$ symmetry, we write down all possible dimension-6 effective operators involving DM-$\nu_R$ as well as $\nu_R$-SM which conserve $Z_2$, global lepton number and SM gauge symmetries. DM thermalization also ensures the thermalization of $\nu_R$, leading to enhanced effective relativistic degrees of freedom $N_{\rm eff}$, within reach of future cosmic microwave background (CMB) experiments. We study the complementarity among DM and CMB related observations for different Lorentz structures of effective operators. We also propose a UV complete gauged $\rm B-L$ symmetric model with Dirac neutrino portal dark matter.