Sterile neutrino Dark Matter in the minimal Dirac Seesaw

Abstract

We study sterile neutrino dark matter in a minimal Type-I Dirac seesaw framework where the states responsible for generating Dirac neutrino masses at tree level can be viable dark matter candidates. A ${\mathcal{Z}}_6$ symmetry, spontaneously broken to a residual ${\mathcal{Z}}_3$ by the vacuum expectation value of a singlet scalar, forbids Majorana mass operators and ensures neutrino Diracness. The lightest sterile neutrino is produced non-thermally via freeze-in from decays of Standard Model particles and an additional scalar state. We show that the presence of an additional right-handed mixing angle, $\hat{I}{¸}_R$, opens up viable regions of parameter space where the observed dark matter relic abundance can be reproduced while maintaining cosmological stability. This mainly stems from the absence of X-ray astrophysical constraints in our scenario. We further find that the freeze-in production of right-handed neutrinos yields a negligible contribution to $\hat{I}”N_{\mathrm{eff}}$, consistent with current cosmological bounds.

13 LaTeX pages, 3 figures. Minor comments added in the main text, Figs. 2 and 3 updated and references added. Matches version published in PLB