$Z^{\prime }$-portal right-handed neutrino dark matter in the minimal U(1)${}_X$ extended Standard Model

Abstract

We consider a concise dark matter (DM) scenario in the context of a non-exotic U(1) extension of the Standard Model (SM), where a new U(1)${}_X$ gauge symmetry is introduced along with three generation of right-handed neutrinos (RHNs) and an SM gauge singlet Higgs field. The model is a generalization of the minimal gauged U(1)${}_{B-L}$ (baryon number minus lepton number) extension of the SM, in which the extra U(1)${}_X$ gauge symmetry is expressed as a linear combination of the SM U(1)${}_Y$ and U(1)${}_{B-L}$ gauge symmetries. We introduce a $Z_2$-parity and assign an odd-parity only for one RHN among all particles, so that this $Z_2$-odd RHN plays a role of DM. The so-called minimal seesaw mechanism is implemented in this model with only two $Z_2$-even RHNs. In this context, we investigate physics of the RHN DM, focusing on the case that this DM particle communicates with the SM particles through the U(1)${}_X$ gauge boson ($Z^{\prime }$ boson). This "$Z^{\prime }$-portal RHN DM" scenario is controlled by only three free parameters: the U(1)${}_X$ gauge coupling (${\alpha }_X$), the $Z^{\prime }$ boson mass ($m_{Z^{\prime }}$), and the U(1)${}_X$ charge of the SM Higgs doublet ($x_H$). We consider various phenomenological constraints to identify a phenomenologically viable parameter space. The most important constraints are the observed DM relic abundance and the latest LHC Run-2 results on the search for a narrow resonance with the di-lepton final state. We find that these are complementary with each other and narrow the allowed parameter region, leading to the lower mass bound of $m_{Z^{\prime }}\gtrsim 2.7$ TeV.

Comments: 20 pages, 8 figures, version to be published in Physical Review D. arXiv admin note: text overlap with arXiv:1601.07526