Dirac Neutrino Dark Matter

TL;DR

Dirac neutrino dark matter is explored as a heavy WIMP with suppressed Z coupling and possible Z' interactions. A model-independent analysis identifies viable relic-density and direct-detection constraints via Z, Z', and Higgs couplings, highlighting resonance regions near $M_Z/2$, $M_H/2$, and $M_{Z'}/2$. The work then presents a warped GUT realization (LZP) where a KK right-handed neutrino plays the DM role, incorporating coannihilations with KK fermions and multiple resonant channels; the relic density and direct-detection prospects are computed with micrOMEGAs/CalcHEP. Key findings show that satisfying both the observed relic density and direct-detection limits typically requires a highly suppressed $Z$-coupling or the presence of a sizable $Z'$ coupling, with substantial phenomenology at colliders through invisible Higgs decays and possible long-lived charged states. These results connect TeV-scale DM with warped extra dimensions, offering concrete collider and direct-detection signatures that future experiments can probe.

Abstract

We investigate the possibility that dark matter is made of heavy Dirac neutrinos with mass in the range [O(1) GeV- a few TeV] and with suppressed but non-zero coupling to the Standard Model Z as well as a coupling to an additional Z' gauge boson. The first part of this paper provides a model-independent analysis for the relic density and direct detection in terms of four main parameters: the mass, the couplings to the Z, to the Z' and to the Higgs. These WIMP candidates arise naturally as Kaluza-Klein states in extra-dimensional models with extended electroweak gauge group SU(2)_L* SU(2)_R * U(1). They can be stable because of Kaluza-Klein parity or of other discrete symmetries related to baryon number for instance, or even, in the low mass and low coupling limits, just because of a phase-space-suppressed decay width. An interesting aspect of warped models is that the extra Z' typically couples only to the third generation, thus avoiding the usual experimental constraints. In the second part of the paper, we illustrate the situation in details in a warped GUT model.

Figures (24)

• Figure 1: Neutrino-neutron scattering cross section due to $Z$-exchange for $g/g_z=10,30,100,300$ where $g=e/(\sin \theta_W)$ is the SM coupling. The dotted line shows the effect of adding the Higgs exchange for $g_H$=0.25, $m_H=120$ GeV, in the case where $g/g_z=300$. Also represented is the CDMS limit as well as the recent XENON limit latest_xenon.
• Figure 2: $\nu^{\prime}$ annihilation diagrams into $f\overline{f}$, $WW$, $Zh$, $hh$ and $ZZ$.
• Figure 3: $\Omega_{\nu^{\prime}} h^2$ versus $\nu^{\prime}$ for $g/g_z=10,30,100,300,1000$ where $g=e/(\sin \theta_W)$ is the SM coupling. In a), only the $Z$-exchange is included. For comparison, we also show in dotted line the relic density of a fourth generation Dirac neutrino with SM coupling to the $Z$ and a Yukawa coupling to the Higgs. For this curve, we set $m_H=200$ GeV. In b) the Higgs exchange is included as well with $g_H$=0.25.
• Figure 4: Annihilation cross sections at freese-out. The effect of a $Z'$ is omitted here and will be shown in Fig. \ref{['fig:cross']}.
• Figure 5: WMAP region, $0.097<\Omega h^2< 0.113$, in the $(M_{\nu'},g/g_Z)$ plane (where $g=e/\sin \theta_W$ is the SM coupling). The blue band includes the $Z$-exchange only, the red band includes the Higgs exchange as well. The region excluded by CDMS and Xenon is also displayed. The dotted line takes into account the Higgs contribution. The region above the direct detection line and below the WMAP band is allowed. In this figure, the effect of the $Z'$ is omitted.