Majorana dark matter in a classically scale invariant model

TL;DR

The paper investigates a scale-invariant extension of the Standard Model with a dark $U(1)_X$ that is broken radiatively via the Coleman–Weinberg mechanism, generating masses and introducing two stable Majorana DM candidates $N_1$ and $N_2$. DM phenomenology hinges on two annihilation channels within the dark sector and a suppressed Higgs-portal coupling to the SM; relic abundance, LUX direct-detection, and LHC constraints bound the parameter space, yielding a viable DM mass range of roughly 470 GeV to a few TeV and a heavy $X$-boson ($m_X \\gtrsim 680$ GeV). In the degenerate-mass scenario both DM species contribute equally, while in the non-degenerate case $N_1$ dominates the relic density and leads to two distinct direct-detection signals that could be tested by XENON1T/LZ. The results link the electroweak and DM scales through radiative symmetry breaking and highlight future experimental tests in direct detection and collider phenomenology.

Abstract

We analyze a classically scale invariant extension of the Standard Model with dark gauge $U(1)_X$ broken by doubly charge scalar $Φ$ leaving a remnant $Z_2$ symmetry. Dark fermions are introduced as dark matter candidates and for anomaly reasons we introduce two chiral fermions. Due to classical scale invariance, bare mass term that would mix these two states is absent and they end up as stable Majorana fermions $N_1$ and $N_2$. We calculate cross sections for $N_aN_a \to φφ$, $N_aN_a \to X^μφ$ and $N_2N_2 \to N_1N_1$ annihilation channels. We put constraints to the model from the Higgs searches at the LHC, dark matter relic abundance and dark matter direct detection limits by LUX. The dark gauge boson plays a crucial role in the Coleman-Weinberg mechanism and has to be heavier then 680 GeV. The viable mass region for dark matter is from 470 GeV up to a few TeV. In the case when two Majorana fermions have different masses, two dark matter signals at direct detection experiments could provide a distinctive signature of this model.

Figures (7)

• Figure 1: Annihilation processes of Majorana dark matter $N_1$ and $N_2$ considered in this work.
• Figure 2: Scattering of Majorana dark matter $N_a$ of a nucleon.
• Figure 3: Left (right) panel: Mass of the DM Majorana fermion $m_{N_1}$ (vev of the $U(1)_X$ charged scalar $v_\Phi$) in the $m_\phi-m_X$ plane for the degenerate mass case. We impose the LHC bound on $\sin\theta$ and the LUX experiment constraints.
• Figure 4: Left (right) panel: The $U(1)_X$ gauge coupling $g_X$ (scalar mixing angle $\sin \theta$) in the $m_\phi-m_X$ plane for the degenerate mass case. We impose the LHC bound on $\sin\theta$ and the LUX experiment constraints.
• Figure 5: The direct detection cross section $\sigma_{SI}$ in the $m_\phi-m_X$ plane for the degenerate mass case. We impose the LHC bound on $\sin\theta$ and the LUX experiment constraints.