Vector Dark Matter in a $U(1)_X$ extended 2HDM

Abstract

We investigate the possibility of having a vector boson dark matter in a $U(1)_X$ extended two-Higgs-doublet model (2HDM) setup. The gauge boson gains mass when a SM singlet complex scalar, which is charged under the dark $U(1)_X$ symmetry, acquires vacuum expectation value (\textit{vev}). This scalar acts as the connection between the SM sector and DM via the Higgs portal. An additional exact charge conjugation symmetry inhibits the mixing of this gauge boson with the photon, thereby confirming the stability of DM. On the other hand, 2HDM with Type I $Z_2$ restriction can offer a non-standard Higgs in the lighter mass range. This freedom allows us to accommodate dark matter mass in the (40-60) GeV regime where the direct detection constraints are strongest. We study the dark matter phenomenology of such a model while taking care of all possible theoretical and experimental constraints.

Figures (10)

• Figure 1: Relevant trilinear and quartic vertices of vector DM candidate $Z^{\prime}$.
• Figure 2: Parameter space in $m_{h_1}$ vs $\tan\beta$ plane allowed from unitarity and BFB conditions.
• Figure 3: Diagrams for thermalisation of $Z^\prime$. In the leftmost diagram, $\Phi$ denotes the $3$ Higgs bosons $h_1, h_2, h_3$ in mass eigenstate.
• Figure 4: Relic abundance vs ${\rm m}_{DM}$ (GeV). The Higgs masses are fixed at $m_{h_1}=80 \,{\rm GeV},m_{h_2}=125 \,{\rm GeV}, m_{h_3}=500\,{\rm GeV}$. In the left panel, two values of $g_x$ are considered when $tan\beta=10$. In the right panel, three values of $\tan\beta$ are considered when $g_x=1.0$. The other parameters are fixed at $\sin\alpha_{1}=0.01$, $\sin\alpha_{2}=0.01$ and $\sin\alpha_{3}=0.01$.
• Figure 5: The Feynman diagram corresponding to the direct detection of the dark matter candidate $Z^{\prime}$. Here, $h_i$ ($i=1,2,3$) refer to the Higgs bosons and $N$ refers to a nucleon (proton or neutron).