Dark Photon mediated Inelastic Dark Matter in Cosmology, Astrophysics and Colliders

Abstract

We provide a systematic discussion of the phenomenology of Dark Photon iDM (A$^{\prime}$iDM) where the Standard Model (SM) is extended by a dark sector containing an additional $U(1)_D$ gauge symmetry under which all SM particles are neutral, and that couples to the SM hypercharge gauge boson through a kinetic mixing parameter $ε$. The model contains two Majorana states $χ_1$ and $χ_2$ with $δ=M_{χ_2}-M_{χ_1}>0$ and $χ_1$ the dark matter candidate, and a dark photon $A^{\prime}$ with mass $M_{A^{\prime}}$. Our analysis represents an integration of existing ones, where only specific benchmarks of the A$^{\prime}$iDM scenario have been discussed. In particular, we fix the $U(1)_D$ coupling $α_D$ equal to the electromagnetic one $α_{EM}$ and $ε$ to its experimental upper bound, and perform a complete scan of the remaining parameters $(M_{χ_1},δ, M_{A^{\prime}})$, discussing the $χ_1$ relic abundance, its direct and indirect searches, as well as potential signals from astrophysics and accelerators. Our systematic scan shows that $α_D$ = $α_{EM}$ is not disfavored, as some previous analyses, limited to specific benchmarks, may suggest. We also find that when the $χ_1$ relic density matches observation direct and indirect searches are not kinematically accessible. On the other hand we find that the projected luminosity of FASER, a detector searching for Long Lived Particles (LLP) decay at the LHC, can probe or rule out the parameters space of the model for $M_{χ_1}\lesssim$ 7 GeV, 100 MeV $\lesssim δ\lesssim$ 300 MeV and $M_{A^{\prime}}\lesssim$ 25 GeV. This range of parameter could be significantly extended by the FASER 2 upgrade proposed for the High-Luminosity phase at the LHC. The parameter space probed by LLP seaches partially overlaps with that probed by $χ_1$ capture in neutron stars.

Figures (4)

• Figure 1: Scatter plots in $M_{\chi_1}-\delta$ plane with color pallets corresponding to $M_{A^\prime}$. Note that all the points satisfy collider and DM observables constraints except the cyan colored points, which are disallowed by BBN. The magenta and green areas can be probed by Long Lived Particle searches at FASER and FASER 2 (see Section \ref{['sec:Collider']}).
• Figure 2: Scatter plot in the $(M_{\chi_1},\,\sigma_{\text{inel}})$ plane showing points consistent with relic density, and known experimental constraints. The horizontal line indicates the geometric limit $\sigma_{\text{geom}}$ for $M=M_*$ and $R=R_*$; all satisfy $\sigma_{\text{inel}}>\sigma_{\text{geom}}$, implying geometric capture saturation.
• Figure 3: Top: Number of signal events in the FASER (left) and FASER 2 (right) decay volumes. Bottom: Range of proper decay length $c\tau$ for FASER (left) and FASER 2 (right). The horizontal dashed black lines show the distances of FASER (left) and FASER 2 (right) from the ATLAS interaction point.
• Figure 4: Normalized transverse (left) and longitudinal (right) boost distributions for the benchmark points in Eq. (\ref{['benchmark points 1']}).