Speaker
Description
We explore the phenomenology of the Dark Photon iDM (A'iDM) model, one of the simplest and most direct realisations of the inelastic Dark Matter (iDM) scenario. In this framework, the Standard Model (SM) is extended by a dark sector with an additional U(1)_D gauge symmetry. All SM particles are neutral under this symmetry, which couples to the SM hypercharge gauge boson via a kinetic mixing parameter ε. Our work expands upon existing A'iDM literature in three key ways: we move beyond specific benchmark points, restrict our focus to cosmologically viable configurations, and evaluate the complementarity between accelerator and astrophysical signals. The model features a dark photon A' with mass M_A' and two Majorana states, χ₁ and χ₂, with a mass splitting δ = M_χ₂ - M_χ₁ > 0, where χ₁ serves as the dark matter candidate. By fixing the dark coupling α_D to the electromagnetic coupling α_EM and setting ε to its experimental upper bound, we perform a comprehensive scan of the remaining parameter space (M_χ₁, δ, M_A'). We analyse the χ₁ relic abundance, the prospects for direct and indirect detection, and potential signals at accelerators and in astrophysics. Contrary to suggestions from previous studies that focused on specific benchmarks, our scan shows that α_D = α_EM is not phenomenologically disfavored. We also find that when the χ₁ relic density matches observational data, direct and indirect searches become kinematically inaccessible. However, we show that the projected luminosity of FASER---a dedicated Long-Lived Particle (LLP) detector at the LHC --- can probe or exclude the model's parameter space for M_χ₁ ≲ 7 GeV, 100 MeV ≲ δ ≲ 300 MeV, and M_A' ≲ 25 GeV. This reach could be significantly extended by the proposed FASER 2 upgrade for the High-Luminosity LHC. Interestingly, the parameter space accessible via accelerator LLP searches partially overlaps with the region probed by χ₁ capture in neutron stars. This capture process is expected to heat nearby neutron stars to approximately 2000 K, offering a promising, though challenging, signature for future infrared telescope observations.
