Abstract
We consider models of decaying spin-1 dark matter whose dominant coupling to the standard model sector is through a dark-Higgs Yukawa portal connecting a TeV-scale vector-like lepton to the standard model (right-handed) electron. Below the electron-positron threshold, dark matter has very slow, loop-suppressed decays to photons and (electron) neutrinos, and is stable on cosmological time-scale for sufficiently small gauge coupling values. Its relic abundance is set by in-equilibrium dark lepton decays, through the freeze-in mechanism. We show that this model accommodates the observed dark matter abundance for natural values of its parameters and a dark matter mass in the ∼ 5 keV to 1 MeV range, while evading constraints from direct detection, indirect detection, stellar cooling and cosmology. We also consider the possibility of a nonzero gauge kinetic mixing with the standard model hypercharge field, which is found to yield a mild impact on the model’s phenomenology.
Highlights
Equilibrium with the SM until its annihilation rate is beaten by Hubble expansion leading to DM freeze-out, within the freeze-in scenario the DM is never in thermal equilibrium with the SM and is gradually produced from scattering or decay of SM particles
In this work we considered models of decaying spin-1 DM χ associated with a spontaneously broken U(1)X gauge symmetry
In the U(1)X broken phase, this portal induces a mass mixing between the dark lepton and the electron
Summary
This section describes the production of VDM in the early universe. We assume that dark sector particles are initially absent from the thermal bath, that is nχ = nE = 0 at T = TR where TR Λ is the reheating temperature. If VDM interactions with the thermal bath are too slow, the relic density will be produced mostly out-of-equilibrium, from a freeze-in mechanism where VDM particles are created by collision of thermal SM (and E) particles. 2 → 2 scattering processes with a photon, like 1 ̄2 → γχ or 1γ → 2χ with i being either e or E Such processes, creating one VDM particle per collision, are allowed since χμ is not stable. VDM can be produced by (in-equilibrium) decay of E particles, E → χe This process is parametrically more efficient than 2 → 2 ones (suppressed by α), since it requires one less power of equilibrium density in the initial state, and largely dominates VDM production [13].
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