Simply split SIMPs

Abstract

Dark Matter which interacts strongly with itself, but only feebly with the Standard Model is a possibility that has been entertained to solve apparent small-scale structure problems that are pertinent to the non-interacting cold Dark Matter paradigm. In this paper, we study the simple case in which the self-scattering rate today is regulated by kinematics and/or the abundance ratio, through the mass-splitting of nearly degenerate pseudo-Dirac fermions χ 1 and χ 2 or real scalars φ 1 and φ 2 . We calculate the relic density of these states in a scenario where self-scattering proceeds through off-diagonal couplings with a vector particle V (Dark Photon) and where the abundance is set through number-depleting 4-to-2 reactions in the hidden sector, or, alternatively, via freeze-in. We study the implications of the considered models and their prospect of solving astrophysical small-scale structure problems. We also show how the introduction of the (meta-)stable heavier state may be probed in future dark matter searches.

The dawn of FIMP Dark Matter: A review of models and constraints

“…[292]. A dark sector not being in thermal equilibrium with the visible sector may also accommodate complicated dynamics of its own [240,241,246,284,293,294]. This results in modifications in both the DM number density and its distribution function, with possibly interesting consequences for both direct and indirect detection, collider signatures, and cosmic structure formation.…”

“…In the first studies of the freeze-in mechanism, the effect of interactions in the dark sector was neglected when calculating the DM abundance [15,19,262,263,295,296]. However, it has later been shown to be important for both determining the initial conditions for DM production [259,275,276] and the subsequent evolution of DM number density after the yield from the visible sector is completed [240,241,284,293,294,297]. Here we concentrate on the evolution of DM abundance produced initially by the decays and annihilations of visible sector particles and discuss the effect of dark sector couplings on the initial conditions for production in Section 5.…”

“…This can happen independently of their interactions with the visible sector and does not require or imply that the dark sector particles would thermalize with the visible sector particles. In case the DM particles reach thermal equilibrium within the dark sector, their final abundance may become determined not by the initial freeze-in but by a freeze-out mechanism operating within the dark sector; the so-called dark freeze-out [240,241,245,284,293,294,298,299]. If the number-changing interactions are fast, they will lead to chemical equilibrium within the dark sector.…”

“…In that case, the s particles may thermalize within itself after the initial DM production from the SM sector has ended and the DM number density can change even after the coupling between DM and the SM has been effectively shut off. That being the case, the final DM abundance is determined not only by the freeze-in, but also by the dark freeze-out occurring in the hidden sector [240,241,284,293,294].…”

“…Multiple experimental setups have recently been suggested for the detection of elastic and inelastic scatterings of DM in the mass range from keV to MeV [55-61, 63-71, 77, 78]. In particular, the typical DM-electron cross sections for an MeV FIMP DM could be tested by some next generation experiments [9,55,61,294]. Finally, due to the possibility of absorbing a dark photon, the direct detection constraints for light vector DM in the U (1) vector portal model extend down to values of the kinetic mixing coupling constant relevant for freeze-in [310].…”