Colder Freeze-in Axinos Decaying into Photons

TL;DR

This paper proposes that 7 keV axino dark matter, produced via freeze-in and entropy production, can explain the 3.5 keV X-ray line and evade Ly-alpha constraints, acting as a sterile neutrino-like DM candidate.

Contribution

It introduces a model where axino DM naturally explains the X-ray excess and remains consistent with cosmological constraints, highlighting the role of entropy production in cooling the phase space distribution.

Findings

Axino decay produces a detectable monochromatic photon.

The phase space distribution of axinos is colder than Fermi-Dirac.

The model satisfies Ly-alpha forest constraints.

Abstract

We point out that 7 keV axino dark matter (DM) in the R-parity violating (RPV) supersymmetric (SUSY) Dine-Fischler-Srednicki-Zhitnitsky model can simultaneously reproduce the 3.5keV X-ray excess, and evade stringent constraints from the Ly-alpha forest data. Peccei-Quinn symmetry breaking naturally generates both axino interactions with minimal SUSY standard model particles and RPV interactions. The RPV interaction introduces an axino-neutrino mixing and provides axino DM as a variant of sterile neutrino DM, whose decay into a monochromatic photon can be detected by X-ray observations. Axinos, on the other hand, are produced by freeze-in processes of thermal particles in addition to the Dodelson-Widrow mechanism of sterile neutrinos. The resultant phase space distribution tends to be colder than the Fermi-Dirac distribution. The inherent entropy production from late-time saxion decay makes axinos even colder. The linear matter power spectrum satisfies even the latest and strongest constraints from the Ly-alpha forest data.