Colder Freeze-in Axinos Decaying into Photons

Understand

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.