3.5-keV X-ray line from nearly-degenerate WIMP dark matter decays

TL;DR

This paper proposes a WIMP dark matter decay model with nearly degenerate particles explaining the 3.5-keV X-ray line, linking it to CP violation and collider signatures.

Contribution

It introduces a novel WIMP decay scenario with nearly degenerate particles and explores its implications for CP violation and collider detection.

Findings

The model can explain the 3.5-keV line with specific mass splittings.

It predicts new CP violation sources affecting electric dipole moments.

Charged scalars could be detected at the LHC.

Abstract

The unidentified emission line at the energy of $\sim$3.5~keV observed in X-rays from galaxy clusters may originate from a process involving a dark matter particle. On the other hand, a weakly interacting massive particle (WIMP) has been an attractive dark matter candidate, due to its well-understood thermal production mechanism and its connection to physics at the TeV scale. In this paper, we pursue the possibility that the 3.5-keV X-ray arises from a late time decay of a WIMP dark matter into another WIMP dark matter, both of which have the mass of $O(100)$~GeV and whose mass splitting is about 3.5~keV. We focus on the simplest case where there are two Majorana dark matter particles and two charged scalars that couple with a standard model matter particle. By assuming a hierarchical structure in the couplings of the two dark matter particles and two charged scalars, it is possible to explain the 3.5-keV line and realize the WIMP dark matter scenario at the same time. Since the effective coupling of the two different Majorana dark matter particles and one photon violates CP symmetry, the model always contains a new source of CP violation, so the model's connection to the physics of electric dipole moments is discussed. The model's peculiar signatures at the LHC are also studied. We show the prospect of detecting the charged scalars through a detailed collider simulation.