Quasi-degenerate dark matter for DAMPE excess and $3.5\,\textrm{keV}$ line

TL;DR

This paper proposes a dark matter model with quasi-degenerate fermions that can explain both the DAMPE cosmic-ray excess and the 3.5 keV X-ray line through specific annihilation and decay processes, offering testable predictions.

Contribution

It introduces a novel dark matter scenario with a dark gauge symmetry that accounts for two astrophysical anomalies simultaneously, with detailed mechanisms for mass splitting, annihilation, and decay.

Findings

Explains the DAMPE excess via dark fermion annihilation into electron-positron pairs.

Accounts for the 3.5 keV X-ray line through slow decay of heavy dark fermion.

Predicts signals for direct detection experiments.

Abstract

We propose a quasi-degenerate dark matter scenario to simultaneously explain the $1.4\,\textrm{TeV}$ peak in the high-energy cosmic-ray electron-positron spectrum reported by the DAMPE collaboration very recently and the $3.5\,\textrm{keV}$ X-ray line observed in galaxies clusters and from the Galactic centre and confirmed by the Chandra and NuSTAR satellites. We consider a dark $SU(2)'\times U(1)'$ gauge symmetry under which the dark matter is a Dirac fermion doublet composed of two $SU(2)'$ doublets with non-trivial $U(1)'$ charges. At one-loop level the two dark fermion components can have a mass split as a result of the dark gauge symmetry breaking. Through the exchange of a mediator scalar doublet the two quasi-degenerate dark fermions can mostly annihilate into the electron-positron pairs at tree level for explaining the $1.4\,\textrm{TeV}$ positron anomaly, meanwhile, the heavy dark fermion can very slowly decay into the light dark fermion with a photon at one-loop level for explaining the $3.5\,\textrm{keV}$ X-ray line. Our dark fermions can be also verified in the direct detection experiments.