Nonthermal two component dark matter model for Fermi-LAT γ-ray excess and 3.55 keV X-ray line

Abstract

A two component model of nonthermal dark matter is formulated to simultaneously explain the Fermi-LAT results indicating a γ-ray excess observed from our Galactic Centre in the 1–3 GeV energy range and the detection of an X-ray line at 3.55 keV from extragalactic sources. Two additional Standard Model singlet scalar fields S 2 and S 3 are introduced. These fields couple among themselves and with the Standard Model Higgs doublet H. The interaction terms among the scalar fields, namely H, S 2 and S 3, are constrained by the application of a discrete ℤ 2 × ℤ 2 ′ symmetry which breaks softly to a remnant ℤ 2 ′ ′ symmetry. This residual discrete symmetry is then spontaneously broken through an MeV order vacuum expectation value u of the singlet scalar field S 3. The resultant physical scalar spectrum has the Standard Model like Higgs as χ 1 with Mχ1∼125 GeV, a moderately heavy scalar χ 2 with 50 GeV ≤ Mχ2≤80 GeV and a light χ 3 with Mχ3∼7 keV. There is only tiny mixing between χ 1 and χ 2 as well as between χ 1 and χ 3. The lack of importance of domain wall formation in the present scenario from the spontaneous breaking of the discrete symmetry ℤ 2 ′ ′, provided u ≤ 10 MeV, is pointed out. We find that our proposed two component dark matter model is able to explain successfully both the above mentioned phenomena — the Fermi-LAT observed γ-ray excess (from the χ2→bb¯ decay mode) and the observation of the X-ray line (from the decay channel χ 3 → γγ) by the XMM-Newton observatory.