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

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## 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 \( {M}_{\chi_1}\sim 125 \) GeV, a moderately heavy scalar *χ*_{2} with 50 GeV ≤ \( {M}_{\chi_2}\le 80 \) GeV and a light *χ*_{3} with \( {M}_{\chi_3}\sim 7 \) 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 \( {\chi}_2\to \mathrm{b}\overline{\mathrm{b}} \) decay mode) and the observation of the X-ray line (from the decay channel *χ*_{3} → *γγ*) by the XMM-Newton observatory.

## Keywords

Cosmology of Theories beyond the SM Beyond Standard Model## Notes

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