Nonthermal Two Component Dark Matter Model for Fermi-LAT $\gamma$-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 $\gamma$-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 ${\mathbb{Z}}_2\times {\mathbb{Z}}_2^{\prime }$ symmetry which breaks softly to a remnant ${\mathbb{Z}}_2^{\prime \prime }$ 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 ${\chi }_{{}_{{}_1}}$ with $M_{{\chi }_{{}_{{}_1}}}\sim 125$ GeV, a moderately heavy scalar ${\chi }_{{}_{{}_2}}$ with $50\mathrm{GeV}\le M_{{\chi }_{{}_{{}_2}}}\le 80\mathrm{GeV}$ and a light ${\chi }_{{}_{{}_3}}$ with $M_{{\chi }_{{}_{{}_3}}}\sim 7$ keV. There is only tiny mixing between ${\chi }_{{}_{{}_1}}$ and ${\chi }_{{}_{{}_2}}$ as well as between ${\chi }_{{}_{{}_1}}$ and ${\chi }_{{}_{{}_3}}$. The lack of importance of domain wall formation in the present scenario from the spontaneous breaking of the discrete symmetry ${\mathbb{Z}}_2^{\prime \prime }$, provided $u\le 10$ 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 $\gamma$-ray excess (from the ${\chi }_{{}_{{}_2}}\to \mathrm{b}\bar{\mathrm{b}}$ decay mode) and the observation of the X-ray line (from the decay channel ${\chi }_{{}_{{}_3}}\to \gamma \gamma$) by the XMM-Newton observatory.

11 eps Figures, 2 Tables, 32 Pages. Minor addition in Abstract. Inclusion in Section 1 of discussion of earlier attempts to explain the concerned phenomena by astrophysical processes. Extension of discussion in Section 6 to the case of a steeper dark matter density profile. Results unchanged. Version accepted for publication in JHEP

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