3.5 keV Galactic Emission Line as a Signal from the Hidden Sector

An emission line with energy of $E\sim 3.5$ keV has been observed in galaxy clusters by two experiments. The emission line is consistent with the decay of a dark matter particle with a mass of $\sim 7$ keV. In this work we discuss the possibility that the dark particle responsible for the emission is a real scalar ($\rho$) which arises naturally in a $U(1)_X$ Stueckelberg of MSSM. In the MSSM Stueckelberg extension $\rho$ couples only to other scalars carrying a $U(1)_X$ quantum number. Under the assumption that there exists a vectorlike leptonic generation carrying both $SU(2)_L\times U(1)_Y$ and $U(1)_X$ quantum numbers, we compute the decay of the $\rho$ into two photons via a triangle loop involving scalars. The relic density of the $\rho$ arises via the decay $H^0\to h^0+ \rho$ at the loop level involving scalars, and via the annihilation processes of the vectorlike scalars into $\rho + h^0$. It is shown that the galactic data can be explained within a multicomponent dark matter model where the 7 keV dark matter is a subdominant component constituting only $(1-10)$\% of the matter relic density with the rest being supersymmetric dark matter such as the neutralino. Thus the direct detection experiments remain viable searches for WIMPs. The fact that the dark scalar $\rho$ with no interactions with the standard model particles arises from a Stueckelberg extension of a hidden $U(1)_X$ implies that the 3.5 KeV galactic line emission is a signal from the hidden sector.