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We show the presence of a potentially detectable gamma-ray line in the FermiGalactic center excess in light of the secluded (vector) dark matter (DM) modelin which the hidden scalar, nearly degenerate with DM in mass, mediates theinteraction of the secluded DM with the Standard Model (SM) due to its mixingwith the SM Higgs. We find that the parameter region $m_X\in[60, 132]$ GeV canprovide a good fit to the Fermi Galactic center gamma-ray excess spectrum,appearing a prominent gamma-ray line with the energy $\in [30, 66]$ GeV. Thebest fit gives $m_X\simeq m_S \simeq 86$ GeV with a $p$-value$\, =0.42$, sothat the resultant gamma-ray line, arising from the decay of the scalarmediator into $\gamma\gamma$, peaks at 43 GeV. We derive constraints on theannihilation cross section from the Fermi-LAT gamma-ray line search, gamma-rayobservations of the Fermi-LAT dwarf spheroidal galaxies, and Planck cosmicmicrowave background measurement. For the secluded vector DM model, theparameter space constrained by the current XENON1T and future LUX-ZEPLIN isshown. Finally, for the mixing angle between the Higgs sectors, we discuss itslower bound, which is required by the big bang nucleosynthesis constraint andrelevant to the hidden sector decoupling temperature.

the journal of high energy physics/the journal of high energy physics

A Potentially Detectable Gamma-Ray Line in the Fermi Galactic Center  Excess -- In Light of One-Step Cascade Annihilations of Secluded (Vector)  Dark Matter via the Higgs Portal