Abstract
Recent evidence for an excess of gamma rays in the GeV energy range about the Galactic Center have refocused attention on models of dark matter in the low mass regime (mχ ≲ m Z /2). Because this is an experimentally well-trod energy range, it can be a challenge to develop simple models that explain this excess, consistent with other experimental constraints. We reconsider models where the dark matter couples to dark photon, which has a weak kinetic mixing to the Standard Model photon, or scalars with a weak mixing with the Higgs boson. We focus on the light (≲1.5 GeV) dark mediator mass regime. Annihilations into the dark mediators can produce observable gamma rays through decays to π0, through radiative processes when decaying to charged particles (e+e−, μ+μ−, . . .), and subsequent interactions of high energy e+e− with gas and light. However, these models have no signals of $$ \overline{p} $$ production, which is kinematically forbidden. We find that in these models, the shape of resulting gamma-ray spectrum can provide a good fit to the excess at Galactic Center. We discuss further constraints from AMS-02 and the CMB, and find regions of compatibility.
Highlights
A great effort has been undertaken to do this, especially for the broad “WIMP” and WIMP-like particles, with masses in the 1−1000 GeV range, and with interaction strengths characterized by the weak scale
We focus on the light ( 1.5 GeV) dark mediator mass regime
Annihilations into the dark mediators can produce observable gamma rays through decays to π0, through radiative processes when decaying to charged particles (e+e−, μ+μ−, . . .), and subsequent interactions of high energy e+e− with gas and light
Summary
The class of models we consider in this article consists of a DM particle χ and a dark force φ with a mass MeV < mφ < GeV, which is lighter than the DM mass, mχ. The DM has a dominant annihilation process, χ + χ → φ + φ, followed by cascade decays of the dark force to the Standard Model particles. The mixing occurs with the EM field strength, and the cascade decay is triggered by the coupling of dark force and the Standard Model currents. The principle obstacles to this is that for a fermion the annihilation of χχ to φφ is p-wave suppressed This can be evaded if the annihilation is into a complex scalar. In this case, either the pseudoscalar would be massless (and would be an additional relativistic degree of freedom), or could mix with the Higgs via a CP-violating mixing term eiQφ |H|2 + h.c. Our points below do not depend crucially on these details,
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