Abstract
We propose that the nearly massless dark photons produced from the annihilation of keV dark fermions in the Galaxy can induce the excess of electron recoil events recently observed in the XENON1T experiment. The minimal model for this is the extension of a $U(1)_{X}$ gauge symmetry, under which the dark photon couples to both dark and visible matter currents. We find that the best-fit parameters of the dark sector are compatible with the most stringent constraints from stellar cooling. We also show that in the freeze-out scenario, the dark fermions can explain the anomaly while contributing $\gtrsim 1\%$ of the DM relic density.
Highlights
One of the outstanding puzzles in particle physics is the nature of dark matter (DM), whose existence has been confirmed from various cosmological and astrophysical observations [1]
The Dine-Fischler-Srednicki-Zhitnitsky [7] type of axionlike particles (ALPs) are excluded by the stellar cooling since they strongly couple to both electrons and photons
We find that the stellar cooling from horizontal branch (HB) stars has restricted the fraction fχ to be in the range of ≳1% if the dark photon obtains its mass via the Higgs mechanism
Summary
One of the outstanding puzzles in particle physics is the nature of dark matter (DM), whose existence has been confirmed from various cosmological and astrophysical observations [1]. If axionlike particles (ALPs) are assumed to be the sole DM that couple predominantly to electrons (the so-called photophobic models), the XENON1T anomaly can be explained without being excluded by the constraints [6]. The Dine-Fischler-Srednicki-Zhitnitsky [7] type of ALPs are excluded by the stellar cooling since they strongly couple to both electrons and photons. The absorption of massive dark photon DM suffers strong constraints from stellar cooling observations [9]. We propose the dark fermion annihilation to nearly massless dark photons in the Galaxy as the origin for the XENON1T anomaly. Our scenario is quite simple and natural, and can survive the most stringent constraints from stellar cooling observations. The XENON1T data can be accommodated even when the dark fermions constitute only a small fraction of the DM
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