Abstract
Recently XENON1T Collaboration announced that they observed some excess in the electron recoil energy around a 2–3 keV. We show that this excess can be interpreted as exothermic scattering of excited dark matter on atomic electron through dark photon exchange. We consider DM models with local dark U(1) gauge symmetry that is spontaneously broken into its Z2 subgroup by Krauss-Wilczek mechanism. In order to explain the XENON1T excess with the correct DM thermal relic density within freeze-out scenario, all the particles in the dark sector should be light enough, namely ∼O(100) MeV for scalar DM and ∼O(1−10) MeV for fermion DM cases. And even lighter dark Higgs ϕ plays an important role in the DM relic density calculation: XX†→Z′ϕ for scalar DM (X) and χχ¯→ϕϕ for fermion DM (χ) assuming mZ′>mχ. Both of them are in the p-wave annihilation, and one can easily evade stringent bounds from Planck data on CMB on the s-wave annihilations, assuming other dangerous s-wave annihilations are kinematically forbidden.
Highlights
XENON1T Collaboration announced that they observed some excess in the electron recoil energy around a 2–3 keV
We shall consider both complex scalar [51] and Dirac fermion dark matter (DM) models [52] with local U (1) dark gauge symmetry which is spontaneously broken into its Z2 subgroup by Krauss-Wilczek mechanism [53]
In particular we will emphasize that the DM thermal relic density and the XENON1T electron recoil excess with a few keV could be simultaneously accommodated if dark Higgs boson is light enough that DM + XDM → Z ∗ → Z φ is kinematically open
Summary
XENON 1T Collaboration reported that they found electron recoil excess around 2-3 keV with 3.5 σ significance analyzing data for an exposer of 0.65 ton-year [1]. We interpret this electron recoil excess in terms of exothermic DM scattering on atomic electron bound to Xe in the inelastic DM models We shall consider both complex scalar [51] and Dirac fermion DM models [52] with local U (1) dark gauge symmetry which is spontaneously broken into its Z2 subgroup by Krauss-Wilczek mechanism [53]. In order to explain the XENON1T excess in terms of XDM + eatomic → DM + efree with a kinetic mixing, both dark photon and (X)DM mass should be sub-GeV, ∼ O(100) MeV, in order to avoid the stringent bounds on the kinetic mixing parameter For such a light DM, one has to consider the DM annihilation should be mainly in p-wave, and not in s-wave, in order to avoid strong constraints from CMB (see [54, 55] and references therein). It can not be emphasized enough that these channels would not be possible without dark Higgs φ, and it would be difficult to make the DM pair annihilation be dominated by the p-wave annihilation
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