http://arxiv.org/abs/2012.07209
A broad excess of keV electron recoil events was reported by XENON1T recently. This excess may be induced by the scattering of electrons and long-lived MeV particles with high-speed. The properties of the new long-lived particles are restricted by the big bang nucleosynthesis (BBN) and cosmic microwave background (CMB). Here we consider a tangible model of two scalar MeV dark matter (DM) particles $S_A$ and $S_B$ to interpret the XENON1T keV excess via boosted $S_B$. A small mass splitting $m_{S_A}-m_{S_B}>0$ is introduced and pairs of boosted $S_B S_B^\ast$ can be produced by the dark annihilation process of $S_A S_A^\ast\to \phi \to S_B S_B^\ast$ via a resonant scalar $\phi$. The $S_B-$electron scattering is intermediated by a vector boson $X$ which may cause the anomalous transitions of $^8$Be and $^4$He. Although constraints from BBN, CMB and low-energy experiments set the $X-$mediated $S_B-$electron scattering cross section to be $\lesssim 10^{-35} \mathrm{cm}^2$, the MeV scale DM with an enhanced dark annihilation today can still provide enough boosted $S_B S_B^\ast$ and induce the XENON1T keV excess. The relic density of $S_B S_B^\ast$ is significantly reduced by the $s-$wave process of $S_B S_B^\ast \to X X$ which is allowed by constraints from CMB and 21-cm absorption. A very small relic fraction of $S_B S_B^\ast$ is compatible with the stringent bounds on un-boosted $S_B$-electron scattering in DM direct detection and the un-boosted $S_A$-electron scattering is also allowed by the direct detection.
L. Jia and T. Li
Tue, 15 Dec 20
76/136
Comments: 5 pages, 4 figures
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