Inelastic Dark Matter Electron Scattering and the XENON1T Excess

Abstract

Detection of electron recoils by dark matter (DM) may reveal the structure of the dark sector. We consider a scenario where a heavier DM particle inelastically scatters off an electron and is converted into a lighter DM particle. A small mass difference between the two DM particles is transferred into electron recoil energy. We investigate the DM-electron interaction mediated by a massive dark photon and evaluate the inelastic DM scattering rate, taking account of the atomic structure. It is found that the scattering rate is significantly enhanced because of the small mass splitting, which allows for a small momentum transfer matched with the size of the electron wave function. We show that there exists a viable parameter space which explains the excess of electron recoil events around 2-3 keV recently reported by the XENON1T experiment.

XENON1T excess in local $Z_2$ DM models with light dark sector

“…Since X R is a dark matter component in our model with the same abundance with X I , its lifetime should be much longer than the age of the universe. It can decay via X R → X I γγγ as shown in [9]. Its decay into three-body final state, X R → X I νν, is also possible in our model.…”

“…Although this channel is much more effective than X R → X I γγγ considered in [9], the lifetime of X R is still much longer than the age of the universe.…”

“…To calculate the relic abundance of the DM we take the X as the physical state with mass m X instead of X I and X R , i.e. m X m R m I , because the mass difference δ is much smaller than the freezeout temperature T f ∼ m X /10 [9]. For this light DM the CMB constraint rules out the s-wave annihilation of the DM pair.…”

“…where E R is the recoil energy, q is the momentum transfer, K(E R , q) is the atomic excitation factor. From energy conservation we obtain the relation [9],…”

Exothermic Dark Matter for XENON1T Excess

“…Quite recently, a tantalizing hint has been announced for the potential dark matter signals from the electron recoil events in the recoil energy, E R = 1−10 keV, from XENON1T experiment [1]. The origin of the Xenon excess has been pondered over by particle physicists as seen from a lot of the already published articles on the arXiv for the last few days [2][3][4][5][6]. A simple explanation with the solar axion or the neutrino magnetic dipole moment has been put forward from the XENON1T collaboration, but both cases are inconsistent with the star cooling constraints, because the electron coupling to the axion or the neutrino magnetic dipole moment required for the Xenon excess exceeds them by the order of magnitude [1].…”