Abstract
We consider searches for the inelastic scattering of low-mass dark matter at direct detection experiments, using the Migdal effect. We find that there are degeneracies between the dark matter mass and the mass splitting that are difficult to break. Using XENON1T data we set bounds on a previously unexplored region of the inelastic dark matter parameter space. For the case of exothermic scattering, we find that the Migdal effect allows xenon-based detectors to have sensitivity to dark matter with ${\cal O}(\mathrm{MeV})$ mass, far beyond what can be obtained with nuclear recoils alone.
Highlights
For the case where the dominant interaction is with nucleons, this problem is exacerbated by the fact that nuclear recoils are more difficult to detect than electron recoils
While we will not explore particular inelastic dark matter models for the purpose of this paper, we provide the following as an illustrative example
Inelastic nuclear scattering is a feature which arises in many classes of dark matter models
Summary
There has been significant recent interest in methods of detecting low-mass dark matter at direct detection experiments [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. Because many direct detection experiments are more sensitive to energetic electrons than to recoiling nuclei, these Migdal electrons can provide the leading channel for the direct detection of low-mass dark matter. Dark matter-nucleus scattering can exhibit inelasticity in two other ways; by exciting a low-lying nuclear state, or by changing the dark matter particle mass. We consider the latter case, by assuming the dark matter particle emerging from the scattering process has a different mass than the incoming dark particle. We will show that, in the case of exothermic scattering, the Migdal effect provides a unique opportunity to probe very low-mass dark matter. V, we conclude with a discussion of our results and future avenues
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