On the relation between Migdal effect and dark matter-electron scattering in isolated atoms and semiconductors

TL;DR

This paper explores the theoretical connection between Migdal effect and dark matter-electron scattering, providing new estimates for semiconductors and updated experimental limits for sub-GeV dark matter detection.

Contribution

It establishes a principal mapping between dark matter-electron and nucleus scattering rates and estimates Migdal ionization in semiconductors based on crystal structure.

Findings

Migdal and dark matter-electron processes are closely related.

New limits on dark matter-nucleus scattering down to 500 keV.

Dark matter-electron scattering dominates for masses below 100 MeV with a dark photon mediator.

Abstract

A key strategy for the direct detection of sub-GeV dark matter is to search for small ionization signals. These can arise from dark matter-electron scattering or when the dark matter-nucleus scattering process is accompanied by a "Migdal" electron. We show that the theoretical descriptions of both processes are closely related, which allows for a principal mapping between dark matter-electron and dark matter-nucleus scattering rates once the dark matter interactions with matter are specified. We explore this parametric relationship for noble-liquid targets and, for the first time, provide an estimate of the "Migdal" ionization rate in semiconductors that is based on evaluating a crystal form factor that accounts for the semiconductor band structure. We also present new dark-matter-nucleus scattering limits down to dark matter masses of 500 keV using published data from XENON10, XENON100, and a SENSEI prototype Skipper-CCD. For a dark photon mediator, the dark matter-electron scattering rates dominate over the Migdal rates for dark matter masses below 100 MeV. We also provide projections for proposed experiments with xenon and silicon targets.