# Electron Ionization via Dark Matter-Electron Scattering and the Migdal Effect

## TL;DR

This paper compares electron ionization signals from direct DM-electron scattering and the Migdal effect, showing the latter can dominate for heavy mediators and certain DM masses, impacting experimental interpretations.

## Contribution

It provides a unified theoretical framework for both ionization processes and clarifies when the Migdal effect becomes significant in dark matter detection.

## Key findings

- Migdal effect scales with Z^2 and q^2, making it more relevant for heavy mediators.
- For light mediators, electron scattering dominates ionization signals.
- Heavy mediators and DM masses in hundreds of MeV can lead to Migdal-induced ionization surpassing direct electron scattering.

## Abstract

There are currently several existing and proposed experiments designed to probe sub-GeV dark matter (DM) using electron ionization in various materials. The projected signal rates for these experiments assume that this ionization yield arises only from DM scattering directly off electron targets, ignoring secondary ionization contributions from DM scattering off nuclear targets. We investigate the validity of this assumption and show that if sub-GeV DM couples with comparable strength to both protons and electrons, as would be the case for a dark photon mediator, the ionization signal from atomic scattering via the Migdal effect scales with the atomic number $Z$ and 3-momentum transfer $\mathbf{q}$ as $Z^2 \mathbf{q}^2$. The result is that the Migdal effect is always subdominant to electron scattering when the mediator is light, but that Migdal-induced ionization can dominate over electron scattering for heavy mediators and DM masses in the hundreds of MeV range. We put these two ionization processes on identical theoretical footing, address some theoretical uncertainties in the choice of atomic wavefunctions used to compute rates, and discuss the implications for DM scenarios where the Migdal process dominates, including for XENON10, XENON100, and the recent XENON1T results on light DM scattering.