The Migdal effect in semiconductors for dark matter with masses below ∼ 100 MeV

Dark matter scattering off a nucleus has a small probability of inducing an observable ionization through the inelastic excitation of an electron, called the Migdal effect. We use an effective field theory to extend the computation of the Migdal effect in semiconductors to regions of small momentum transfer to the nucleus, where the final state of the nucleus is no longer well described by a plane wave. Our analytical result can be fully quantified by the measurable dynamic structure factor of the semiconductor, which accounts for the vibrational degrees of freedom (phonons) in a crystal. We show that, due to the sum rules obeyed by the structure factor, the inclusive Migdal rate and the shape of the electron recoil spectrum is well captured by approximating the nuclei in the crystal as free ions; however, the exclusive differential rate with respect to energy depositions to the crystal depends on the phonon dynamics encoded in the dynamic structure function of the specific material. Our results now allow the Migdal effect in semiconductors to be evaluated even for the lightest dark matter candidates (m(chi) (sic) 1 MeV) that can kinematically excite electrons.

Automated summary

Using an effective field theory, we expand the calculation of the Migdal effect in semiconductors to describe the scattering of dark matter off a nucleus with small momentum transfer. The measurable dynamic structure factor of the semiconductor, which includes vibrational degrees of freedom, fully quantifies our analytical result. We demonstrate that approximating the nuclei in the crystal as free ions can accurately capture the Migdal rate and the shape of the electron recoil spectrum, but the differential rate of energy deposition to the crystal depends on the phonon dynamics encoded in the dynamic structure function of the specific material. Our findings enable evaluation of the Migdal effect in semiconductors, even for the lightest dark matter candidates kinematically capable of exciting electrons.