Ionization of atoms by slow heavy particles, including dark matter

TL;DR

This paper investigates atomic ionization caused by slow heavy particles, including dark matter WIMPs, revealing that relativistic effects and atomic structure can significantly enhance ionization probabilities, challenging previous assumptions of exponential suppression.

Contribution

It demonstrates the applicability of the Born approximation to WIMP-induced ionization and uncovers the dominant role of relativistic effects and atomic structure enhancements.

Findings

Relativistic effects increase ionization cross section up to 1000 times.

Atomic structure effects remove exponential suppression of ionization probability.

Born approximation is valid for describing WIMP-atom interactions.

Abstract

Atoms and molecules can become ionized during the scattering of a slow, heavy particle off a bound electron. Such an interaction involving leptophilic weakly interacting massive particles (WIMPs) is a promising possible explanation for the anomalous 9 sigma annual modulation in the DAMA dark matter direct detection experiment [R. Bernabei et al., Eur. Phys. J. C 73, 2648 (2013)]. We demonstrate the applicability of the Born approximation for such an interaction by showing its equivalence to the semiclassical adiabatic treatment of atomic ionization by slow-moving WIMPs. Conventional wisdom has it that the ionization probability for such a process should be exponentially small. We show, however, that due to nonanalytic, cusp-like behaviour of Coulomb functions close to the nucleus this suppression is removed, leading to an effective atomic structure enhancement. Crucially, we also show that electron relativistic effects actually give the dominant contribution to such a process, enhancing the differential cross section by up to 1000 times.