Intrinsic Fano factor of nuclear recoils for dark matter searches

TL;DR

This paper investigates the intrinsic variance in electron-hole production during nuclear recoils in germanium and silicon, revealing larger fluctuations than electron recoils, which impacts dark matter detection experiments.

Contribution

It introduces the intrinsic nuclear recoil Fano factor for germanium and silicon, quantifying the variance in electron-hole production.

Findings

Fano factor of 24.3±0.2 for silicon at 25 keV

Fano factor of 26±8 for germanium at 25 keV

Significant impact on signal shape predictions for dark matter searches

Abstract

Nuclear recoils in germanium and silicon are shown to have much larger variance in electron-hole production than their electron-recoil counterparts for recoil energies between 10 and 200\,keV. This effect--owing primarily to deviations in the amount of energy given to the crystal lattice in response to a nuclear recoil of a given energy--has been predicted by the Lindhard model. We parameterize the variance in terms of an intrinsic nuclear recoil Fano factor which is 24.3$\pm$0.2 and 26$\pm$8 at around 25\,keV for silicon and germanium respectively. The variance has important effects on the expected signal shapes for experiments utilizing low-energy nuclear recoils such as direct dark matter searches and coherent neutrino-nucleus scattering measurements.