Collective excitations and low-energy ionization signatures of relativistic particles in silicon detectors

TL;DR

This paper calculates the low-energy ionization signatures of relativistic particles in silicon detectors, incorporating collective effects like plasmons, to improve detection sensitivity for beyond-the-Standard-Model particles.

Contribution

It generalizes the energy-loss function for relativistic particles and benchmarks it against experimental data, highlighting the importance of collective effects in detector response.

Findings

Including collective effects shifts the recoil spectrum to higher energies.

Proper modeling enhances sensitivity to millicharged particles, neutrinos, and dark matter.

Detector backgrounds peak at lower energies, making the effects significant.

Abstract

Solid-state detectors with a low energy threshold have several applications, including searches of non-relativistic halo dark-matter particles with sub-GeV masses. When searching for relativistic, beyond-the-Standard-Model particles with enhanced cross sections for small energy transfers, a small detector with a low energy threshold may have better sensitivity than a larger detector with a higher energy threshold. In this paper, we calculate the low-energy ionization spectrum from high-velocity particles scattering in a dielectric material. We consider the full material response including the excitation of bulk plasmons. We generalize the energy-loss function to relativistic kinematics, and benchmark existing tools used for halo dark-matter scattering against electron energy-loss spectroscopy data. Compared to calculations commonly used in the literature, such as the Photo-Absorption-Ionization model or the free-electron model, including collective effects shifts the recoil ionization spectrum towards higher energies, typically peaking around 4--6 electron-hole pairs. We apply our results to the three benchmark examples: millicharged particles produced in a beam, neutrinos with a magnetic dipole moment produced in a reactor, and upscattered dark-matter particles. Our results show that the proper inclusion of collective effects typically enhances a detector's sensitivity to these particles, since detector backgrounds, such as dark counts, peak at lower energies.