Sources of Low-Energy Events in Low-Threshold Dark-Matter and Neutrino Detectors

Abstract

The reach of sub-GeV dark-matter detectors is at present severely affected by low-energy events from various origins. We present the theoretical methods to compute the single-and few-electron events that arise from secondary radiation emitted by highenergy particles as they pass through detector materials and perform a detailed simulation to quantify them at (Skipper) CCD-based experiments, focusing on the SENSEI data collected at Fermilab near the MINOS cavern. The simulations account for the generation of secondaries from Cherenkov and luminescent recombination radiation; photo-absorption in the bulk, backside layer, pitch adapter, and epoxy; the photon reflection and refraction at interfaces; thin-film interference; the roughness of the interfaces; the dynamics of charges produced in the highly doped CCD-backside-layers; and the partial charge collection on the CCD backside. We consider several systematic uncertainties, notably those stemming from the backside modeling, which we estimate with a "fiducial" and an "extreme" charge-diffusion model, with the former model being preferred due to better agreement with partial-charge collection data. We find that Cherenkov photons constitute about 40% of the observed single-electron events for both diffusion models; radiative recombination contributes negligibly to the event rate for the fiducial model, although it can dominate over Cherenkov for the extreme model. We also estimate the fraction of 2-electron events that arise from 1-electron event coincidences in the same pixel, finding that the entire 2-electron rate can be explained by coincidences of radiative events and spurious charge. Accounting for both radiative and non-radiative backgrounds, we project the sensitivity of future Skipper-CCD-based experiments to different dark-matter models. For light-mediator models with dark-matter masses of 1, 5, and 10 MeV, we find that future experiments with 10-kg-year exposures and successful background mitigation could have a sensitivity that is larger by 9, 3, and 2 orders of magnitude, respectively, when compared to an experiment without background improvements.

Constraints on the electron-hole pair creation energy and Fano factor below 150 eV from Compton scattering in a Skipper-CCD

“…The γ-ray images contained a higher rate of single-electron events, about 4 × 10 −2 e − /pix/day, with respect to previous measurements at the surface [26]. This is expected due to the high-energy events producing a halo of single-electron events around the γ-ray hit [11,27]. In addition, the CCD was operated in a continuous readout mode, and the clocks were optimized to maximize the charge transport efficiency needed to move thousands of electrons from pixel to pixel; this leads to a higher rate of singleelectron events as reported in [28].…”

The Oscura Experiment

“…Near-infrared (NIR) photon emission, transport, and absorption High-energy charged particles interacting with the silicon of the CCD and with the surrounding structures can produce NIR photons from Cherenkov radiation. In addition, charged particles produce electron-hole pairs in the highly-doped gettering 3 layer on the backside of the CCD that subsequently recombine radiatively and emit near-infrared NIR photons [53]. These NIR photons have a long attenuation length in silicon and, once absorbed, produce a singleelectron event.…”

“…(v) Rapid progress is being been made in understanding the origin and the mitigation strategies of single-and few-electron backgrounds [52][53][54][55].…”

The physics potential of a reactor neutrino experiment with Skipper-CCDs: searching for new physics with light mediators

“…Reducing the rate of muons arriving at the detector. It will impact the one-electron events produced by Cherenkov and bremsstrahlung [97] and the rate of neutron-induced events. Although we do not have enough information at the moment to calculate the expected background rate after applying all these features, we will work in a slightly optimistic scenario with a baseline background of 1 kdru.…”

“…As a result, a exposure time of about a couple of hours corresponds to an expected pixel occupancy of the CCD of around 10-15 [6]. In addition, photons produced by high energy cosmic particles via Cherenkov and bremsstrahlung processes would also contribute to the one-electron event rate as their energies could lead to ionized electrons in silicon [97]. Besides, there is a contribution due to the dark current, but is quite low in this kind of sensor (∼10 −2 e − /pix/day).…”