Characterization of the Low Energy Excess using a NUCLEUS $Al_2O_3$ detector

TL;DR

This study investigates the low energy excess observed in the NUCLEUS experiment, analyzing background rates with sapphire detectors under various conditions to understand its origin and inform mitigation strategies.

Contribution

It provides a comprehensive characterization of the low energy excess using a sapphire detector, revealing its independence from background levels and its relation to cooling procedures.

Findings

No dependence of LEE rate on background level.

Slower cooling reduces initial LEE rate.

LEE rate follows a power law with a common exponent.

Abstract

The NUCLEUS experiment aims to detect coherent elastic neutrino-nucleus scattering of reactor antineutrinos using low-threshold, gram-scale cryogenic calorimeters. Similar to other low-threshold experiments, NUCLEUS observes a sharp rise in the event rate below a few hundred eV, referred to as the low energy excess (LEE), whose origin remains yet unidentified. Building on results from the NUCLEUS testing and commissioning at the Technical University of Munich and from previous characterization campaigns, we present a comprehensive study of the background rate measured with a sapphire detector equipped with two transition-edge sensors under various experimental conditions. We find no evidence for a dependence of the LEE rate on the particle background level, whereas the results indicate that slower cooling-down procedures lead to lower initial LEE rates. The behavior of the LEE rate during the same cooldown is comparable across the measurements and is best described by a power law with a common exponent across datasets of $(-0.59 \pm 0.06)$, when time is expressed from the moment the detector reaches the 4 K temperature. These findings provide valuable guidance for future LEE mitigation strategies in the NUCLEUS experiment.