Low Energy Backgrounds and Excess Noise in a Two-Channel Low-Threshold Calorimeter

TL;DR

This paper investigates low energy excess events and noise sources in a two-channel silicon athermal phonon detector, revealing distinct populations of background events and higher-than-expected noise levels.

Contribution

It identifies two types of low energy excess events and characterizes their origins, providing insights into background and noise mechanisms in low-threshold calorimeters.

Findings

Two distinct low energy excess populations identified

Measured noise exceeds theoretical expectations

Noise components linked to substrate and sensor energy depositions

Abstract

We describe observations of low energy excess (LEE) events, background events observed in all light dark matter direct detection calorimeters, and noise in a Transition Edge Sensor based two-channel silicon athermal phonon detector with 375 meV baseline energy resolution. We measure two distinct LEE populations: ``shared'' multichannel events with a pulse shape consistent with substrate athermal phonon events, and sub-eV events that couple nearly exclusively to a single channel with a significantly faster pulse shape. These ``singles'' are consistent with events occurring within the aluminum athermal phonon collection fins. Similarly, our measured detector noise is higher than the theoretical expectation. Measured noise can be split into an uncorrelated component, consistent with shot noise from small energy depositions within the athermal phonon sensor itself, and a correlated component, consistent with shot noise from energy depositions within the silicon substrate's phonon system.