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.
@article{arxiv.2410.16510,
title = {Low Energy Backgrounds and Excess Noise in a Two-Channel Low-Threshold Calorimeter},
author = {Robin Anthony-Petersen and Clarence L. Chang and Yen-Yung Chang and Luke Chaplinsky and Caleb W. Fink and Maurice Garcia-Sciveres and Wei Guo and Scott A. Hertel and Xinran Li and Junsong Lin and Marharyta Lisovenko and Rupak Mahapatra and William Matava and Daniel N. McKinsey and David Z. Osterman and Pratyush K. Patel and Bjoern Penning and Mark Platt and Matt Pyle and Yinghe Qi and Maggie Reed and Ivar Rydstrom and Roger K. Romani and Bernard Sadoulet and Bruno Serfass and Peter Sorensen and Burkhant Suerfu and Vetri Velan and Gensheng Wang and Yue Wang and Samuel L. Watkins and Michael R. Williams},
journal= {arXiv preprint arXiv:2410.16510},
year = {2025}
}