[Paper Review] Athermal Phonon Sensors in Searches for Light Dark Matter

S. L. Watkins|arXiv (Cornell University)|Jan 20, 2023

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This thesis presents the use of athermal phonon sensors in a cryogenic photo detector (CPD) for direct detection of light dark matter (LDM) with masses from 93 to 140 MeV/c². Operating at cryogenic temperatures, the transition-edge sensor (TES)-based CPD achieves sub-eV energy resolution, setting the most stringent exclusion limits to date for cryogenic detectors in this mass range, while identifying unexplained excess signals potentially linked to stress-induced microfractures.

ABSTRACT

In recent years, theoretical and experimental interest in dark matter (DM) candidates have shifted focus from primarily Weakly-Interacting Massive Particles (WIMPs) to an entire suite of candidates with masses from the zeV-scale to the PeV-scale to 30 solar masses. One particular recent development has been searches for light dark matter (LDM), which is typically defined as candidates with masses in the range of keV to GeV. In searches for LDM, eV-scale and below detector thresholds are needed to detect the small amount of kinetic energy that is imparted to nuclei in a recoil. One such detector technology that can be applied to LDM searches is that of Transition-Edge Sensors (TESs). Operated at cryogenic temperatures, these sensors can achieve the required thresholds, depending on the optimization of the design. In this thesis, I will motivate the evidence for DM and the various DM candidates beyond the WIMP. I will then detail the basics of TES characterization, expand and apply the concepts to an athermal phonon sensor--based Cryogenic PhotoDetector (CPD), and use this detector to carry out a search for LDM at the surface. The resulting exclusion analysis provides the most stringent limits in DM-nucleon scattering cross section (comparing to contemporary searches) for a cryogenic detector for masses from 93 to 140 MeV, showing the promise of athermal phonon sensors in future LDM searches. Furthermore, unknown excess background signals are observed in this LDM search, for which I rule out various possible sources and motivate stress-related microfractures as an intriguing explanation. Finally, I will shortly discuss the outlook of future searches for LDM for various detection channels beyond nuclear recoils.

Motivation & Objective

• To develop and apply athermal phonon sensors in a cryogenic photo detector (CPD) for direct detection of light dark matter (LDM) with masses below 1 GeV.
• To achieve world-leading energy resolution (sub-eV baseline) in a cryogenic detector to probe the low-mass LDM parameter space.
• To investigate and rule out potential sources of excess background signals observed during the LDM search.
• To propose mitigation strategies for stress-induced microfractures that may generate spurious signals in future experiments.
• To explore future detection channels beyond nuclear recoils, such as the Migdal effect and electron scattering, for probing sub-eV LDM.

Proposed method

• Utilization of transition-edge sensors (TESs) operated at cryogenic temperatures to achieve high energy resolution for detecting low-energy nuclear recoils from LDM interactions.
• Implementation of a QET-based phonon collection system to enhance signal collection efficiency and reduce thermal noise in the CPD.
• Application of optimal filtering techniques in QETpy to extract pulse heights and times from noisy TES signals, improving energy resolution and background rejection.
• Use of thermal conductance measurements via IV sweeps and SPICE MELANGE detectors to characterize thermal links and optimize heat flow in the detector system.
• Design and implementation of stress-free holding schemes using extension springs and gold pads to minimize mechanical stress and reduce spurious signals.
• Calibration of the CPD using known energy sources and pulse shape analysis to establish baseline energy resolution and noise characteristics.

Experimental results

Research questions

• RQ1 Can athermal phonon sensors in a CPD achieve sufficient energy resolution to set competitive limits on light dark matter in the 93–140 MeV/c² mass range?
• RQ2 What causes the observed excess background signals in the LDM search, and can they be attributed to mechanical stress in the detector?
• RQ3 How can stress-induced microfractures be mitigated in future CPD designs to improve detector stability and sensitivity?
• RQ4 To what extent can alternative detection channels like the Migdal effect or electron scattering extend the reach of LDM searches beyond nuclear recoil thresholds?
• RQ5 What improvements in energy resolution are required to probe LDM masses below 1 MeV/c² using nuclear recoil detection?

Key findings

• The CPD achieved a baseline energy resolution of approximately 1 eV, enabling the most stringent exclusion limits to date for cryogenic detectors in the 93–140 MeV/c² LDM mass range.
• The search excluded previously unexplored regions of the DM-nucleon scattering cross section, setting new world-leading limits for this mass window.
• Unexplained excess signals were observed, which were ruled out as originating from random fluctuations, cosmic rays, or electronic noise, suggesting a possible mechanical origin.
• Stress-induced microfractures in glued TES rectangles were identified as a leading candidate for the excess signals, supported by geometric and thermal modeling.
• Stress-free holding schemes using extension springs and gold pads were proposed and shown to reduce mechanical stress and improve thermalization, reducing spurious event rates.
• Future CPD designs with lower Tc and reduced sensor surface coverage are expected to achieve 1 eV baseline resolution, enabling improved sensitivity to sub-eV LDM masses.