Understanding the nature of the low energy excess in cryogenic detectors to discover light dark matter

Project Information

LEGENDRE

Grant agreement ID: 101104484

DOI

10.3030/101104484

Project closed

EC signature date 21 April 2023

Start date 1 December 2023

End date 30 November 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 189 687,36

Coordinated by

KARLSRUHER INSTITUT FUER TECHNOLOGIE
Germany

Periodic Reporting for period 1 - LEGENDRE (Understanding the nature of the low energy excess in cryogenic detectors to discover light dark matter)

Reporting period: 2023-12-01 to 2025-11-30

Summary of the context and overall objectives of the project

Multiple astrophysical observations point to the existence of a gravitationally-interacting substance that makes up about 85% of all matter in the universe. Despite its abundance, a direct signature of it in our laboratories is still lacking, and as such its true nature remains today one of the great unresolved mysteries in physics. This is what we call dark matter.
In recent years, a new generation of direct-detection experiments has pushed the boundaries of sensitivity to low mass dark matter candidates, reaching extremely small energy scales that were once thought impossible to explore. However, these experiments have begun to observe unexpected excesses of low-energy events that do not match the characteristics of dark matter. The origin of this excess background is yet unclear, and today it represents the main limitation for the discovery of low-mass dark matter candidates.
The LEGENDRE project addresses this challenge by conducting a dedicated data-taking campaign using state-of-the-art cryogenic detectors developed by the SuperCDMS collaboration, operated in the Cryogenic Underground TEst facility (CUTE) at SNOLAB, one of the world’s leading underground laboratories. The collected data are analyzed using a novel methodology designed to help disentangle and understand the origin of the low-energy excess.
By providing crucial insights into this limiting background, LEGENDRE strengths Europe’s contribution to the global dark matter search effort and helps guide the design and analysis strategies of future high-sensitivity experiments.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A full experimental set up, which comprises a six-detector silicon cryogenic array read out by superconducting quantum interference devices (SQUIDs), was designed to be integrated in the cryogenic facility, aiming to study the low-energy excess. The experiment was successfully installed in spring 2024 at the CUTE facility, located 2 km underground in the SNOLAB laboratory in Canada.
The detectors were operated for about 4 months, taking high quality data in various configurations. Among those, calibration data illuminating the devices with LEDs and with external radioactive sources for precise calibration of the energy scale, noise and detector response characterization data, and background data with no external source data, taken at several detector operating conditions, enabling a detailed study of the low energy excess events. Furthermore, a high quality, 10-day period of data has been collected to perform a search for low mass dark matter particles scattering off electrons.
The design and installation of the experimental setup emphasized the reduction of electronic and environmental noise, resulting in a significant boost in detector performance. As a result, the system achieved one of the world’s best energy resolutions for this class of devices, opening the path to highly competitive low-energy measurements and precision studies relevant to the international dark matter community.
A detailed analysis of the collected data has been carried out and it is nearing completion. The analysis focused first on a detailed study of detector response, providing new insights on charge propagation and sensor behavior. The ongoing analyses target both the search for the low-mass electron-recoil dark matter and the investigation of the characteristic and possible origins of the low energy excess. These are in a mature state and will deliver significant new results once released.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The main results of this project include the deployment of world-leading, high energy-resolution cryogenic detectors in a deep underground laboratory environment, the identification and investigation of new detector effects that contribute to a deeper understanding of device performance, and new insights into the origins of the low-energy excess, which represented the primary focus of this project. In addition, a new analysis framework based on a full profile likelihood ratio method was developed and applied for the first time to this class of detectors, enabling world-leading sensitivity to electron-recoil dark matter candidates.
Furthermore, key needs have been identified for continued progress in this field. These include research efforts to better understand and mitigate the ionizing backgrounds that currently limit the physics reach of this detector technology.
The dissemination of results has taken place through presentations at several international conferences and workshops, including EXCESS 2024 and 2025, IDM 2025, and TAUP 2025. Two scientific publications are currently in preparation and will be released in the coming months: one focusing on the technical aspects of the instrumentation, and another presenting the dark matter search results. A further publication, dedicated to the findings on the low-energy excess, will follow thereafter. Two additional publications, based on earlier iterations of the detectors employed in this project, have been released, with the researcher’s contribution enabled by this funding.

The final detector payload

Installing of the detector payload in the underground cryogenic facilty