SENSEI collaboration reports first findings


Findings of the first search for sub-GeV dark matter using the SENSEI detector at SNOLAB
The SENSEI detector at SNOLAB. Left: Two CCD modules in their copper tray. Middle: Copper CCD box and trays deployed inside the vessel. Right: Closed SENSEI vacuum vessel, before installing the outer copper, lead, and poly-water shields. Credit: Prakruth Adari et al

Detecting dark matter particles and understanding their underlying physics is a long-standing research goal for many researchers worldwide. Dark matter searches have been aimed at detecting different possible signals that could be associated with the presence of these elusive particles or with their interaction with regular matter.

A promising technology for conducting dark matter searches is the SENSEI (Sub-Electron Noise Skipper-CCD experimental instrument) detector, a highly sensitive imaging sensor located at the SNOLAB research facility in Canada.

The research group analyzing data collected by this detector, dubbed the SENSEI collaboration, have published the results of their first search for sub-GeV dark matter at SNOLAB in the journal Physical Review Letters.

“The primary objective of our recent paper was to search for dark matter particle candidates with a mass below the proton, which we refer to as ‘sub-GeV dark matter,’ since the mass of the proton is about 1 GeV,” Rouven Essig, co-author of the paper, told Phys.org.

“The results we presented came after several years of effort in which the SENSEI collaboration improved the sensitivity of their detectors to sub-GeV dark matter and reduced the impact of other types of events, which mimic dark matter events (i.e., ‘backgrounds’). This is the first SENSEI study utilizing data collected at SNOLAB, one of the world’s deepest laboratories, situated over 2 km underground in Sudbury, Canada.”

Bringing the SENSEI detector to the underground SNOLAB in Canada was a long-standing goal for Essig and his colleagues, as the collection of data in this location could allow them to advance their search for sub-GeV dark matter. Like other dark matter candidates, sub-GeV dark matter is believed to interact weakly with ordinary matter, which would make it incredibly difficult to detect.

“SENSEI uses ultrasensitive silicon ‘Skipper Charge Coupled Devices’ (Skipper CCDs), which allow us to search for dark matter particles that scatter off an electron in the silicon,” said Kelly Stifter, co-author of the paper.

“Such a scatter would release only a small number of electrons (approximately 1-10) from the silicon atoms in one of the pixels in the Skipper CCD. The revolutionary advance afforded by the Skipper CCD (when compared to an ordinary CCD) occurred in 2017 and allows us to measure precisely the number of electrons in each of the millions of pixels across the device. “

Via ultrasensitive Skipper CCDs, the SENSEI detector allows researchers to search for sub-GeV dark matter with high sensitivity. The detector’s first experimental run at SNOLAB and the subsequent analysis of collected data allowed researchers to set unprecedented constraints on the interactions of this dark matter candidate with electrons and nuclei.

“We obtained the first dark matter search results with a Skipper-CCD in 2018, and several others over the next few years,” explained Javier Tiffenberg, co-author of the paper.

“Notably, these experiments were run near the surface of the Earth, which is inundated with cosmic rays that can occasionally mimic events that look like dark matter. Our PRL paper presents our collaboration’s first result obtained with an experiment that is being operated at SNOLAB, which is deep underground and well shielded.”

The findings of the first search for Sub-GeV dark matter using the SENSEI detector at SNOLAB
Photo showing the inner parts of the SENSEI detector. Credit: SENSEI collaboration.

The experimental run that gathered the data analyzed by the researchers as part of this recent study was performed over a 7-month period, spanning from 2022 to 2023.

To set new constraints on sub-GeV dark matter interacting with electrons and nuclei, the SENSEI collaboration specifically measured the number of events picked up by the detector that contained one or more electrons, which allowed them to set limits on the dark matter particles that could create these events.

“One of our goals for future work is to use more Skipper-CCDs so that we can detect more dark matter particles,” said Sho Uemura, co-author of the paper.

“We have now shown that we can operate an array of Skipper-CCDs and continue to improve their performance over our previous results with a single Skipper-CCD. Our understanding of background events, and our ability to remove them from the data, is keeping pace with the increased detector size.”

The paper by the SENSEI collaboration could inform future efforts aimed at detecting dark matter, potentially leading to even more sensitive searches for sub-GeV .

The researchers are now planning to further enhance the sensitivity of the detector, which may contribute to the detection of these elusive particles or could allow them to set even more stringent constraints on their interactions with .

“We are confident that we can further reduce the backgrounds in our Skipper-CCDs, and we also plan to increase the number of Skipper-CCDs that we operate,” said Ana Botti, co-author of the paper. “Both will improve the sensitivity of our detector to dark matter.”

A key aspect of the research efforts by the SENSEI collaboration involves understanding how new high-sensitivity sensors are best operated, maximizing their potential for detecting dark matter-related signals. This is because effects (e.g., dark counts or spurious charges) produced without particles interacting, can often dominate the background signals collected while trying to pick up rare events, such as dark matter interactions,

“As Skipper CCDs are a new technology, there is no instruction manual for their use,” added Botti.

“Developing strategies to reduce these rates and mitigate their impact on the analysis has been crucial for the continuous improvement of SENSEI’s results. We are also considering developing new technologies similar to the Skipper-CCD for light-dark matter detection to improve sensitivity further.

“It is worth mentioning that our work in SENSEI has positioned us at the forefront of this technology development, with applications that have extended beyond particle physics to fields such as astronomy and quantum imaging.”

More information:
Prakruth Adari et al, First Direct-Detection Results on Sub-GeV Dark Matter Using the SENSEI Detector at SNOLAB, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.011804. On arXiv: DOI: 10.48550/arxiv.2312.13342

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Sub-GeV dark matter hunt: SENSEI collaboration reports first findings (2025, January 30)
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