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Over the past decades, many research teams worldwide have been trying to detect dark matter, an elusive type of matter that does not emit, reflect or absorb light, using a variety of highly sensitive detectors. Ultimately, these detectors should be able to pick up the very small signals that would indicate the presence of dark matter or its weak interactions with regular matter.

The QROCODILE (Quantum Resolution-Optimized Cryogenic Observatory for Dark matter Incident at Low Energy) experiment, a research effort by researchers at the University of Zurich, the Hebrew University of Jerusalem and Massachusetts Institute of Technology (MIT), has recently introduced a promising approach for conducting dark matter searches. In a recent paper in Â鶹ÒùÔºical Review Letters, they demonstrated the potential of this method and the high sensitivity of the detector it relies on.

"The idea for the QROCODILE experiment took shape a few years ago, when our colleague and expert on superconducting nanowires, Ilya Charaev, moved from MIT to Zurich, joining the group of Andreas Schilling," Laura Baudis and Andreas Schilling, who are part of the QROCODILE collaboration, told Â鶹ÒùÔº.

"Ilya had already collaborated with our theory colleagues, Yonit Hochberg and Ben Lehmann, on early proposals for using superconducting sensors to detect light dark matter. Building on their pioneering work, and together with condensed matter physicists in our department, we set out to design a new experiment based on superconducting nanowire single-photon detectors (SNSPDs)."

The primary objective of the QROCODILE experiment was to develop an advanced SNSPD, a detector made of superconducting nanowires that acts both as the target material (i.e., the material that dark matter will collide with) and as a sensor to pick up energy emerging from the collisions. Concurrently, Baudis, Schilling and their colleagues wanted to study the origin of the event rate that they could observe with their detector, which is roughly one count per day, which could arise from cosmic rays, natural radioactivity, or other effects.

"QROCODILE uses SNSPDs, which are normally used in quantum optics, as extremely sensitive dark matter sensors," explained Baudis and Schilling. "Our devices are made of thin tungsten silicide (WSi) microwires cooled to just 0.1 degrees above absolute zero. In this superconducting state, electrons form pairs called Cooper pairs."

If even a small amount of energy, such as the one that would originate from a dark matter particle, breaks the Cooper pairs in the nanowires, a tiny resistive 'hot spot' is created. This disturbance in turn produces a measurable electrical pulse.

"What makes this approach powerful is that breaking a Cooper pair requires thousands of times less energy than the ionization or scintillation processes used in traditional detectors," said Baudis and Schilling. "As a result, our sensors can detect energy deposits as low as 0.1 eV, which allows us to probe dark matter particles with masses down to tens of keV, far lighter than what other direct detection experiments can currently access."

The QROCODILE detector is still at the proof-of-principle stage and has not yet been used to conduct dark matter searches spanning across long periods of time. Nonetheless, the results of the team's first test run were highly promising, as the SNSPD attained a very low energy threshold for a dark matter detector (0.11 eV) and set new leading constraints on dark matter-electron scattering down to 30 keV masses.

"We also showed that the same device is sensitive to interactions with both electrons and nuclei, thanks to phonon coupling in the material," said Baudis and Schilling. "Perhaps most exciting is that our sensor geometry naturally provides directional sensitivity: the detector responds differently depending on the incoming direction of the particle.

"This is a powerful feature, because a true dark matter signal should align with Earth's motion through the Milky Way, while backgrounds such as radioactivity would not. We believe that establishing directional sensitivity at these energy scales is a unique and promising step toward a future unambiguous detection of dark matter."

In the future, the QROCODILE experiment and the new detector it relies on could contribute to ongoing efforts aimed at observing signals associated with dark matter and uncovering its underlying nature. The researchers are now working on improving their detector by increasing the effective mass of the target it relies on, further lowering its energy threshold and characterizing background signals at these low energies.

"To achieve this, we plan to build larger-area sensors and adjust the material composition for optimal performance, while also performing dedicated low-energy calibrations, for example, with an internal 55-Fe source," added Baudis and Schilling.

"A major step will be to move underground to the Gran Sasso Laboratory in Italy, where shielding from cosmic rays is expected to reduce backgrounds. We have already partnered with a local group operating a low-background cryostat there, and preparations for our first underground physics run are underway."

Baudis, Schilling and their colleagues hope that the improvements they are working on will further expand the reach of the QROCODILE experiment. This could in turn allow them to probe regions of the light dark matter parameter space that have not yet been explored.