First WIMP search results from the XENONnT experiment

PhD student Daniel Wenz from Mainz University presents data at a seminar at the Laboratori Nazionali del Gran Sasso in Italy


23 March 2023

On behalf of the XENON collaboration, PhD student Daniel Wenz from the group of Professor Uwe Oberlack at the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU) presented results from XENONnT, the latest-generation experiment of the XENON Dark Matter project dedicated to the direct search for dark Matter in the form of Weakly Interacting Massive Particles (WIMPs). With an initial exposure slightly larger than one tonne x year, a blind analysis shows that the data is consistent with the expectations from the background-only hypothesis. XENONnT thus sets new limits on interaction of WIMPs with ordinary matter. Thanks to the five times lower background, XENONnT improved on the results from the former XENON1T experiment obtained with a similar exposure. An article has been submitted to Physical Review Letters.

The XENONnT experiment was designed to search for dark matter particles with an order of magnitude higher sensitivity than its predecessor. The detector at the core of the experiment is a cylindrical Dual-Phase Xenon Time Projection Chamber (TPC), of approximately 1.5-meter height and diameter, filled with ultra-pure liquid xenon kept at -95 degrees Celsius. A mass of 5,900 kilograms of xenon out of the 8,600 kilograms total required to operate the detector constitutes the active target for particle interactions. It is installed inside a water Cherenkov active muon and neutron veto, deep underground at the INFN Laboratori Nazionali del Gran Sasso in Italy. XENONnT was constructed and subsequently commissioned between spring 2020 and spring 2021 and took this first science data over 97.1 days, from July 6 to November 10, 2021.

The signature of a WIMP interaction with a xenon atom is a tiny flash of scintillation light together with a handful of ionization electrons that are drifted by an applied electric field towards the top of the TPC, where they are extracted by a stronger electric field into the gas xenon above the liquid, producing a second scintillation signal. Both light signals are detected with ultra-sensitive photodetectors, providing energy and 3-D position information on an event-by-event basis.

Experiments searching for dark matter require the lowest possible level of natural radioactivity, both from sources intrinsically present in the liquid xenon target and from construction materials and the environment. The former is dominated by radon atoms that are constantly emitted from detector materials and which are extremely difficult to reduce. The XENON collaboration has pioneered the technologies to lower radon to an unprecedented low level, from extensive material selection campaigns to an online cryogenic distillation system that actively removes radon from the xenon. Another important radioactive background comes from neutrons that are generated by the radioactivity of detector materials. In XENONnT, its impact has been reduced by a novel neutron veto detector installed inside the water tank around the xenon cryostat, which allows for recognition and removal of those neutron events that can otherwise mimic the WIMP signature. The XENONnT detector is so sensitive to rare interactions that even neutrinos, the most elusive particles known so far, have to be considered in the background model.

"With this result, XENONnT strengthens previous constraints already with a first short exposure," said Professor Uwe Oberlack, who is making important contributions with his Mainz research group, especially in the vetodetectors and the calibration for the XENON experiments. "The PhD thesis of Daniel Wenz provides a special contribution to the measurement of the efficiency of the neutron veto and the first calibration of the nuclear recoils in the TPC with a coincident gamma signal in the neutron veto. He has thus contributed in a special way to the present result. XENONnT is now collecting more data with improved detector conditions and an even lower background level due to a further improvement of the radon control and online removal system, aiming for an increased WIMP sensitivity over the following years."