Short answers, pulled from the story.
XENON is a deep underground detector facility at the Gran Sasso National Laboratory in Italy, designed to search for dark matter particles called WIMPs (weakly interacting massive particles) using liquid xenon as a target. The underground location provides 3,100 meters of water-equivalent shielding against cosmic ray backgrounds. The project is led by Elena Aprile, an Italian professor of physics at Columbia University.
The XENON detector uses a dual phase time projection chamber filled with liquid xenon. When a particle collides inside the liquid, it produces two signals: a prompt ultraviolet scintillation flash (S1) and a secondary electroluminescence flash (S2) created by electrons drifted into a gas layer above. The ratio of S2 to S1 distinguishes nuclear recoils (expected from WIMPs) from electronic recoils (background), suppressing electronic recoil backgrounds by more than 99% while retaining 50% of nuclear recoil events.
In June 2020, XENON1T reported 285 electron recoil events where only 232 were expected, a surplus of 53 events at 3.5 sigma significance. The three main candidate explanations were solar axions, an unexpectedly large neutrino magnetic moment, and tritium contamination. In July 2022, the successor detector XENONnT found no such excess in new data, discarding the anomaly.
In April 2019, the XENON Collaboration reported in Nature the first direct observation of two-neutrino double electron capture in xenon-124 nuclei. The measured half-life of this process is several orders of magnitude larger than the age of the universe, demonstrating the sensitivity of liquid xenon detectors to extremely rare events.
XENONnT contains more than 8 tonnes of xenon in total. This compares to 3.2 tonnes in XENON1T, 165 kilograms in XENON100, and just 15 kilograms in the original XENON10 prototype installed in 2006.
XENONnT published its first WIMP search results on the 28th of July 2023, excluding spin-independent cross sections above a set threshold at 28 GeV with 90% confidence. On the same date, the competing LZ experiment independently published its own first results, excluding cross sections above a comparable threshold at 36 GeV.