Despite the short duration of the preliminary run, the sensitivity of XENON1T has already surpassed that of any other experiment in the field, probing unexplored dark matter territory.
“The best result on dark matter so far! … and we have only just started!”
This is how scientists behind XENON1T, currently the most sensitive dark matter experiment worldwide, commented on their first result from a short 30-day run, presented today to the scientific community.
Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a worldwide effort to directly observe interactions between dark matter particles with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties. However, these interactions are so weak that they have escaped direct detection up to this point, forcing scientists to build detectors with unprecedented sensitivity. The XENON Collaboration, which has been leading the field for years with XENON100, is now back on the frontline with XENON1T. The result from a first short 30-day run shows that this detector has a new record-low radioactivity level, many orders of magnitude below that of surrounding matter on Earth. With a total mass of around 3200 kg, XENON1T is, at the same time, the largest detector of this type ever built. The combination of significantly increased size with much lower background radioactivity levels implies an excellent discovery potential in the years to come.
The XENON Collaboration consists of 135 researchers from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and Abu Dhabi.
The latest detector of the XENON family has been in operation at the Gran Sasso Underground Laboratory (LNGS) since autumn 2016. The underground experimental site contains a gigantic cylindrical metal tank filled with ultra-pure water to shield the detector at its center, and a three-story transparent building teeming with equipment to keep the detector running. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), sits within a cryostat fully submersed in the middle of the water tank in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat enables the xenon to be kept at a temperature of -95°C without freezing the surrounding water. The mountain above the laboratory further shields the detector, preventing it from being perturbed by cosmic rays. But shielding it from the outer world is not enough since all matter on Earth contains tiny traces of natural radioactivity. Thus, extreme care was taken to find, select and process the materials that were used in the construction of the detector to achieve the lowest possible radioactive level. Prof. Laura Baudis of the University of Zürich and Prof. Manfred Lindner of the Max Planck Institute for Nuclear Physics in Heidelberg emphasize that this allowed XENON1T to achieve a record level of “silence,” necessary for the detector to be able to pick up the very weak “voice” of dark matter.
A particle interaction in liquid xenon leads to tiny flashes of light. This is what the XENON scientists are recording and studying to infer the position and the energy of the interacting particles and whether it might be dark matter or not.
The Israeli team, headed by Weizmann Institute of Science’s Dr. Ran Budnik, is responsible for the experiments’ control system, the active Muon Veto water shield surrounding the detector and the external calibration system. They also provided the statistical analysis and final results of this 30-day run. Budnik took over the helm from Weizmann colleagues Profs. Amos Breskin, Ehud Duchovni and Eilam Gross, who initiated the Israeli team’s involvement in the experiment from 2010-2014.
Despite the short duration of the preliminary run, the sensitivity of XENON1T has already surpassed that of any other experiment in the field, probing unexplored dark matter territory. “As we expected, WIMPs [weakly interacting massive particles] did not show up in this first search with XENON1T,” says Elena Aprile, Professor at Columbia University and spokesperson of the project. “The best news is that the experiment continues to accumulate excellent data, which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before. A new phase in the race to detect dark matter using massive detectors with ultra-low background radiation has just began with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”
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