UvA-physicists present promising results of dark matter detector XENON1T
A group of international researchers has presented the first results of research that is conducted with XENON1T, the most sensitive dark matter detector in the world. Several UvA- and Nikhef-scientists are involved in the experiment. Among them are Patrick Decowski and PhD students Jelle Aalbers, Sander Breur and Erik Hogenbirk.
Corresponding author of the paper with the first results is Jelle Aalbers from the Center of Excellence for Gravitation and Astroparticle Physics Amsterdam (GRAPPA). Aalbers, a GRAPPA PhD Fellow, made important contributions to the analysis of the new results. Patrick Decowski (UvA-GRAPPA), Auke-Pieter Colijn (UvA-IHEF), UvA PhD students Sander Breur and Erik Hogenbirk, and Nikhef postdoc Andrew Brown are also involved in the project. The researchers are enthusiastic about their short 30-day measurement: 'The best result on dark matter so far! … and we have just started!'
Prof. Patrick Decowski: 'With this short run, we have been able to show that XENON1T is the most sensitive detector. In doing so, we put the strictest limit worldwide on dark matter interactions. It is great to see that our three PhD students played a crucial role in analyzing the results.'
The run was cut short because of an earthquake close to the experiment on 18 January 2017. Two weeks later, the experiment could be resumed. 'The XENON1T detector that we built over the past five years, does exactly what it is supposed to do. The earthquake provided some tense moments, but now we are continuously collecting data again. Of course, we are hoping for a future discovery', says prof. Auke-Pieter Colijn.
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 world-wide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties.
However, these interactions are so feeble that they have escaped direct detection up to this point, forcing scientists to build detectors that are more and more sensitive. The XENON Collaboration, having led the field with the smaller XENON100 detector for years in the past, 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 the surrounding materials on Earth. With a total mass of about 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 provides an excellent discovery potential in the years to come.
The XENON Collaboration consists of 135 researchers from the Netherlands, US, Germany, Italy, Switzerland, Portugal, France, Israel, Sweden and the United Arab Emirates. The latest detector of the XENON family has been in science operation at the INFN Laboratori Nazionali del Gran Sasso underground laboratory (Italy) since autumn 2016.
The only things you see when visiting the underground experimental site now are a gigantic cylindrical metal tank, filled with ultra-pure water to shield the detector at its center, and a three-story-tall, transparent building crowded with equipment to keep the detector running. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), is not visible. It sits within a cryostat in the middle of the water tank, fully submersed, in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat allows keeping the xenon at a temperature of -95°C without freezing the surrounding water.
The mountain above the laboratory further shields the detector, preventing perturbations from cosmic rays. But shielding from the outer world is not enough since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content.
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 particle and determining whether it might be dark matter or not. The spatial information allows selection of interactions occurring in the central one ton core of the detector. The surrounding xenon further shields the core xenon target from all materials which already have tiny surviving radioactive contaminants. Despite the shortness of the 30-day science run, the sensitivity of XENON1T has already overcome that of any other experiment in the field, probing un-explored dark matter territory.
'WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon!', 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 with ultra-low background massive detectors on Earth has just begun with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.'