Scientists of the XENON-collaboration, which involves several UvA physicists, have presented new results of the most sensitive dark matter experiment in the world, eight times more sensitive than similar earlier experiments. The new results are based on an unprecedented number of measurements, and agree with the expectation of only background noise in the detector.
The XENON1T detector, filled with liquid xenon, is located deep underground in the Laboratori Nazionali del Gran Sasso in Italy. Here, researchers search for dark matter in the form of WIMPs (Weakly interacting Massive Particles). WIMPs form a category of dark matter particles that scientists search for with experiments such as XENON1T, but also at the Large Hadron Collider (LHC) at CERN in Geneva, as well as in space. Scientists all over the world have been eagerly awaiting the XENON1T results. The fact that this measurement did not produce any indications for WIMPs will form a basis for further theoretical and experimental research.
Patrick Decowski, program leader of the Dark Matter research group at Nikhef and professor at the University of Amsterdam: 'With these results, XENON1T sets a new limit on the possible interactions between WIMPs and ordinary matter. The experiment itself is performing great, but possibly WIMPs have an even smaller chance to collide with ordinary matter than we already thought. Another possibility is that dark matter consists of a different type of subatomic particle. With XENON1T, we are looking for those particles as well.'
The XENON1T results are based on a 279 day measurement with no less than 1300 kilograms of liquid xenon . The experiments has achieved the lowest background noise ever in this type of experiment.
Auke-Pieter Colijn, senior researcher at Nikhef as well as the University of Amsterdam and Utrecht University: 'Of course, we hoped to find dark matter particles, but they are hard to catch. One year from now, we will have an even better detector, ten times more sensitive. Then, hopefuly, we'll be able to outsmart nature after all.'
Dark matter is one of the basic components of the universe. There exists five times as much dark matter as ordinary matter. Many astronomical measurements have confirmed the existence of dark matter. Several research groups worldwide attempt to observe collisions of dark matter particles with ordinary matter, using extremely sensitive detectors. Observing these collisions would not only confirm the existence of dark matter particles directly, but would also allow researchers to measure important properties. It is expected that roughly a billion dark matter particles pass through each of us every second. However, the interactions between dark matter and ordinary matter are so weak that they have so far escaped direct detection. This put scientists on a quest to build increasingly sensitive detectors.
XENON1T is located in the Laboratori Nazionali del Gran Sasso in Italy, the largest underground laboratory in the world. The central XENON1T detector is a cylindrical container filled with liquid xenon. It is embedded in a cryostate in the middle of a 10 meters wide and high water tank, used to protect the experiment against natural radioactivity as much as possible. The cryostate keeps the xenon at a temperature of -95 °C. The mountain on top of the laboratory further protects the detector against disturbances coming from cosmic rays. A collision between dark matter and the xenon would result in a tiny flash of light. These light flashes provide information about the location and the energy of the particle that collided; they are used to determine whether the flash came from a dark matter particle or not.
XENON1T became operational at the end of 2015, and is four orders of magnitude more sensitive that XENON10. The latter detector was the first dark matter experiment built by the XENON collaboration in 2005. The higher sensitivity of XENON1T was acieved by increasing the amount of liquid xenon for the dark matter analysis from 5 kilograms to 1300 kilograms, and to reduce the background noise coming from natural radioactivity by a factor of 5000.
XENON1T will keep collecting data with high sensitivity and will keep searching for dark matter particles until the experiment will be upgraded to XENONnT. With four times the amount of xenon and ten times less background noise, XENONnT will continue the search for dark matter particles in 2019.