Dark matter is still a big mystery for scientists; it's hard to observe. But we have come a step further: the XENON collaboration has presented the first results of a 30-day test with the XENON1T, the most sensitive dark matter detector ever. And those results are promising.
Exist
Dark matter is one of the basic constituents of the universe and is five times more abundant than ordinary matter composed of atoms (stars, planets and other visible material). However, as much as 95% is made up of dark energy (68%) and dark matter (27%). Dark matter does not produce and reflect light: therefore, it cannot be seen directly. Nevertheless, several astronomical measurements have demonstrated the existence of dark matter. The search for the zcithability of dark matter has therefore been going on for decades.
Several studies have already established with considerable certainty that dark matter contains the Weakly Interacting Massive Particle (WIMP), a heavy particle that hardly collides with other particles. This may be one reason why collision experiments on Earth have not yet yielded any information about dark matter. The interaction with other particles is simply too weak. Initiatives are therefore under way worldwide with the aim of developing ever more sensitive detectors. The XENON1T is the best example of this to date.
Underground
The experiment takes place in a cryostat vessel, according to the website of the Dutch company Nikhef, which is collaborating on the XENON1T. The cryostat ensures that the xenon is kept at a temperature of -95 °C without freezing the surrounding water. It contains 3.5 tonnes of liquid xenon and detectors to observe the light signals. A water tank has been built around it to shield the experiment from background radiation as much as possible. The setup is underground; it was built in a mountain near the Gran Sasso National Laboratory in Italy. The presence of the mountain ensures that as few cosmic rays as possible interfere with the experiment. But shielding from the outside world alone is not enough as all materials on Earth contain small traces of natural radioactivity. The materials the detector itself is made of have been very carefully selected to achieve the lowest possible radioactivity. Particle interactions in liquid xenon lead to flashes of light that the XENON scientists measure and study. The flashes of light provide information about its position and energy and are used to determine whether it comes from a dark-matter particle or not. The detector has now been tested for 30 days, with an interruption due to an earthquake.
Dutch contribution
The results of that test show that this detector set a record with the lowest radioactivity level. Moreover, the XENON1T (with a total mass of about 3200kg) is the largest detector of this type ever built. The combination between the detector's increased size and low background radiation offers great potential for the discovery of dark material in the coming years.
The partnership consists of 135 scientists from America, Germany, Italy, Switzerland, Portugal, France, Israel, Sweden, the United Arab Emirates and the Netherlands. The Dutch company Nikhef is collaborating in the project: researchers and engineers from Nikhef designed the cryostat vessel, as well as the support and compensation structure used to attach the vessel. Furthermore, they developed the algorithms to determine which signals are kept in the detector, and wrote the software to make the electronic signals suitable for further analysis. Prof Patrick Decowski, programme leader Dark Matter at Nikhef, says in a press release, "With this short run, we have been able to demonstrate that XENON1T is the most sensitive detector. With this, we have set the best limit worldwide on dark-matter interactions."
By: Kelly Bakker
Source: Nikhef/XENON
