01 Dec XENON1T: the next generation dark matter detector
On the 11th of the 11th, while kids in the Netherlands were swarming the streets with their lanterns, singing songs for candy (the Dutch Halloween), somewhere deep down in a mountain in Italy another party was taking place. The occasion was the inauguration of XENON1T, an experiment designed to be the world’s most sensitive direct dark matter detector. What will this brand new machinery be used for? And how did the Science-Park-Amsterdam-based research institute Nikhef contribute? Read on to find out!
Scientists might be a bit embarrassed to admit this, but only 4% of the Universe exists of matter we know; 73% is the unknown dark energy and 23% is dark matter. This is what XENON1T will be looking for. All we know about this mysterious matter is that it doesn’t reflect or emit light, because we cannot see it. But how can we be so sure it exists if we haven’t observed it (yet)?
Well, the dark substance is needed to explain the movement of stars and galaxies. These astronomical objects move in ways which cannot be described by combining the known gravitational laws with the observed amount of matter in our Universe. This can mean two things: either our laws of gravity are wrong (sorry Einstein) or there is stuff floating around that we cannot see. Changing fundamental laws of nature is always a bit tricky; therefore a lot of physicists hope the answer can be found in the hypothesized dark matter particles: WIMPs (Weakly Interacting Massive Particles). As a nice bonus, introducing WIMPs also solves several other problems in physics.
‘We expect that every second about one hundred thousand dark matter particles pass through an area the size of a thumbnail,’ says Patrick Decowski, professor at the UvA Institute of Physics and programme leader of the Dark Matter research group at Nikhef. So why haven’t we found them yet? Unfortunately, these heavy particles are theorized to only interact through the weak force, meaning they rarely interact with other particles. Because of this anti-social nature it is very difficult to detect them.
Detecting the invisible
A tough challenge never stopped physicists from trying and several detectors were built to look for the mysterious dark matter. Because WIMPs rarely collide with matter we know, the detectors need a material with a high atomic number and high density to increase the chance of an interaction. A suitable substance is liquid xenon. When a dark matter particle collides with a xenon nucleus, a flash of light is created which can be detected. It should be noted that using xenon comes with a price: 1kg of the liquid costs over a 1000 euros!
One of the research groups lucky enough to work with this liquid is the XENON-project. Their first, XENON10, was built in 2005 and contained 15kg of the high density liquid. It was followed up by XENON100 (165 kg liquid) in 2008. Neither of these experiments nor any other detector has found WIMPs. XENON1T, the third generation in the XENON-project, joined the game on 11 November 2015. This new kid on the block contains over 3500 kg of xenon, increasing its chances to see a glimpse of the missing 23% of our Universe. So, the bigger (and heavier), the better!
Now take a look at the impressive set-up of the experiment. As you can see in the picture on the right, the detector consists of a vessel filled with liquid xenon. Light detectors are placed on the top and bottom to detect the light which can be created when a WIMP impolitely bumps into a xenon atom. To make sure the flash is caused by the exotic dark matter and not by cosmic rays or other boring ordinary matter the vessel is placed 1400 meters underground and is also shielded by water in a tank with a 10 meter diameter. Most of the radiation we know will be stopped by all these filters and (almost) only the WIMPs will happily fly through and reach the xenon. There they will, hopefully, leave a signal to let us know they exist.
Why is XENON1T such a big improvement? Well, it doesn’t only have better shielding and more xenon than its predecessors, the liquid and all the other materials are also cleaner (resulting in less background radiation) and the light detectors are better. These improvements mean that it will not just be the most sensitive dark matter detector in the world, it is also a few hundred times more sensitive than all the other experiments.
Did you know that this awesome project is partly made possible by the Faculty of Science’s neighbor institute? The XENON collaboration consists of twenty research groups from ten different countries of which Nikhef at Amsterdam Science Park is one. The enthusiastic physicists from Nikhef are responsible for, among other things, the vibration-free suspension of the xenon vessel, as well as the data acquisition and data analysis software. So, there will be plenty to do for the Science Park based scientists when the first data from XENON1T arrives in the beginning of 2016. We will keep you posted about the results!
Questions about dark matter or XENON1T? Or do you suggest another approach than looking for WIMPs to solve this problem of missing mass? Let us know in the comments below.