In the beginning of November 2014, The Oskar Klein Centre officially joined the XENON dark matter project. The idea is to detect dark matter particles scattering of heavy nuclei.
Since one of the strongest limitation of dark matter detection is due to cosmic ray induced background, it is important to shield the detectors. For this reason XENON is situated in the Gran Sasso National Laboratory, on the side of a ten kilometer long freeway tunnel crossing the Gran Sasso mountain, about 120 km from Rome. Thus, there is about 1500 meters of rock protecting the laboratory from cosmic ray backgrounds.
The particular approach of the XENON project is to use XENON noble gas in liquid and gaseous phase. The noble element xenon has the right mass to be optimally sensitive to the heavy WIMP (Weakly Interacting Massive Particle) dark matter which constitute the current paradigm for dark matter particles. Interactions of WIMPs in the gaseous phase xenon yield scintillation light and electrons, which are drifted to the liquid phase xenon, where they are extracted to give a ionisation signal.
Signals read-out by arrays of photomultiplier tubes on the top and bottom of the detector volume, can be used to reject background coming from recoils of particles on the electrons of the xenon atom. Radioactive impurities in xenon and the light sensors are the main trouble makers. The collaboration has therefore developed new techniques for screening and purifiying all materials involved in the experiment.
Less than 100 kg of XENON were enough to provide the most stringent limit on WIMP existence so far, only superseeded recently by results of the LUX collaboration, employing the same principle. We are now in the process of building the next generation of this experiment, with 1 tonne of ultra-pure Xenon.
In the end of November, I was in Gran Sasso for my first collaboration meeting. It was a lot of fun, the collaboration consists of very dedicated and highly competent people. Discussions were vivid and boisterous at times, but always constructive and in a cordial astmosphere.
Right now I am the only person involved from Stockholm, thanks to a grant by the Wallenberg foundation, but from new year we welcome Alfredo Ferella, who has long experience in the direct detection business and will recruit a postdoc and a graduate student. At first, we are going to build a data analysis and calibration center in Stockholm and will meet with representatives of the Swedish National Infrastructure for Computing (SNIC) very soon. We are also very eager to get our hands on data to try out some new methods and prepare for the next phase. Later-on, we will get more involved in instrumentation, focusing on the behaviour of special photosensors with low radioactivity at low temperatures. Together with colleagues at Chalmers University in Gothenburg, we will also be working on the theoretical ingredients necessary to turn the experimental measurements into solid knowledge about WIMPs. The spokesperson of XENON, Elena Aprile from Columbia University, made quite clear what the goal is: “Now, I hope we will discover the damn thing”.