There is five times more dark matter in the Universe than 'normal' matter, the atoms and molecules that make up all we know. Yet, it is still unknown what this dominant dark component actually is. Today, an international collaboration of scientists inaugurated the new XENON1T instrument designed to search for dark matter with unprecedented sensitivity, at the Gran Sasso Underground Laboratory of INFN in Italy. 

Dark matter is one of the basic ingredients of the Universe, and searches to detect it in laboratory-based experiments are being conducted since decades. However, until today dark matter has been observed only indirectly, via its gravitational interactions that govern the dynamics of the Cosmos at all length-scales. It is expected that dark matter is made of a new, stable elementary particle, which has escaped detection so far. About 100'000 dark matter particles are expected to pass through an area of 1 cm² per second. These were not yet directly detected, putting stringent constraints on their tiny interaction probability with the atoms of ordinary matter. This implies that more sensitive instruments are required to find the rare signature of the dark matter particle. The international XENON Collaboration, consisting of 21 research groups from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and Abu Dhabi celebrated the inauguration of their new XENON1T instrument today, which will search for dark matter with unprecedented sensitivity. 

The event took place at the Gran Sasso National Laboratory (INFN-LNGS), the largest underground laboratories in the world for astroparticle physics, where about 1400m of solid rock shield the detector from cosmic rays. The inauguration was attended by the XENON scientists along with guests from funding agencies as well as journalists and colleagues. About 80 visitors were able to join the ceremony directly at the experimental site in the 100m long, 20m wide and 18m high hall B of the LNGS laboratory. Here, the new XENON1T instrument is installed inside a 10m-diameter water shield to protect it from radioactive background radiation which originates from the environment. Even more guests were following the introductory presentations in the LNGS auditorium, where Elena Aprile, Professor at Columbia University (New York) and founder of the XENON project, illustrated the evolution of the XENON program from the early beginnings with a 3kg detector 15 years ago to the present-day instrument XENON1T with a total mass of 3500kg.

Fighting against radioactivity

XENON1T employs the ultra-pure noble gas xenon as dark matter detection material, cooled down to –95°C to make it liquid. The large-mass instrument features an extremely low radioactive background in order to be able to identify the rare events from a dark matter interaction. For this reason, the XENON scientists have carefully selected all materials used in the construction of the detector, ensuring that their intrinsic contamination with radioactive isotopes meet the low-background experiment's requirement. 

The XENON scientists measure tiny flashes of light and charge to reconstruct the position of the particle interaction within their detector, as well as the deposited energy and whether it might be induced by a dark matter or not. The light is observed by 248 sensitive photosensors, capable to detect even single photons. A vacuum-insulated double-wall cryostat, in some sense a gigantic version of a thermos flask, contains the cryogenic xenon and the dark matter detector. The xenon gas is cooled down and purified from impurities in the three-floors high XENON building, a fancy installation with a transparent glass facade right next to the water shield, which allows visitors to actually see what the scientist are doing inside. A gigantic stainless-steel sphere equipped with pipes and valves is installed on the ground floor. It can accommodate 7.6 tons of xenon in liquid and gaseous form, more than two times the capacity needed for XENON1T. This will allow the collaboration to swiftly increase the sensitivity of the experiment with a larger mass detector in the near future.

Aiming for a dark matter detection

Once fully operational, XENON1T will be the most sensitive dark matter experiment world-wide. The detector installation has been completed just a few days ago and first tests on its performance have been already started. First science results are expected for early 2016, as already one week of good data is sufficient to take the lead in the field again. The design goal of the experiment will be reached after two years of data taking, as the collaboration explains in a detailed sensitivity study published at the same time as the inauguration. The goal is the detection of the dark matter particle, however, even if there will be only found some hints after two years, the XENON collaboration is in an excellent position to move on as the next step of the project, XENONnT, is already being prepared.  It will largely use already existing infrastructures, improving the sensitivity to dark matter by another order of magnitude.

For further information and contacts: 

www.xenon1t.org                                 The XENON collaboration

www.lngs.infn.it                                    The underground laboratory
 

Photos:

1. HallB with illuminated building and water tank in the back

2. People in cleanroom gear working on the XENON1T TPC