The discovery of dark matter is of fundamental importance to cosmology, astrophysics, and particle physics. Despite the extreme challenges, the search for Weakly Interacting Massive Particles (WIMPs) believed to constitute the dark matter, continues with accelerator, space and underground based experiments. Ultimately, only astrophysical observations can determine whether WIMPs exist in nature, but the importance of direct detection of WIMP-nucleus scattering in laboratory experiments is well recognized. The US is in a world-leading position in direct detection; however, better-sensitivity experiments with sizeable increase in target mass and sophisticated background rejection schemes are key to future progress and discovery.
This award provides support to the Columbia and Rice groups for their continuation of the XENON Dark Matter Program, with the construction of a 100 kg fiducial mass detector (XENON100). The sensitivity goal is 2x10^45 cm^2. This is achieved through background discrimination provided by the simultaneous detection of ionization and scintillation signals in pure liquid xenon. XENON100 is built on the design and technology established with the XENON10 prototype, with a factor of ~10 increase in total Xe mass. XENON100 is an experiment that can be built with minimum risk, giving first physics results by 2009, when the LHC will be searching for Supersymmetric dark matter.
Among the Broader Impacts of this work, the XENON science has all the ingredients to captivate the interest and imagination of students and the general public alike. XENON technical-related work can impact society in a number of ways. The liquid xenon imaging detector technology is a prime candidate for applications in several fields outside particle astrophysics, including national security and medical imaging research.
Most of the matter in the Universe is actually not in the form of known atoms, but is made from some new, unknown kind of matter. The nature of this so-called Dark Matter is one of the greatest outstanding questions of contemporary physics. As we fly with the Solar System through the Milky Way, we constantly plow through a halo of this Dark Matter. Thus there is a constant head-wind of Dark Matter particles that penetrate everything and everybody on Earth, much as it is the case for better known particles such as, for example, neutrinos and cosmic rays. One way to try to unravel what Dark Matter is made of is then to try to detect some of these particles as they pass through a detector. This proposal provided the funds to build the XENON100 detector, the currently most sensitive detector to search for such Dark Matter particles. XENON100 uses ultra-pure liquid xenon in a cryostat made from carefully selected materials to reduce the radioactive background from known particles. It is located about a mile deep underground in the Laboratori Nazionali del Gran Sasso in central Italy to shield it from cosmic radiation, and behind a thick shield from polyethylene, lead and other materials to shield other ambient radioactivity. The liquid xenon is equipped with photomultipliers in a so-called Time Projection Chamber so that each particle interaction can be localized in three dimensions. This helps to further reduce radioactive backgrounds, which are mostly located near the surfaces, from Dark Matter interactions, which are expected to occur throughout the target volume. Furthermore, radioactive beta- and gamma-radiation can be distinguished from neutrons or Dark Matter events based on the light and charge signal that is left by each interaction. With their detector, the international XENON100 collaboration was able to set the most stringent constraints on the properties of Dark matter particles ever obtained. Although no Dark Matter was found by the time of writing, the detector continues to take data to search for these elusive particles.
