XENON Dark Matter Search Experiment
XENON is a next-generation Dark Matter Direct Detection experiment that will use liquid xenon as a sensitive detector medium to search for WIMPs (Weakly Interacting Massive Particles), the leading candidates for dark matter in the universe. These particles, postulated by Supersymmetry (SUSY), are believed to have a mass in the neighborhood of 100 GeV, to ignore electromagntic interactions, and to permeate the Universe. XENON plans to probe a substantial fraction of SUSY parameter space, with a sensitivity of 1 event/100kg/year.
A schematic representation of the XENON design is shown above. An array of 10 such position-sensitive liquid xenon time projection chambers (LXe TPCs) will make the detector a tonne-scale experiment. Each TPC will contain 100 kg of active xenon mass and be self-shielded with additional LXe.
Technique
As shown above, the LXeTPC module containing the active xenon target is formed by a sandwich of Teflon spacers (UV diffuse reflector) and copper rings for electric field shaping. The structure is closed at the bottom by a copper plate, which is coated with CsI to convert Xe scintillation photons into free electrons. The light collection of the detector is optimized in order to see the tiny signal produced by WIMPs in liquid xenon.
On the top, the structure is hermetically sealed with a cylindrical copper vessel of larger diameter, housing the PMTs and the wire structure for the proportional scintillation process in the gas phase. The LXeTPC structure is enclosed in a copper vessel containing the liquid xenon for active shielding. With both detectors at the same temperature and similar pressure, the amount of material for the inner detector walls is minimized.
Xe recoils (from WIMPs and neutrons) and electron recoils (from background gammas and betas) have different recombination rate for their ionization in liquid xenon. This provides excellent background rejection based on the ionization/scintillation ratio.
Sensitivity
Liquid xenon is an attractive target for a sensitive WIMP search. Its high density (~3 g/cm3) and high atomic number (Z = 54, A = 131) allow for a compact detector geometry. The high mass of the Xe nucleus is favorable for WIMP scalar interactions, providing that a low recoil energy threshold is achieved (less than 20 keV). As detector material LXe has excellent ionization and scintillation properties. With the simultaneous measurement of charge and light with 3D position resolution, event information can be maximized to achieve effective and redundant background identification and discrimination power. Xenon, which contains both odd and even isotopes for coherent and purely spin-dependent WIMP interactions, is available in large quantities at reasonable cost. Various techniques have demonstrated ultra pure LXe in which an electron lifetime in excess of 1 ms allows the drift of free electrons over 30 cm and longer. The reduction of the krypton contamination in natural xenon to the required part per billion (ppb) level has also been verified with a distillation tower and cold traps.
The XENON experiment has finished a three-year R&D program, demonstrating the capability of its proposed technique for WIMP dark matter detection. The first detector (XENON10) with 15 kg of active LXe target is currently (as of March 2006) under installation at the Gran Sasso Underground Laboratory in Italy. Successful operation of XENON10 at the underground lab for one month will allow reaching a SUSY parameter space down to 2 x 10-44 cm2 (the dark green dotted line in the above plot), a factor of ten times lower than the current lowest limit reached by the Cryogenic Dark Matter Search (CDMS) experiment (the red line in the above plot). The experience and technique from the design and opeartion of XENON10 detector will pave the way for a larger scale detector (XENON100), to probe more SUSY parameter space (shaded area in the plot) and perhaps to finally detect positive signals from WIMPs and to study their properties.
XENON @ Yale
Currently, more than 30 researchers from nine institutions are participating in the XENON project (see XENON Collaboration page). The Yale group is actively involved in data analysis, simulation, 3D position reconstruction and photomultilplier calibration for the current detector, and various R&D projects for a future larger scale xenon detector. The Yale group is also responsible for the XENON slow control system, to monitor the stability and to ensure the safety of detector operation.
The figure above is the nuclear recoil scintillation efficiency, relative to that from electron recoils with the same energy, measured by the Yale group and collaborators at Columbia Univeristy. Our results resolved inconsistencies from previous measurements and filled up the empty values at low energy recoils, the most interesting energy range for dark matter detection using liquid xenon. This work has been published as Phys. Rev. D. 72 (2005) 072006.
The following two pictures show the liquid xenon test facility at Yale Sloane Physics Lab. They show a xenon gas purification system (above) and a cryogenic refrigerator with the LXe cell (below). Photomultiplier tubes (PMTs) are installed in the cell to detect the scintillation light. The facility has been running for different R&D projects for the XENON experiment.
