XENON Dark Matter Search Experiment

Expected Signal

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, provided a low recoil energy threshold, as shown in the above figure. The expected event rate, integrated above the energy threshold, is calculated for Xe, Ge and Si, assuming a 100 GeV WIMP with a cross-section = 3.6E-42 cm2. As detector material LXe has excellent ionization and scintillation properties. With the simultaneous measurement of charge and light and 3D position resolution, event information can be maximized to achieve effective and redundant background identification and discrimination power, while maintaining most of the target active. 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.

Most events will provide three signals, with a timing scheme shown schematically in above Figure. The first is the prompt scintillation signal detected directly by the PMTs. The last is the proportional scintillation signal from the CsI photoelectrons drifting the entire 30 cm liquid gap. These two signals are separated by exactly 150 microseconds, i.e. the maximum drift time. The proportional scintillation signal from the drift of ionization electrons can occur anywhere in between these two. The difference in arrival time between the primary scintillation pulse and the proportional pulse from the electron drift measures the interaction depth (Z-coordinate) of the event. Since electron diffusion in LXe is small, the proportional scintillation pulse is produced in a small spot with the same X-Y coordinates as the interaction site. The photons in the proportional scintillation will spread over several PMTs in the vicinity of this spot. By applying a position reconstruction algorithm, the X-Y position can be reconstructed to about 1 cm precision. The X-Y information, along with the absolute Z, gives a 3D localization which will permit further background discrimination via fiducial volume cuts. For events where the rest pulse is below detection threshold, the third pulse will always be present since the CsI has high quantum efficiency and the signal is amplified by proportional scintillation. The Z-coordinate can still be inferred from the relative drift time difference, since the third pulse contains the same information as the first. In such events with incomplete signatures, the sum signal from the wire structure will be present only if a proportional scintillation pulse is detected. Based on the redundant information in our design and the 3D position sensitivity, we expect to achieve a background rejection efficiency better than 99.5% and energy threshold as low as few keV.