Direct Dark Matter Search with the XENON100 Experiment
Doctor of Philosophy
Dark matter, a non-luminous, non-baryonic matter, is thought to constitute 23 % of the matter-energy components in the universe today. Except for its gravitational effects, the existence of dark matter has never been confirmed by any other means and its nature remains unknown. If a hypothetical Weakly Interacting Massive Particle (WIMP) were in thermal equilibrium in the early universe, it could have a relic abundance close to that of dark matter today, which provides a promising particle candidate of dark matter. Minimal Super-Symmetric extensions to the standard model predicts a stable particle with mass in the range 10 GeV/c 2 to 1000 GeV/c 2 , and spin-independent cross-section with ordinary matter nucleon σ x ∠ 1 × 10 -43 cm 2 . The XENON100 experiment deploys a Dual Phase Liquid Xenon Time Projection Chamber (LXeTPC) of 62 kg liquid xenon as its sensitive volume, to detect scintillation ( S1 ) and ionization ( S2 ) signals from WIMP dark matter particles directly scattering off xenon nuclei. The detector is located underground at Laboratori Nazionali del Gran Sasso (LNGS) in central Italy. 1.4 km of rock (3.7 km water equivalent) reduces the cosmic muon background by a factor of 10 6 . The event-by-event 3D positioning capability of TPC allows volume fiducialization. With the self-shielding power of liquid xenon, as well as a 99 kg liquid xenon active veto, the electromagnetic radiation background is greatly suppressed. By utilizing the difference of ( S2/S1 ) between electronic recoil and nuclear recoil, the expected WIMP signature, a small nuclear recoil energy deposition, could be discriminated from electronic recoil background with high efficiency. XENON100 achieved the lowest background rate (∠ 2.2 × 10 -2 events/kg/day/keV) in the dark matter search region (∠ 40 keV) among all direct dark matter detectors. With 11.2 days of data, XENON100 already sets the world's best spin-independent WIMP-nucleon cross-section limit of 2.7 × 10 -44 cm 2 at WIMP mass 50 GeV/c 2 . With 100.9 days of data, XENON100 excludes WIMP-nucleon cross-section above 7.0 × 10 -45 cm 2 for a WIMP mass of 50 GeV/c 2 at 90% confidence level.