Search for Dark Matter with PICO (PICASSO/COUPP)

点击翻译按钮获取中文摘要

The quest for the mysterious missing mass of the Universe has become one of the big challenges for today's particle physics and cosmology. Astronomical observations show that only 1% of the matter of the Universe is luminous and there is now convincing evidence that 85% of all gravitationally traceable matter in the Universe is of a new exotic kind, different from the "ordinary" matter surrounding us with its protons, neutrons and electrons. This so-called Cold Dark Matter (CDM) is thought to consist of so-called WIMPs (Weakly Interacting Massive Particles), neutral particles, about ten to thousand times heavier than a proton and which interact extremely weakly with each other and ordinary matter. The PICASSO* dark matter experiment is tuned to the search for WIMPs. It is based on liquid-to-vapour phase transitions which can be induced by WIMP interactions with nuclei in metastable superheated droplets. In its present phase the PICASSO collaboration has installed 32 detectors with a total target mass of 2.6 kg data in the SNOLAB underground laboratory at Sudbury, Ontario, Canada. and is continuously taken data. Results published in 2012 had placed PICASSO at the forefront of direct spin-dependent dark matter searches. A new physics analysis of an entire set of detectors is presently finalized and a substantial improvement with respect to our 2012 results is anticipated. The new analysis extends also to the spin-independent sector where the new PICASSO data start to probe the joint CoGENT/DAMA/CRESST annual modulation region. The powerful background discrimination features of the superheated liquid technique discovered by PICASSO promise “background blind” operation for up to tonne - year exposures. Therefore in December 2012 the two leading groups in the field of superheated liquids, PICASSO and COUPP joint forces within the new PICO collaboration to work together towards a 500 kg dark matter experiment at SNOLAB. With superheated C4F10 or C3F8 as the active target in a 250L bubble chamber, PICO-250L would focus on spin-dependent proton interactions, with an expected sensitivity at the level of 3 x 10-7 pb for one year of live data taking. In the spin-independent sector the sensitivity reach would be around 4 x 10-10 pb for WIMP masses around 20 GeVc-2, with an increased sensitivity down to masses of a few GeVc-2. PICO-250L meets the definition of a next generation dark matter experiment (G2) and no other G2 experiment would have comparable sensitivities in the sectors of spin-dependent interactions and light WIMP masses. At the moment background sources appear well under control; the engineering design of PICO 500 is progressing well. Work during FY14 / 15 will concentrate on construction, with commissioning foreseen in FY15/16. The Canadian PICASSO / PICO contingent consists of research groups of five institutions: University of Alberta, Laurentian University, Université de Montréal, Queen’s University and University of Toronto. In this grant request we ask for operating funds for the completion of the present phase of PICASSO, which should be concluded by summer 2013, and for operating funds to support our work in PICO-250L. *Project In Canada to Search for Supersymmetric Objects Projet d'Identification de CAnditats Supersymetriques Sombres