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EDELWEISS-II, direct Dark Matter search experiment, first data analysis and results

Abstract : One of the greatest mysteries of the universe that, for the present, puzzles the mind of most astronomers, cosmologists and physicists is the question: "What makes up our universe?". This is due to how a certain substance named Dark Matter came under speculation. It is believed this enigmatic substance, of type unknown, accounts for almost three-quarters of the cosmos within the universe, could be the answer to several questions raised by the models of the expanding universe astronomers have created, and even decide the fate of the expansion of the universe. There is strong observational evidence for the dominance of non-baryonic Dark Matter (DM) over baryonic matter in the universe. Such evidence comes from many independent observations over different length scales. The most stringent constraint on the abundance of DM comes from the analysis of the Cosmic Microwave Background (CMB) anisotropies. In particular, the WMAP (Wilkinson Microwave Anisotropy Probe) experiment restricts the abundance of matter and the abundance of baryonic matter in good agreement with predictions from Big Bang Nucleosynthesis. It is commonly believed that such a non-baryonic component could consist of new, as yet undiscovered, particles, usually referred to as WIMPs (Weakly Interacting Massive Particles). Some extensions of the standard model (SM) of particle physics predict the existence of particles that would be excellent DM candidates. In particular great attention has been dedicated to candidates arising in supersymmetric theories: the Lightest Supersymmetric Particle (LSP). In the most supersymmetric scenarios, the so-called neutralino seems to be a natural candidate, being stable in theories with conservation of R-parity and having masses and cross sections of typical WIMPs. The EDELWEISS collaboration is a direct dark matter search experiment, aiming to detect directly a WIMP interaction in a target material, high purity germanium crystal working at cryogenic temperatures. It relies in the measurement of nuclear recoils that produce measurable effects in the crystal such ionization and heat. My PhD thesis is organized as follows. The first chapter aims to provide an introduction to the theoretical framework and the scientific motivation for the following work. The nature of DM has been one of the most challenging topics in contemporary physics since the first evidences of its existence had been found in the 1930s. Cosmologists and astrophysicists on one side, together with particle theorists on the other have put a lot of effort into this field: I will briefly account for their achievements and for the experimental strategies which can be set in this scenario. Since this thesis work was carried out within the EDELWEISS-II direct dark matter experiment, I will focus the next chapter on this topic, describing the main features. The second chapter is related to the set-up of the EDELWEISS-II, the current stage of the EDELWEISS experiment necessary after a first phase that achieved the best upper limit on the WIMP elastic scattering on nucleon as a function of WIMP mass in 2004. The set-up was conceived to reduce radioactive background observed in the first experiment phase. Thus, describing the starting point for this second stage, I will present detectors involved in, with a peculiar regard to the Ge-NTD type, the same implied in EDELWEISS-I, on which I have focused my thesis work. In the third chapter the performed Ge-NTD analysis chain is presented. Starting with the signal processing of the recorded data, I will enter in the essential analysis steps from calibration signals passing through measurements of thresholds and resolutions in order to predict nuclear and electronic recoil band and definition of fiducial zone to conclude determining a selection for likely WIMP candidate. These suggestions are applied in the fourth chapter, which presents the analysis and the results of the 8th cool down that takes places from November 2007 to March 2008. This cool down allows a first real look at the EDELWEISS-II environment and it represents a first real test of Ge-NTD type detectors at large scale. Thus, it follows that two goals are envisaged: a better understanding of radiative background overwhelming the experiment and an improvement of current upper limit on the WIMP scattering cross section.
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Submitted on : Thursday, June 10, 2010 - 11:32:41 AM
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Siliva Scorza. EDELWEISS-II, direct Dark Matter search experiment, first data analysis and results. High Energy Physics - Experiment [hep-ex]. Université Claude Bernard - Lyon I, 2009. English. ⟨tel-00490990⟩

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