Improving background discrimination in the LZ dark matter detector
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Astrophysics | Astroparticle Physics
Claudio Frederico Pascoal da Silva
Universidade de Coimbra
Laboratório de Instrumentação e Física Experimental de Partículas
According to the modern cosmological models, the dark matter accounts for about 85% of the matter of the Universe while its nature is one of the major current scientific questions. Among approximately 20 dark matter direct search experiments worldwide, that are looking for the evidence of galactic dark matter in the form of Weakly Interacting Massive Particles (WIMPs), the LUX (Large Underground Xenon) experiment is the most sensitive up to date. The detector is currently running 1500 meters underground at Sanford Underground Research Facility (SURF), South Dakota, USA. It is installed in the place formerly occupied by the Davis experiment that discovered solar neutrinos back in 1970s. The successor of LUX, the LZ experiment, is expected to improve the current limit on WIMP-nucleon cross section by at least two orders of magnitude. LZ is currently in the design stage, which will be followed by the construction, assembly and deployment (2016-2018), commissioning is scheduled for 2017, with the science run occurring in 2019-2021. LZ collaboration includes more than 200 scientists from USA, UK, Portugal, Russia and Korea. Among the member institutions are several national laboratories (SLAC, Fermilab, Lawrence Livermore, Lawrence Berkeley and RAL) as well as world-renowned Universities (UC Berkeley, UC Davis, Texas A&M, Imperial College London, Oxford University, Edinburgh University, etc.) One of the main challenges of the WIMP search is extremely low expected rate of the events of interest (less than one event per kg*year). These rare events must be discriminated from the background events produced by ionizing radiation passing through the detector. Even the very low background that will be achieved in LZ by careful selection of radio-pure construction materials is by several orders of magnitude higher than expected rate of WIMP interactions. The background discrimination is possible due to the fact that a WIMP interacting in the LZ detector is expected to produce a recoil of the xenon nucleus while most of the background appear in the form of electron recoil events. These different types of recoils can be differentiated by the ratio between ionization charge and scintillation light produced by the recoil. Using this technique, the LUX detector achieves background suppression factor of about 1000. The proposed work will focus on increasing this suppression factor. For this the following directions of research are envisioned: (i) improve the current signal processing and analysis in order to reduce errors in the estimation of the light/charge ratio; (ii) include in the analysis the temporal structure of the scintillation pulse (pulse shape); (iii) perform thorough calibration of the detector aiming to create the most adequate physics model of liquid xenon response to nuclear and electron recoils.