Studies of calibration and electron recoil background modelling for the XENON100 dark matter experiment
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Abstract
According to cosmological studies, there are hints for the existence of Dark Matter. Latest results from the Planck satellite mission suggest that our universe consist of 4.9% baryonic matter, 26.8% Dark Matter and68.3% Dark Energy. Many theories, such as a super symmetric extension (SUSY) of the standard model, provide a natural candidate for particle Dark Matter which has the following properties: It is charge-less and has a mass but its interaction probability with baryonic matter is small. This particle is summarised as WIMP, a weakly interacting massive particle.
In order to detect the WIMP, the XENON collaboration started in 2007 to build an experiment which is designed for direct Dark Matter detection. The experimental setup is located at Laboratori Nazionali del Gran Sasso (Italy). The detector is a two-phase Time Projection Chamber (TPC) filled with 161 kg of liquid xenon that aims at observing Dark Matter by looking at nuclear recoils produces by WIMPs scattering off xenon nucleons. The XENON collaboration archive the best limit in 2012 on the spin-independent WIMP-nucleon cross-section with a minimum of σχ = 2 × 10−45 cm2 at a WIMP mass of mχ = 55 GeV/c2 (90% C.L.). In 2013, using the same Dark Matter data, the XENON collaboration published a more stringent limit for the spin-
dependent cross-section minimum of σχ = 3.5 × 10−40 cm2 at a WIMP mass of m χ = 45 GeV/c2 (90% C.L.) for neutrons. Further Dark Matter candidates, such as axions or axion-like particles, are tested with electronic recoil data obtained during the same Dark Matter data taking period. This lead in 2014 to the best upper limit for axion-electron couplings in the 5 − 10 keV/c2 mass range, assuming that axion-like particles constitute all of the galactic Dark Matter.
Thanks to the underground laboratory and a careful material selection, the XENON100 experiment archived an ultra-low electro-magnetic background of 5.3 × 10 −3 events/kg/day in the region of interest. In order to identify a WIMP event properly, it is important to understand the background. Therefore a set of calibration sources is used: 241AmBe (Americium-Beryllium) as a neutron source and 60 Co (Cobalt) and 232 Th (Thorium) as a gamma source (electronic recoil). The neutron source is used to understand how a WIMP signal would look like in the detector and the gamma sources are used to understand the electronic recoil signal, similar to the background one. Weekly photo multiplier-tube calibrations are used, in addition, to determine the gain values and assure a stable read-out condition.
This work is dedicated to a subset of three data analysis topics. An alternative photo-multiplier tube calibration is developed from raw LED data. This method is able to support the actual calibration technique by searching for single photo-electron pulses with the same peak-finder algorithm which is used for Dark Matter data reprocessing. This allows to determine the single photo-electron response directly. An already used method is improved: The signal acceptance for the ionisation energy threshold is developed for the data taking periods 2011/12 and 2013/14. It is also tested with simulated neutrons and further uncertainties on the signal acceptance are discussed. Furthermore there is an alternative phenomenological electronic recoil background model developed from 60 Co and 232 Th data. This model is compared to the already published model in 2012 and the impact on the WIMP exclusion limit is evaluated.