An improved signal model for a dual-phase xenon TPC using Bayesian inference and studies on the software trigger efficiency of the XENON1T DAQ system
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Abstract
Understanding nature has always been one of the most driving factors for mankind to invest huge amounts of effort and resources into research and development. The most fundamental questions have always been how the universe came to be and in what direction it will develop in the future. To answer these questions, we have to step back and first find the answer to the even more fundamental question: What exactly is our universe made of.
Recent experiments, like the Planck satellite mission, gave us already a sophisticated plan on what to expect of the composition of the universe. But even with today's advanced technology, where everybody carries around a powerful computer in his pocket in form of a smartphone, we are only able to grasp less than 5% of the whole: Ordinary baryonic matter. While studying these ~5% ordinary matter is still a very active field of research, there is already a rapidly growing community that tries to tackle the next question: What is the rest of the Universe made of and what is Dark Matter? With all the observational evidences being present, the science community has long accepted the fact, that there has to be a form of matter that has not been detected yet. It is also known, that it has a roughly 5 times higher abundance than ordinary baryonic matter. Discovering these next ~25% would be a major step towards a more substantial understanding of how our universe developed since its sudden appearance after a big bang roughly 14 billion years ago.
This thesis has been written while being part of the XENON Dark Matter search project. This collaboration of scientists is on the hunt for WIMP, one of the most promising candidates for Dark Matter, using time projection champers (TPCs) filled with liquid xenon (LXe). These ultra low background detectors are located in the underground facilities of the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The most recent generation is the XENON1T experiment, using a total 3.2t of xenon and being equipped with 248 photo multiplier tubes (PMTs) used for measuring particle interactions through flashes of light. Right now, the commissioning of the next generation experiment XENONnT is already ongoing. It will use even more xenon, will be equipped with additional PMTs and even a novel neutron veto.
This work has two focus topics: First, the data driven determination of the software trigger efficiency of the XENON1T data acquisition (DAQ) system. It has been an important cross-check of the performance and adjustment of the software part of the DAQ system. The trigger efficiency is a main factor, that greatly influences the sensitivity of the detector. If the detector is tuned to be too sensitive in the wrong range, e.g. too much noise from coincidental dark counts would be recorded. On the other hand will a badly tuned efficiency lead to many missed low energy events, which is the main energy region WIMP interactions are expected. The second part of this work deals with the development of an improved signal model and a more general introduction of the signal efficiency together with an extended spatial signal dependence. Dealing with these topics, this work tries to add new pieces to the puzzle and support the efforts of the XENON collaboration to solve the mysteries of Dark Matter.