A Search for Neutrino-Less Double Beta Decay and Lepton Number Violation with the nEXO Experiment

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

Investigating the rarest of processes between elementary particles is a very powerful tool in understanding Nature at its most fundamental level. The Enriched Xenon Observatory (EXO) program investigates the fundamental nature of the neutrino, that intriguing elementary particle that for a long time was thought to have no mass, doesn't interact with much of anything, and may hold the key to why there is more matter than antimatter in the Universe. The EXO program carries out its study of the neutrino by searching for the existence of a nuclear decay called neutrino-less double beta decay. Beta decay involves the emission of an electron from the nucleus, and is always accompanied by the emission of an antineutrino. Neutrino-less double beta decay would be a process in which a nucleus (xenon-136 in this case) transforms by emitting two electrons (and nothing else). According to the Standard Model of particle physics, this can only happen if neutrinos and antineutrinos are exactly the same. Such a direct overlap between matter and antimatter could prove essential in answering one of the most fundamental questions in science: why is our Universe made exclusively of matter? The first experiment of the program, EXO-200, uses 200 kg of liquefied xenon enriched in the 136 isotope as a source and has operated at the underground Waste Isolation Pilot Plant in New Mexico since 2010. EXO-200 is set to run for another 3 years, and this award will support the PI and his group for up to three years. The accumulated data will allow scientists to probe the existence of neutrino-less double beta decay should this process happen with a characteristic half-life of up four million billion times the current age of the universe. The detector technology used in EXO-200 pioneers the use of selected materials with ultra-low levels of residual radioactivity in order to produce the most radio-quiet environment for the detection of rare events. Some of these cleanliness protocols and requirements are now reaching the electronics and pharmaceutical industries. In addition, the large noble liquid detectors, such as the one used in EXO-200, are finding increased applications outside of particle physics, in particular in nuclear reactor fuel cycle and composition monitoring and in medical imaging (e.g. PET scan machines). The EXO-200 project is proving an ideal stage for graduate students to grow into researchers with both data analysis and hardware expertise. The UMass group has a track record of successfully involving a diverse population of undergraduate students in core areas of the project. The newly established Amherst Center for Fundamental Interactions provides a stimulating and productive environment for a broad scientific exchange between experimentalists and theorists. The UMass group plays a central role in fully exploiting the data from the EXO-200 experiment. The chosen xenon Time Projection Chamber (TPC) technology allows the project to take advantage of the radiation self-shielding properties of high-Z xenon, in a homogeneous layout with excellent event tracking capability and energy resolution. The work includes a comprehensive study of processes with spatial topology, such as bb decay of 136Xe to excited states of 136Ba, 137Xe background, and high energy gamma-rays originating from cosmic muon interactions with detector components. An integral part of the proposal is a laboratory program at UMass to test optical properties of various detector materials and surfaces in liquid xenon and to use the results from these measurements in a ray-tracing optical simulation of EXO-200. Together with focused tests of novel detectors for the Vacuum Ultra-Violet (VUV) scintillation light produced in xenon (silicon photo-multipliers in particular), they are part of investigations under way towards a tonne-scale, next generation experiment (nEXO) for neutrinoless double beta decay.