Analysis tools and thin film development for nEXO, XENON1T and the NAPOLEON concept device

Abstract

One, as of yet, undiscovered attribute of the neutrino is whether it is a Dirac type fermion (like all the known leptons) or if it is a Majorana fermion. One of the experimentally accessible processes to probe this is the search for a rare type of radioactive decay called neutrinoless double beta decay (0vbb). This form of decay is only possible if neutrinos are Majorana fermions, and present results have shown that if the decay occurs, then the half-life is many orders of magnitude larger than the age of the universe($\lambda_{0\nu\beta\beta} >10^{25} years$).

The last two pieces of research presented here are both computational as well. The first is a summary of all work done on the design, simulation, and analysis of signals for the NAPOLEON detector. The simulation is built off the GEANT4 framework, whereas the experimental reach and reconstruction performance studies were completed using python to analyze the simulated detector responses. Finally, a statistical analysis of the kinematics of $0\nu\beta\beta$-decay in a left-right symmetric model is performed. The analysis shows the improvement of neutrino parameter limits, which should be expected for an experiment with sensitivity to the difference in kinetic energy of the electrons in the decay, as well as the traditional resolution of time of decay.

In support of the nEXO experiment, thin films were developed on cylindrical substrates to demonstrate the feasibility of replacing traditional electronics within the detector with thin films coated on already existing detector components. For the XENON1T experiment, the work performed was purely computational, with the first part being an analysis of lone signal rates and the convolution of multiple lone signals to induce an artificial background in the experiment called accidental coincidence. The second part is a study of a novel data-driven position reconstruction algorithm as an alternative to algorithms trained on simulated signals and detector responses.

The work presented is split in support of three different experiments of varying degree of completion or operation. The first is the next Enriched Xenon Observatory (nEXO), which is a future, 5-tonne liquid Xe-136 detector focused on discovering $0\nu\beta\beta$-decay. The second is the XENON1T dark matter experiment, which used 1-tonne of liquid xenon to look for WIMP scatters with an exposure of 1-tonne-year. The last experiment is a computational prototype of the NAPOLEON detector, which is designed to perform both discovery and post-discovery neutrino physics using 0vbb-decay in a Cadmium Telluride segmented calorimeter.

While the physics being investigated in the search for $0\nu\beta\beta$-decay and WIMP dark matter is drastically different, due to the incredibly large half-life of the former and the incredibly small cross-section of the latter, the technology of their respective experiments has converged. The research presented in the latter half of this thesis(chapters five and beyond) is focused on developing technologies or extending studies from current theory to inform decisions on experimental design.

Within particle physics and cosmology, there are only a handful of puzzles as long sought after and of such import as dark matter and neutrinos. Both of these topics are entwined in how our universe evolved and came to be. Similarly, both have the potential to expand known physics beyond the standard model of particle physics and challenge some of our certainties about the universe.

In the case of dark matter, one of the most promising candidates to explain current cosmological observations is a hypothetical class of new particle called a Weakly Interacting Massive Particle, or more commonly WIMP. With current limits on the mass of WIMP dark matter, there could be potentially hundreds of the particles on average within any cubic meter of space in our galaxy. Despite this large and ubiquitous source of potential particles for discovery, WIMPs have not been experimentally confirmed due to their weakly interacting nature and a cross-section for interaction well below any other particle to date.