MuSun: A Complete Analysis of the Precision Measurement of Nuclear Muon Capture in Deuterium.

Abstract

The MuSun experiment aims to measure the muon capture rate on deuterium to 1.5% precision. MuSun will be the first experiment to meaningfully constrain the weak axial current inthe two-nucleon system, which is relevant for Effective Field Theory approaches to studying light nuclei and is also related to solar proton-proton fusion and to the neutrino-deuterium scattering reactions measured at the Sudbury Neutrino Observatory. The capture rate is determined via a 10 ppm measurement of the negative muon disappearance rate in a deuterium target, which differs from the free muon decay rate by approximately 0.1% due to muon capture providing an additional decay channel. Such precision is achieved by measuring over 10^10 muons, using a cryogenic time-projection chamber to ensure clean stops in deuterium gas. This thesis describes the experimental design and hardware, and gives an overview of the main production runs in 2014 and 2015 that collected the full statistics needed for the experiment. It then explains the operation of the MuSun analysis software and some aspects of software development. Finally several in-depth analyses constraining specific sources of systematic uncertainty are presented. These focus primarily on characterizing the effects of mis-reconstructed muon stop positions, as well as background signals produced by capture on heavier elements when muons stop outside the target. These analyses establish uncertainty estimates near the target level, and a final unblinded result will be produced in 2023.