Sensitivity and background estimates for phase-II of the COMET experiment

Abstract

Conservation of Lepton Flavour in the Standard Model (SM) requires that neutrino emission accompanies muon decay. COMET is one experiment looking for Charged Lepton Flavour Violation. It searches for COherent Muon to Electron Transitions, where a muon converts to a 105 MeV electron in the presence of an atomic nucleus, without emitting neutrinos. The current limit on this process is $7\times10^{-13}$ at 90% C.L., which COMET intends to improve by four orders of magnitude.

To realise such an improvement, COMET will use several novel techniques to produce a very intense, low-energy muon beam, with very high signal acceptance and strong background suppression. Given the challenge this presents, COMET will run in a staged approach. Phase-I is currently under construction with first data-taking due in JFY 2018, and the goal of measuring $\mu$-e conversion with a Single-Event Sensitivity (SES) of $3\times10^{-15}$. Phase-II should follow at the start of the next decade and achieve a SES of $3\times10^{-17}$.

This thesis provides an overview of CLFV, $\mu$-e conversion, and the COMET experiment itself. It sets out the software and simulation that has been developed to help understand and analyse the experiment, and then uses this to perform a comprehensive optimisation of the Phase-II set-up, providing a new baseline configuration. The expected performance of this baseline is assessed, with studies on the signal sensitivity demonstrating that an SES of $2.6\times10^{-17}$ can be achieved in $1.57\times10^{7}$ s of beam. Background rates are also estimated and, although subject to large uncertainties, predict 0.662 background events can be expected during Phase-II. Suggestions for future performance studies and experiment improvements are also discussed, with a possible improvement in the SES of a factor of 2.5 likely achievable.