Initial Tests Of The Pion Beta Decay Detector

1.6 Summary

A precise measurement of the pion beta decay rate would provide a third method for determining the V_ud element of the CKM providing a more stringent unitarity test of the top row. Since pion beta decay represents a transition between two spin zero members of an isotriplet, it is analogous to superallowed nuclear beta decays allowing for a comparison of [florin]t values. Additionally, a stringent test of CVC would be made by a measurement of the pion beta decay rate at the <0.5% level of uncertainty. Pion beta decay provides the most direct test of CVC because it is a pure vector transition occurring outside of a nucleus and thus free from the re-normalizing effects of the strong force.

The branching ratio for the p -> e n ( g ) decay has been measured accurately to an uncertainty of <0.4%. There have been, however, only two accurate measurements of the p ⁺ -> p ⁰e⁺ n branching ratio. These had uncertainties of >3% [McF 85] and >8% [Dep 68]. The pibeta project will measure the pion beta decay rate using a stopped pion method not unlike the one used in [Dep 68] but with a modern data acquisition system and a calorimeter with much greater angular resolution. The pibeta project anticipates an uncertainty of <0.5% on the initial measurement of the p ⁺ -> p ⁰e⁺ n decay rate with hopes of reducing the uncertainty to the level of <0.2%.

This document focuses on a measurement of the p -> e n ( g ) branching ratio and the preparation of the plastic veto array supplied by Arizona State University for the pibeta project. A review of the theoretical background related to the pibeta experiment as well as an overview of the detector apparatus is presented. Results from cosmic ray tomography of three plastic veto detectors are presented. Finally, results from the analysis of pion decay data are given which yielded a measurement of the p -> e n ( g ) branching ratio.

For this work, the author assembled and prepared the plastic veto scintillator staves for the pion beta decay experiment. Data were taken by the author using cosmic rays and three plastic veto scintillator staves. Results of the analysis of that data are presented in Chapter 4. The author participated in performing in-beam tests of several components of the pion beta decay detector in 1994, 1995, 1996, and 1997. Analysis done by the author of data acquired in 1996 and 1997 is presented in Chapter 5, resulting in a determination of the p -> e n ( g ) branching ratio.