6 Conclusion

In this work, elements of the detector to be used in measuring the pion beta decay rate have been discussed. Tests have been performed using several detector components and a pion beam at the Paul Scherrer Institute in Villigen, Switzerland. Analysis of data taken during these tests has been presented.

Pion beta decay is a fundamental electroweak process that provides insight in two important areas. First, pion beta decay is a 0^- -> 0^- transition which is completely analogous to the 0⁺ -> 0⁺ transitions of superallowed Fermi decay of nucleons. The Conserved Vector Current Hypothesis (CVC) states that the vector part of an electroweak interaction is conserved. A measurement of the p ⁺ -> p ⁰e⁺ n decay rate with an uncertainty of <0.5%, thus, will provide a stringent test of the CVC and radiative corrections.

The second area in which pion beta decay provides insight is the unitarity of the Cabbibo-Kobayashi-Maskawa quark mixing matrix (CKM). The CKM transforms between the mass eigenstates of the strong interaction and the eigenstates of the weak interaction. The CKM is a unitary matrix, providing only three quark generations exist. Currently, the most stringent test of the unitarity of the CKM is done on the top row. The largest element in the top row is , the term that represents up and down quark mixing and can be determined from the p ⁺ -> p ⁰e⁺ n decay rate. A stringent test of the unitarity of the CKM is thus obtained from a precise measurement of the p ⁺ -> p ⁰e⁺ n decay rate at the uncertainty level of ~0.3%.

The pibeta detector centers on a calorimeter composed of 240 pure CsI crystals. The plastic veto, active target and degrader, and cylindrical wire chambers compose the rest of the detector. The CsI crystals must pass exacting geometric and performance standards.

Arizona State University supplied the plastic veto scintillator array. The array is composed of 20 long, thin staves that form a cylinder that fits between the calorimeter and target. Cosmic ray tomography tests on three plastic veto scintillator staves resulted in measurements of cm for the attenuation length and m/s for the average signal velocity.

A study of the pion decay data from 1997 using a partial setup of the pibeta detector resulted in a measurement of the p -> e n ( g ) branching ratio of (1.26 ± 0.06) × 10^-4. This is consistent with the currently accepted value of (1.230 ± 0.004) × 10^-4 [PDG 96].

In-beam tests of the pibeta detector components have shown them to perform as expected. The trigger electronics and logic have been used in comjunction with these detectors to successfully measure the p -> e n ( g ) branching ratio. Several minor obstacles have been identified during the 1993-1997 tests and overcome. The partial pion beta decay detector has performed at a level consistent with expectations giving confidence that the full pion beta decay detector will do likewise.

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