The pion beta () decay, , is one of the most fundamental weak interaction processes. It is analogous to superallowed Fermi transitions in nuclear decays. The analysis of nuclear decay involves nuclear corrections which are uncertain at the level of a few tenth of a percent. These corrections do not appear in the process, which, therefore, presents a more stringent test of the weak interaction theory.
The difficulty in measuring the decay is due to the small branching ratio of . The most recent and precise determination of the decay rate was carried out by McFarlane et al. , and is in good agreement with theory. However, the measurement uncertainty, , is an order of magnitude higher than the theoretical uncertainties .
The goal of this experiment is a measurement of the decay rate with a precision of 0.5 %. We have designed a stopped-pion detector system to detect the two 's from the decay, as well as the . Due to the large Michel positron background, the detector must be very efficient in the detection and have excellent background suppression. This is achieved by using a pure CsI calorimeter with a large solid angle coverage and good energy resolution together with an active target and two cylindrical wire chambers for charged particle tracking (Figure 1).