Detector for measuring the $\pi^+\to e^+\nu_e$ branching fraction

TL;DR

The PIENU experiment at TRIUMF developed a detector system to precisely measure the pion decay branching ratio to electrons and muons, aiming for less than 0.1% uncertainty.

Contribution

This paper describes the design, construction, and performance of the PIENU detector system for measuring the pion decay branching ratio with high precision.

Findings

Achieved precise detection of pion decay products.

Demonstrated detector performance suitable for sub-0.1% measurement.

Validated experimental setup for future high-precision measurements.

Abstract

The PIENU experiment at TRIUMF is aimed at a measurement of the branching ratio $R^{e/\mu}$ = ${\Gamma\big((\pi^{+} \rightarrow e^{+} \nu_{e}) + (\pi^{+} \rightarrow e^{+} \nu_{e}\gamma)\big)}/{\Gamma\big((\pi^{+} \rightarrow \mu^{+} \nu_{\mu})+(\pi^{+} \rightarrow \mu^{+} \nu_{\mu}\gamma)\big)}$ with precision $<$0.1\%. Incident pions, delivered at the rate of 60 kHz with momentum 75 MeV/c, were degraded and stopped in a plastic scintillator target. Pions and their decay product positrons were detected with plastic scintillators and tracked with multiwire proportional chambers and silicon strip detectors. The energies of the positrons were measured in a spectrometer consisting of a large NaI(T$\ell$) crystal surrounded by an array of pure CsI crystals. This paper provides a description of the PIENU experimental apparatus and its performance in pursuit of $R^{e/\mu}$.