A new measurement of the branching ratio, $R_{e/mu} =Gamma (pi^+ rightarrow mbox{e}^+ u + pi^+ rightarrow mbox{e}^+ u gamma)/ Gamma (pi^+ rightarrow mu^+ u + pi^+ rightarrow mu^+ u gamma)$, resulted in $R_{e/mu}^{exp} = (1.2344 pm 0.0023 (stat) p
m 0.0019 (syst)) times 10^{-4}$. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1 %.
The PIENU experiment at TRIUMF is aimed at a measurement of the branching ratio $R^{e/mu}$ = ${Gammabig((pi^{+} rightarrow e^{+} u_{e}) + (pi^{+} rightarrow e^{+} u_{e}gamma)big)}/{Gammabig((pi^{+} rightarrow mu^{+} u_{mu})+(pi^{+} rightarrow mu^{
+} u_{mu}gamma)big)}$ 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 $R^{e/mu}$.
Using a narrow band positron beam, the response of a large high-resolution NaI(Tl) crystal to an incident positron beam was measured. It was found that nuclear interactions cause the appearance of additional peaks in the low energy tail of the deposited energy spectrum.
An extension of the TRIUMF M13 low-energy pion channel designed to suppress positrons based on an energy-loss technique is described. A source of beam channel momentum calibration from the decay pi+ --> e+ nu is also described.
