μ-Meson Decay Spectrum

Abstract

The energy spectrum of positrons from the decay of muons at rest has been measured, using a synchrotron as the meson source and a magnetic-field cloud chamber as the spectrometer. A total of 830 tracks was obtained, and the relation between this sample and the true decay spectrum is discussed in some detail; difficulties which may have led to the discrepancies among previously measured spectra are pointed out. A strict set of selection criteria reduced the sample to 280 tracks; from this it is found that the spectral intensity does not go to zero at the upper energy limit, but retains a large value. In terms of Michel's parameter, $\rho =0.50\mathrm{}\pm 0.13$.