Measurement of the Muon-Decay Spectrum with a Wire Spark-Chamber Spectrometer

Abstract

The momentum spectrum of muon-decay positrons has been investigated, in the region above 25 MeV, with a homogeneous magnet containing wire spark chambers with magnetic-core readout. At a given field, this spectrometer accepts, with essentially constant probability and a momentum resolution of ±1%, a 30% momentum range. On the basis of 2.8×${10}^5$ events we find for the Michel parameter the value $\rho =0.760\mathrm{}\pm 0.009$ (in agreement with $V-A$) provided we constrain, as has been customary, the low-energy parameter $\eta$ to be zero. Since the two-component neutrino hypothesis ($V+\epsilon A$) predicts $\rho =\frac{3}{4}$ while leaving $\eta$ free, it is more sensible to impose the constraint $\rho =\frac{3}{4}$ and fit for $\eta$; doing this, we obtain $\eta =-0.7\mathrm{}\pm 0.6$, again in agreement with $V-A$, which predicts $\eta =0\mathrm{}$.