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

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×{}10⁵ events we find for the Michel parameter the value ρ=0.760±0.009 (in agreement with V-A) provided we constrain, as has been customary, the low-energy parameter η to be zero. Since the two-component neutrino hypothesis (V+εA) predicts ρ=3/4 while leaving η free, it is more sensible to impose the constraint ρ=3/4 and fit for η; doing this, we obtain η=-0.7±0.6, again in agreement with V-A, which predicts η=0.