An Improved Measurement of the Number
of Light Neutrino Families by Single Photon
Counting with the OPAL Detector at LEP
by
Edward Gerard Heflin
Doctor of Philosophy, Graduate Program in Physics
University of California, Riverside
December, 1993
This thesis describes the results of a measurement of the number of
elementary light neutrinos.
Using the 1991 data collected by the OPAL detector at LEP, a measurement
of the number of neutrinos, Nn, with mn £ MZ0/2 and the Z0 invisible
width, Ginv, is made.
Cross-sections for the single photon process e+e-® n[`(n)]g are determined at each
center-of-mass energy point by single photon counting.
In a data sample of 13.076±0.089 pb-1, 201 events with corrected energy,
Egcorr, greater than 1.5 GeV are observed.
The number of light neutrino species is determined by the maximum-likelihood
fit to the number of observed events with Egcorr ³ 1.5 GeV for all
center-of-mass energy points.
Using the 1991 data, the number of neutrino species is determined to be
3.01±0.25±0.15 (the first error is statistical and the second error is systematic).
The weighted average of this result and the 1990 result yields 3.01±0.22±0.16.
In the Standard Model, this number of neutrino species corresponds to an
invisible width of 502±42±25 for the 1991 result and 502±35±27 for the combined
result.