Wood, Nicholas; (1990) Determination of the number of light neutrino generations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In 1989, the OPAL detector recorded 1.4 pb-1 of data from electron positron collisions in the centre of mass energy range 88< √s <96 GeV, allowing precision measurements of the Z° resonance for first time. This thesis describes the extraction of the number of neutrino generations from this data. Hadronic decays of the Z° are identified by large energy deposition spread over many clusters in the electromagnetic calorimeters. Tracking information is used to aid rejection of r+r-, two photon, cosmic ray, and beam related backgrounds. Hadronic events are selected with a systematic error of 0.8%. The machine luminosity is measured with the forward detectors, using small angle Bhabha scattering as the normalization process. The components of the forward detector used in this analysis are a lead-scintillator sampling elecromagnetic calorimeter and an array of proportional tube chambers. The tube chambers were used to accurately measure the position of electromagnetic showers, by doing so identifying Bhabha events in a well defined fiducial region, and measuring the absolute luminosity to an accuracy of 2.2%. The calorimeter, having a higher statistical precision, was used to determine a luminosity for each energy point with a systematic point to point error of less than 1.0%. By combining the hadronic events selected with the luminosity measurement, hadronic cross sections were determined at each energy. The hadronic cross sections were fitted in two ways to the Z° lineshape, the difference between the two methods being the parameters left free in the fit. With a model independent fit, the values of the mass. Mz, width, Γz, and peak cross section σpolehad were found to be:- Mz = 91.145 ± 0.022 (exp)±0.030 (LEP) GeV Γz = 2.526 ±0.047 GeV σpolehad = 41.2 ± 1.1 nb and from the width, the number of light neutrinos Nv was found to be Nv = 3.26 ± 0.28 (exp.)-0.28+0.13 (theor.). A fit to the standard model with the number of neutrino generations as a free parameter gave:- Nv, = 3.09 ± 0.19(exp)+0.06-0.12(theor.). From the second measurement, a fourth generation of standard model light neutrinos is excluded with greater than 99% confidence.

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