Evidence for WZ production and a measurement of the WZ production cross section.

Abstract

This dissertation describes a test of the Standard Model (SM) of particle physics by measuring the probability, or cross section, of simultaneously producing a <italic>W</italic> boson and a <italic>Z</italic> boson from proton-antiproton collisions. The SM predicts the cross section of <italic>WZ</italic> production to be 3.68 +/- 0.25 pb. The SM and physics of <italic>WZ</italic> production are described in Chapter 2 of this dissertation. The 1.96 TeV center-of-mass energy proton-antiproton collisions are provided by the Fermi National Accelerator Laboratory (FNAL) Tevatron Collider. The <italic>W</italic> and <italic>Z </italic> particles are detected using the DO detector, which is described in Chapter 3. The data were collected by the detector during 2002-2006 corresponding to 1 fb<super>-1</super> of <italic>pp&macr;</italic> collisions. This data set is described in Chapter 6. The measurement uses the trilepton (<italic> e</italic>nu<italic>ee</italic>, munu<italic>ee</italic>, <italic>e </italic>numumu, and munumumu) decay channels, in which a <italic> W</italic> decays to a charged lepton plus a neutrino and a <italic>Z</italic> decays to a pair of charged leptons. The <italic>W</italic> and <italic>Z </italic> particle selection criteria, detection efficiency, and background determination are described in Chapter 7. We observe 13 candidate events in 1 fb<super>-1</super> of <italic>pp&macr;</italic> collisions. In this data set we expect to see 4.5 +/- 0.6 background events, and we expect to see 9.2 +/- 1.0 signal events. The probability of 4.5 +/- 0.6 background events to fluctuate to 13 or more events is 1.2 x 10<super>-3</super> which is a 3.0 sigma deviation from the background estimate. A log likelihood method is used to determine the most likely cross section as determined by the measured signal efficiencies, the expected backgrounds, and the observed data. Presented in Chapter 8 is a measurement of the cross section for <italic> pp&macr;</italic> &rarr; <italic>WZ</italic> + <italic>X</italic> at s = 1.96 TeV. The <italic>WZ</italic> diboson production cross section is measured to be sigma_WZ = 2.7+1.7-1.3 pb. This is in agreement with the predicted Standard Model cross section.