Measurement of the helicity of <italic>W</italic> bosons in top quark decays.

Abstract

The top quark is the sixth member of the quark family. Quarks are the fundamental particles which form the protons and neutrons that are at the nucleus of all atoms. The top quark is the most massive of the six quarks, and study of its kinematic properties may provide important clues about the mechanism whereby fundamental particles acquire mass. In this thesis we describe a measurement of one such kinematic property: the helicity of W bosons which are produced in the decay of the top quark. We use the transverse momentum spectrum of leptons in the decay chain <italic> t</italic> &rarr; <italic>bW</italic> with <italic>W</italic> &rarr; <italic>l</italic>nu to measure the helicity of the <italic>W</italic> bosons in the top quark rest frame. Our measurement uses a <italic>tt&macr; </italic> sample isolated in 106 +/- 4pb<super>-1</super> of data collected in <italic>pp&macr;</italic> collisions at s = 1.8 TeV with the CDF detector at the Fermilab Tevatron. Assuming a standard V-A weak decay, we find that the fraction of <italic>W</italic>'s with zero helicity in the top rest frame is F0 = 0.91 +/- 0.37(stat) +/- 0.13(syst), consistent with the standard model prediction of F0 = 0.70 for a top mass of 175GeV/<italic>c</italic><super>2</super>.