A measurement of the helicity of <italic>W</italic> bosons produced in top -quark decays.

Abstract

The Standard Model of particle physics is a remarkably successful description of nature. One aspect of the theory that is not well-understood is the nature and the origin of the mechanism which breaks the gauge symmetry of the electroweak interaction. According to the theory, this mechanism gives rise to the masses of elementary particles. However, we have yet to directly probe these phenomena. The top quark is the most massive known elementary particle; it decays almost exclusively via the electroweak interaction. By studying the kinematics of top-quark decays, we can indirectly probe the electroweak symmetry breaking mechanism at the highest energies presently attainable. We measure the fraction of longitudinally-polarized <italic>W</italic> bosons produced in top-quark decays by analyzing the transverse momentum spectrum of charged-lepton arising from the process <italic>t</italic> &rarr; <italic> W</italic><super>+</super><italic>b</italic> &rarr; &ell;<super>+</super>nu_&ell;<italic>b</italic>. Top-quark pairs are produced in proton-antiproton collisions with a center-of-mass energy s = 1.96 GeV at the Tevtron synchrotron at the Fermi National Accelerator Laboratory in Batavia Illinois. Top-quark candidate events are isolated in 200 pb<super>-1</super> of data using the newly-upgraded CDF II detector. These data indicate that the fraction of <italic>W</italic> bosons with longitudinal polarization is <italic>F</italic>₀ = 0.88+0.12-0.47 (stat. + syst.), <italic>F</italic>₀ > 0.24 95% CL in events where only one <italic>W</italic> decays leptonically; <italic>F</italic>₀ < 0.52 95% CL, <italic>F</italic>₀ < 0.94 99% CL in events where both <italic>W</italic>'s decay leptonically, and <italic> F</italic>₀ = 0.27+0.35-0.21 (stat. + syst.), <italic>F</italic>₀ < 0.88 95% CL in the combined analysis. The Standard Model prediction, given a top-quark mass of 175 GeV, is <italic>F</italic>₀ = 0.703. The discrepancy in the dilepton sample is suggestive of new phenomena, while the result in the single-lepton sample is fully consistent with the Standard Model expectation. Clearly, these results warrant further investigation.