Measurement of the W Boson Polarization in Top Decay at CDF at s**(1/2) = 1.8 TeV

TL;DR

This paper measures the polarization of W bosons from top quark decays at the Tevatron, testing the Standard Model prediction and setting limits on possible non-standard V+A contributions to the tWb vertex.

Contribution

It provides the first measurement of W polarization in top decay at Tevatron energies and constrains non-standard V+A couplings.

Findings

No evidence for non-standard V+A vertex.

Limits set on V+A coupling: f_{V+A} < 0.80 at 95% CL.

Combined analysis yields f_{V+A} < 0.61 at 95% CL.

Abstract

The polarization of the $W$ boson in $t \to W b$ decay is unambiguously predicted by the Standard Model of weak interactions and is a powerful test of our understanding of the $tbW$ vertex. The subsequent $W \to l \nu$ decay serves as a polarimeter for the $W$, and the invariant mass of the lepton and $b$ quark in top decay provides a measurement of the $W$ decay angle. In this paper we present a measurement of the decay rate of the W produced from the decay of the top quark in the hypothesis of V+A structure of the tWb vertex. We find no evidence for the non-standard V+A vertex and set a limit on $f_{V+A}$ $<$ 0.80 @ 95% confidence level. By combining this result with a complementary observable in the same data, we assign a limit of $f_{V+A}$ $<$ 0.61 @ 95% CL.