Measurement of the Helicity Fractions of W Bosons from Top Quark Decays Using Fully Reconstructed top-antitop Events with CDF II

TL;DR

This paper measures the fractions of different helicity states of W bosons from top quark decays using data from the CDF II detector, providing results consistent with the Standard Model and setting an upper limit on right-handed W bosons.

Contribution

First measurement of W boson helicity fractions in top decays using full event reconstruction with CDF II data.

Findings

F_0 = 0.85 +0.15 -0.22 (stat) +- 0.06 (syst)

F_+ = 0.05 +0.11 -0.05 (stat) +- 0.03 (syst)

F_+ < 0.26 at 95% CL

Abstract

We present a measurement of the fractions F_0 and F_+ of longitudinally polarized and right-handed W bosons in top quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 318 pb -1. We select ttbar candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle theta*, which is defined as the angle between the momentum of the charged lepton in the W boson rest frame and the W momentum in the top quark rest frame. The cos(theta*) distribution in the data is determined by full kinematic reconstruction of the ttbar candidates. We find F_0 = 0.85 +0.15 -0.22 (stat) +- 0.06 (syst) and F_+ = 0.05 +0.11 -0.05 (stat) +- 0.03 (syst), which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed W bosons of F_+ < 0.26 at the 95% confidence level.