Measurement of the W boson polarization in top quark decays with the ATLAS detector

TL;DR

This paper reports on the measurement of W boson polarization in top quark decays using ATLAS data, confirming Standard Model predictions and constraining possible anomalous Wtb couplings.

Contribution

First measurement of W boson polarization in top decays at 7 TeV with ATLAS, providing data consistent with the Standard Model and limits on anomalous couplings.

Findings

Measured polarization fractions: F_0=0.67±0.07, F_L=0.32±0.04, F_R=0.01±0.05.

Results agree with Standard Model predictions.

Set limits on anomalous Wtb couplings.

Abstract

This paper presents measurements of the polarization of W bosons in top quark decays, derived from ttbar events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb^-1. The measured fractions of longitudinally, left- and right-handed polarization are F_0=0.67+/-0.07, F_L=0.32+/-0.04 and F_R=0.01+/-0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the Wtb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the Wtb couplings.