Measurement of the W polarization in top decays with semileptonic ttbar events

Stringent tests on the top decay mechanism are provided by the measurement of the W boson polarization. This note presents a detailed study of this measurement with the ATLAS detector. We take advantage of the high statistics, high signal over background ratio and relatively easy event topology reconstruction of the semileptonic ttbar events (ttbar-> W b W bbar -> l nu b j1 j2 bbar) to measure the three helicity components of the W: the longitudinal, the left-handed and the right-handed F0, FL and FR fractions. This study is based on leading-order Monte-Carlo generators and a fast simulation of the detector. An absolute precision around 0.01, 0.02 and 0.03 on the Standard Model FR, F0 and FL, respectively, can be achieved in one LHC year at low luminosity (10 fb^{-1}), including pile-up events. This promising result, largely driven by systematic uncertainties, is converted into a sensitivity to the magnitude of the effective tWb couplings that parametrize new physics beyond the Standard Model.