| dc.description.abstracteng | A measurement of the $W$ boson polarisation in top quark decays is presented. The top anti-top pairs were produced at a centre of mass energy of $\sqrt{s}=8$ \tev \ using proton-proton collisions at the LHC. The data sample corresponds to an integrated luminosity of$\int{\mathcal{L}dt}=20.2$ fb$^{-1}$ recorded by the ATLAS detector in 2012. The measurement was performed in the dilepton channel, which is characterised by two leptons (electron or muon) with opposite electric charge, missing transverse energy from two neutrinos, and at least two jets. One of these jets is required to be tagged as $b$ jet. The $W$ boson polarisation was studied using the angular distribution of the charged lepton in the rest frame of the $W$ boson. The event is fully reconstructed using a kinematic reconstruction method. The three existing $W$ boson polarisation state fractions are longitudinal, left-handed, and right-handed. The fractions were estimated from the data distribution using a binned likelihood fit. The $W$ boson helicity fractions are obtained from a combined likelihood fit separately for different requirements on $b$-tag jet multiplicity. The best precision is obtained by requiring at least two $b-$tag jets at the final state. The corresponding fractions yield\\
\begin{center}
\begin{align*}
F_0 = 0.695 \pm 0.014 \textnormal{(stat.+bkg)}^{+0.023}_{-0.024} \textnormal{ (syst.)}, \\
F_L = 0.291 \pm 0.009 \textnormal{(stat.+bkg)}^{+0.011}_{-0.010} \textnormal{ (syst.)}, \\
F_R = 0.012 \pm 0.006 \textnormal{(stat.+bkg)}^{+0.020}_{+0.021} \textnormal{ (syst.)}. \\
\end{align*}
\end{center}
As the polarisation of the $W$ bosons in top quark decays is sensitive to the $Wtb$ vertex structure, limits on anomalous $Wtb$ couplings are set. All results are in good agreement with the Standard Model predictions. | de |