ATLAS sensitivity to top quark and W boson polarization in $t\bar{t}$ events

TL;DR

This paper evaluates ATLAS's ability to measure top quark and W boson polarization in ttbar events, achieving high precision in polarization components and asymmetry, which can probe new physics beyond the Standard Model.

Contribution

It provides a detailed analysis of polarization measurements in ttbar events with the ATLAS detector, including systematic uncertainties and background estimation, demonstrating potential sensitivity to new physics.

Findings

W polarization components measured with 2% and 1% precision.

Top pair spin asymmetry measured with 4% precision.

Sensitivity to new physics such as anomalous couplings and heavy resonances.

Abstract

Stringent tests on top quark production and decay mechanisms are provided by the measurement of the top quark and W boson polarization. This paper presents a detailed study of these two measurements with the ATLAS detector, in the semileptonic (ttbar -> W W b bbar -> l nu j1 j2 b bbar) and dileptonic (ttbar -> W W b bbar -> l nu l nu b bbar) ttbar channels. It is based on leading-order Monte Carlo generators and on a fast simulation of the detector. A particular attention is paid to the systematic uncertainties, which dominate the statistical errors after one LHC year at low luminosity (10 fb^{-1}), and to the background estimate. Combining results from both channel studies, the longitudinal component of the W polarization (F\_0) can be measured with a 2% accuracy and the right-handed component (F\_R) with a 1% precision with 10 fb^{-1}. Even though the top quarks in ttbar pairs are not polarized, a large asymmetry is expected within the Standard Model in the like-spin versus unlike-spin pair production. A 4% precision on this asymmetry measurement is possible with 10 fb^{-1}, after combining results from both channel studies. These promising results are converted in a sensitivity to new physics, such as tWb anomalous couplings, top decay to charged Higgs boson, or new s-channels (heavy resonance, gravitons) in ttbar production.