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

F. Hubaut, E. Monnier, P. Pralavorio, V. Simak, K. Smolek

TL;DR

The paper assesses ATLAS's ability to probe $W$ boson and top quark polarization in $t\bar{t}$ events at the LHC, focusing on semileptonic and dileptonic channels with LO Monte Carlo and fast detector simulation. By defining and employing polarization observables such as the $W$ helicity fractions $F_0$, $F_L$, $F_R$ and top-spin correlation measures, it quantifies expected precisions under systematic uncertainties and backgrounds. The study finds that, with $10\,\mathrm{fb}^{-1}$, $F_0$ can be measured to about 2% and $F_R$ to about 1%, while the like-/unlike-spin asymmetry can be constrained to roughly 4%, enabling sensitivity to anomalous $tWb$ couplings, charged Higgs decays, and new $s$-channel resonances in $t\bar{t}$ production. These results illustrate ATLAS's potential to test the electroweak structure of top quark interactions and to probe scenarios beyond the Standard Model.

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.

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

TL;DR

The paper assesses ATLAS's ability to probe boson and top quark polarization in events at the LHC, focusing on semileptonic and dileptonic channels with LO Monte Carlo and fast detector simulation. By defining and employing polarization observables such as the helicity fractions , , and top-spin correlation measures, it quantifies expected precisions under systematic uncertainties and backgrounds. The study finds that, with , can be measured to about 2% and to about 1%, while the like-/unlike-spin asymmetry can be constrained to roughly 4%, enabling sensitivity to anomalous couplings, charged Higgs decays, and new -channel resonances in production. These results illustrate ATLAS's potential to test the electroweak structure of top quark interactions and to probe scenarios beyond the Standard Model.

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.

Paper Structure

This paper contains 4 sections, 9 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. $W$ boson and top quark polarization in $t\bar{t}$ events
  3. $W$ polarization observables
  4. Top polarization observables

Figures (4)

  • Figure 1: Sketches of angular momentum conservation in $t \rightarrow W^+ b$ decay in the top rest frame. Simple (open) arrows denote particle direction of motion (spin). As a massless $b$-quark must be left-handed, the rightmost plot is forbidden in the SM at tree level.
  • Figure 2: Sketches of the different $W^+$ polarization modes in $t \rightarrow W^+ b$ decay and resulting lepton directions. Simple (open) arrows denote particle direction of motion (spin). For $W^-$, left and right-handed components are inverted.
  • Figure 3: Angular distribution of Equation (\ref{['eq:cosphi']}) in the SM. The predicted contributions from longitudinal (0) and left-handed (L) helicity states are shown separately with dashed lines. The right-handed contribution is null in the SM. The sum (0+L+R) is depicted with a full line.
  • Figure :