On measurement of top polarization as a probe of $t \bar t$ production mechanisms at the LHC

TL;DR

This paper proposes a method to measure top quark polarization via decay-lepton azimuthal asymmetry at the LHC, providing a new probe for $t ar t$ resonance dynamics and potential new physics signals.

Contribution

It introduces an azimuthal asymmetry based on decay-lepton angles as a top polarization probe, unaffected by anomalous couplings, and demonstrates its effectiveness for $Z'$ models at the LHC.

Findings

Net top polarization can reach up to 10% for certain $Z'$ masses.

Azimuthal asymmetry deviations from the Standard Model can be up to 7%.

The method offers 3-sigma sensitivity for $Z'$ detection at 14 TeV LHC with 10 fb$^{-1}$.

Abstract

In this note we demonstrate the use of top polarization in the study of $t \bar t$ resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of $\cos\phi_\ell$) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive $z$ axis is not uniquely defined. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous $tbW$ couplings, to linear order. We study, for purposes of demonstration, the case of a $Z'$ as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: $\sqrt{s} = 14$ TeV and $\sqrt{s} = 7$ TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for $M_{Z'} \simeq 1200 $ GeV, being as high as $10 %$ for a smaller mass of the $Z'$ of $700$ GeV and for the largest allowed coupling in the model, the values being higher for the $7$ TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about $4%$ ($7%$) for $14$ ($7$) TeV LHC, whereas for the Tevatron, values as high as $12%$ are attained. For the $14$ TeV LHC with an integrated luminosity of 10 fb$^{-1}$, these numbers translate into a $3 \sigma$ sensitivity over a large part of the range $500 \lesssim M_{Z'} \lesssim 1500 $ GeV.