Azimuthal Angular Correlation as a Boosted Top Jet Substructure

TL;DR

This paper introduces a new jet substructure observable based on azimuthal angular correlation in boosted top quark decays, which can measure top polarization and distinguish top jets from background without reconstructing decay products.

Contribution

It proposes a novel $ ext{cos}2 heta$ angular correlation observable related to top polarization, applicable in the boosted regime, and develops an experimental method to measure it without full decay reconstruction.

Findings

The $ ext{cos}2 heta$ correlation exists only in the boosted regime.

The proposed observable can measure top polarization.

Numerical simulations agree with Standard Model predictions.

Abstract

We propose a novel jet substructure observable of boosted tops that is related to the linear polarization of the $W$ boson in boosted top quark decay, which results in a $\cos2\phi$ angular correlation between the $t\to bW$ and $W\to f\bar{f'}$ decay planes. We discuss in detail the origin of such linear polarization by applying Wigner's little group transformation. We show that the unique $\cos2\phi$ angular correlation only exists in the boosted regime but not in the top quark rest frame. We construct an experimental observable for such correlation based on the transverse energy deposition asymmetry in the top jet that does not require the reconstruction of $W$ decay products. The degree of this asymmetry can be used to measure the longitudinal polarization of the top quark, which is an important probe of new physics that couples to the top sector, and can discriminate a boosted top quark jet from its background events, such as QCD jets. A numerical simulation is also performed and found to agree well with the analytic prediction of the Standard Model.