Dilepton azimuthal correlations in $t \bar t$ production

TL;DR

This paper introduces a Fourier series parameterization of the dilepton azimuthal correlation in top-antitop production, simplifying the comparison of theoretical predictions with experimental data, especially in the presence of new physics effects.

Contribution

A new Fourier series-based method for modeling the dilepton azimuthal correlation in top pair production, bridging theory and experimental measurements efficiently.

Findings

Third-order Fourier expansion accurately approximates the distribution.

Application to anomalous top couplings predictions.

Facilitates analysis of deviations from the Standard Model.

Abstract

The dilepton azimuthal correlation, namely the difference $\phi$ between the azimuthal angles of the positive and negative charged lepton in the laboratory frame, provides a stringent test of the spin correlation in $t \bar t$ production at the Large Hadron Collider. We introduce a parameterisation of the differential cross section $d\sigma / d\phi$ in terms of a Fourier series and show that the third-order expansion provides a sufficiently accurate approximation. This expansion can be considered as a `bridge' between theory and data, making it very simple to cast predictions in the Standard Model (SM) and beyond, and to report measurements, without the need to provide the numbers for the whole binned distribution. We show its application by giving predictions for the coefficients in the presence of (i) an anomalous top chromomagnetic dipole moment; (ii) an anomalous $tbW$ interaction. The methods presented greatly facilitate the study of this angular distribution, which is of special interest given the $3.2(3.7)\sigma$ deviation from the SM next-to-leading order prediction found by the ATLAS Collaboration in Run 2 data.