Comparison of spin-correlation and polarization variables of spin density matrix for top quark pairs at the LHC and New Physics implications

TL;DR

This paper compares various Monte Carlo simulation methods for top-quark pair spin correlations at the LHC, assessing their impact on understanding Standard Model consistency and potential New Physics signals.

Contribution

It provides the first comprehensive comparison of MC generators' effects on top-quark spin observables, including LO/NLO and electroweak corrections, and explores implications for New Physics scenarios.

Findings

SM MC predictions align with experimental data within uncertainties.

Certain observables may distinguish signals of New Physics from SM predictions.

Electroweak corrections influence spin correlation measurements.

Abstract

Precise determination of top-quark pairs is an essential tool for understanding the overall consistency of the standard model (SM) expectations, understanding limited New Physics (NP) models, through spin-spin correlation and polarization parameters, and has a critical impact on the analyses strategies at upcoming LHC programs. In this work, we review and discuss various state-of-the-art Monte Carlo (MC) methodologies as \textsc{MadGraph5}\_aMC@NLO, \textsc{Sherpa}, \textsc{Powheg-Box} and \textsc{Pythia8}, which are Matrix Element (ME)$/$Parton Shower (PS) matching generators including a complete set of spin correlation and polarization in top quark pair production with dileptonic final states. This is the first such study that not only compares the effects of different MC event generator approaches on spin density matrix elements and polarization parameters, but also investigates the effects of leading order (LO) and next-to-leading order (NLO) accuracy in QCD, and electroweak (EW) corrections via \textsc{Sherpa}. Moreover, as a continuation of the work, the prospects for possible NP scenarios through top-quark spin-spin correlation and polarization measurements for Supersymmetry (R parity conserved and violated models) and Dark Matter (top quarks associated mediator) models during upcoming LHC runs are briefly outlined. We find that all SM MC predictions for the defined set of variables are generally consistent with the experimental data and theoretical predictions within the uncertainty variations. Besides, for the distributions of the $\cos\varphi$, laboratory-frame observables ($\cos\varphi_{lab}$ and $|\Delta\phi_{ll}|$) and the observables generated by parity (P) and charge-parity (CP) conserving interactions, we conclude some clues that the considered signals and beyond may well be separated from experimental data and located above the SM predictions.