[Paper Review] Measurements of inclusive and differential cross sections for top quark production in association with a Z boson in proton-proton collisions at $ \sqrt{s} $ = 13 TeV

Hayrapetyan, Aram, Aram Hayrapetyan+2398 more|arXiv (Cornell University)|Jan 1, 2024

Particle physics theoretical and experimental studies1 citations

TL;DR

This paper presents the first simultaneous measurement of inclusive and differential cross sections for top quark production in association with a Z boson (ttZ + tWZ and tZq) in proton-proton collisions at √s = 13 TeV using 138 fb⁻¹ of CMS data. Using a multiclass deep neural network to classify events into ttZ+tWZ, tZq, and background categories, and a profile likelihood fit to extract cross sections while accounting for systematic uncertainties and correlations, the inclusive cross sections are measured as 1.14 ± 0.07 pb for ttZ + tWZ and 0.81 ± 0.10 pb for tZq, in good agreement with standard model predictions.

ABSTRACT

Measurements are presented of inclusive and differential cross sections for Z boson associated production of top quark pairs ($\mathrm{t\bar{t}}$Z) and single top quarks (tZq or tWZ). The data were recorded in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with three or more leptons, electrons or muons, are selected and a multiclass deep neural network is used to separate three event categories, the $\mathrm{t\bar{t}}$Z and tWZ processes, the tZq process, and the backgrounds. A profile likelihood approach is used to unfold the differential cross sections, to account for systematic uncertainties, and to determine the correlations between the two signal categories in one global fit. The inclusive cross sections for a dilepton invariant mass between 70 and 110 GeV are measured to be 1.14 $\pm$ 0.07 pb for the sum of $\mathrm{t\bar{t}}$Z and tWZ, and 0.81 $\pm$ 0.10 pb for tZq, in good agreement with theoretical predictions.

Motivation & Objective

• To measure inclusive and differential cross sections for top quark production in association with a Z boson in proton-proton collisions at √s = 13 TeV.
• To simultaneously extract cross sections for ttZ + tWZ and tZq processes, enabling direct measurement of their correlation.
• To improve sensitivity to new physics by combining data from multiple top-Z production channels in a global fit.
• To validate theoretical predictions in perturbative QCD and Monte Carlo generators using high-precision data.
• To reduce background contamination from nonprompt leptons and WZ production through advanced multiclass classification.

Proposed method

• Events are selected with three or more isolated leptons (electrons or muons), with two forming a Z boson candidate.
• A multiclass deep neural network (DNN) assigns likelihood scores to three categories: ttZ + tWZ signal, tZq signal, and background.
• The DNN is trained to distinguish between signal processes and dominant backgrounds, including nonprompt leptons and WZ production.
• A profile likelihood fit is used to extract inclusive and differential cross sections, simultaneously constraining nuisance parameters for systematic uncertainties.
• Unfolding of differential distributions is performed using the profile likelihood approach to correct for detector effects and resolution.
• Correlations between the two signal processes are determined directly through a global fit, enabling improved constraints in effective field theory analyses.

Experimental results

Research questions

• RQ1 What are the inclusive and differential cross sections for top quark pair and single top quark production in association with a Z boson at √s = 13 TeV?
• RQ2 How do the measured cross sections for ttZ + tWZ and tZq compare to standard model predictions?
• RQ3 What is the correlation between the ttZ + tWZ and tZq signal processes, and how can it be leveraged in global fits?
• RQ4 To what extent do nonprompt leptons and WZ production contribute to the background in this final state?
• RQ5 Can a multiclass deep neural network effectively separate overlapping top-Z production channels with high purity?

Key findings

• The inclusive cross section for ttZ + tWZ production is measured to be 1.14 ± 0.07 pb, consistent with the standard model prediction.
• The inclusive cross section for tZq production is measured to be 0.81 ± 0.10 pb, in good agreement with theoretical expectations.
• The measured cross sections show good agreement with next-to-leading-order quantum chromodynamics predictions.
• The correlation between the ttZ + tWZ and tZq signal categories is determined directly through a global fit, enabling improved sensitivity in future effective field theory analyses.
• The multiclass DNN achieves effective separation of signal and background categories, significantly reducing contamination from nonprompt leptons and WZ events.
• The differential cross sections are unfolded using a profile likelihood approach, accounting for detector effects and systematic uncertainties with high precision.