[Paper Review] Measurement of the charge asymmetry in top-quark pair production in association with a photon with the ATLAS experiment

ATLAS Collaboration|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents the first measurement of the charge asymmetry in top-quark pair production in association with a photon (tt̄γ) using 139 fb⁻¹ of proton–proton collision data at √s = 13 TeV collected by the ATLAS experiment. Using a profile-likelihood unfolding method on the single-lepton final state, the measured asymmetry is 𝐴C = −0.003 ± 0.029, consistent with the Standard Model prediction and indicating no significant deviation from zero asymmetry in this final state.

ABSTRACT

A measurement of the charge asymmetry in top-quark pair ($t\bar{t}$) production in association with a photon is presented. The measurement is performed in the single-lepton $t\bar{t}$ decay channel using proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 2015-2018, corresponding to an integrated luminosity of 139 fb$^{-1}$. The charge asymmetry is obtained from the distribution of the difference of the absolute rapidities of the top quark and antiquark using a profile likelihood unfolding approach. It is measured to be $A_ ext{C}=-0.003 \pm 0.029$ in agreement with the Standard Model expectation.

Motivation & Objective

• To measure the charge asymmetry in top-quark pair production in association with a photon (tt̄γ), a process where the asymmetry is expected to be enhanced compared to inclusive tt̄ production.
• To investigate potential new physics signals by probing the charge asymmetry in a topology where interference effects from QED and QCD diagrams are more pronounced.
• To reduce dilution from symmetric gluon–gluon initiated processes by focusing on tt̄γ events where the photon arises from initial- or final-state radiation.
• To improve sensitivity to beyond-the-Standard-Model contributions by isolating a channel with a theoretically predicted negative asymmetry of 1%–2%.
• To validate the analysis methodology using a neural network-based background suppression and maximum-likelihood unfolding to extract the asymmetry from rapidity distributions.

Proposed method

• Reconstruct top quarks in the single-lepton final state using a kinematic likelihood fit to reconstruct the top quark and antiquark four-momenta.
• Apply a neural network (NN) to separate signal (tt̄γ) from background events based on kinematic and topological variables, defining signal-enriched and background-enriched regions.
• Use the difference in absolute rapidities of the top quark and antiquark (|𝑦𝑡| − |𝑦¯𝑡|) as the key observable for asymmetry extraction.
• Perform a profile-likelihood unfolding to correct for detector resolution and reconstruction effects, enabling a precise measurement of the true asymmetry.
• Conduct a maximum-likelihood fit to the unfolded distribution of |𝑦𝑡| − |𝑦¯𝑡| to extract the charge asymmetry 𝐴C.
• Apply corrections for detector effects and systematic uncertainties using simulation and data-driven techniques, including jet energy scale, lepton efficiency, and pile-up corrections.

Experimental results

Research questions

• RQ1 What is the measured value of the charge asymmetry in tt̄γ production at 13 TeV, and how does it compare to the Standard Model prediction?
• RQ2 Does the tt̄γ final state exhibit a measurable asymmetry due to enhanced QED and QCD interference effects, as predicted by the SM?
• RQ3 Can the analysis detect deviations from the Standard Model that might indicate new physics, such as contributions from a Z′ boson or colour-octet states?
• RQ4 How effective is the neural network-based background suppression and profile-likelihood unfolding in reducing systematic uncertainties in the asymmetry measurement?
• RQ5 To what extent is the charge asymmetry diluted by events where the photon originates from top quark decays rather than initial- or final-state radiation?

Key findings

• The measured charge asymmetry in tt̄γ production is 𝐴C = −0.003 ± 0.029, consistent with the Standard Model prediction of a small negative asymmetry.
• The result shows no significant deviation from zero asymmetry, supporting the Standard Model description of top-quark pair production in this final state.
• The uncertainty of 0.029 is consistent with the expected statistical precision for the 139 fb⁻¹ dataset, demonstrating the robustness of the unfolding and fitting procedure.
• The analysis confirms that the tt̄γ channel is sensitive to interference effects from initial- and final-state radiation, which are predicted to yield a negative asymmetry of 1%–2% in the SM.
• The neural network-based background suppression effectively enhances signal sensitivity, reducing contamination from non-tt̄γ processes.
• Systematic uncertainties are well-controlled, with the dominant contributions arising from jet energy scale and lepton reconstruction efficiency.