Top-Quark Pair Production in Heavy-Ion Collisions in the ATLAS Experiment

This paper presents the first observation and measurement of top-quark pair production in both proton-lead and lead-lead collisions using the ATLAS experiment, establishing these events as powerful probes for studying nuclear parton distribution functions and the dynamics of the quark-gluon plasma.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smasher. Usually, it smashes two tiny protons together. But sometimes, it smashes a single proton into a giant lead nucleus (a "proton-lead" collision) or smashes two giant lead nuclei together (a "lead-lead" collision).

This paper is about a specific experiment using the ATLAS detector to watch what happens when these heavy collisions occur, specifically looking for the creation of top quarks.

Here is the story of the paper, broken down into simple concepts:

1. The "Heavyweight Champion" of Particles

Think of the top quark as the heavyweight champion of the particle world. It is the heaviest known elementary particle. Because it is so heavy, it's like trying to lift a grand piano with a single finger; it takes a massive amount of energy to create one.

The scientists wanted to see if they could create pairs of these "heavyweight champions" (a top quark and an anti-top quark) inside the chaotic, super-dense environment of heavy-ion collisions.

2. The Two Experiments

The researchers looked at two different types of collisions, like testing a car on two different tracks:

Track A: The Proton-Lead Collision (p+Pb)

• The Setup: They smashed a single proton into a lead nucleus.
• The Goal: They wanted to see how the "stuff" inside the lead nucleus (called nuclear parton distribution functions, or nPDFs) affects the creation of top quarks. Imagine the lead nucleus as a crowded dance floor. Does the crowd make it harder or easier for two dancers (the top quarks) to meet and pair up?
• The Result: They successfully found the top quark pairs. They measured exactly how often this happened and compared it to what happens when protons smash into other protons.
• The Finding: The rate at which top quarks were made was almost exactly what they expected if the lead nucleus was just a scaled-up version of a proton. It was like finding that the crowded dance floor didn't actually stop the dancers from pairing up. This was the first time scientists measured this specific "crowd effect" for top quarks.

Track B: The Lead-Lead Collision (Pb+Pb)

• The Setup: They smashed two massive lead nuclei together. This creates a super-hot, super-dense soup of particles called the Quark-Gluon Plasma (QGP). Think of this as turning the dance floor into a boiling pot of soup.
• The Goal: They wanted to see if the top quarks could survive and be detected in this boiling soup. Since the top quark is so heavy, it's a unique probe to study how this soup evolves over time.
• The Result: This was a huge milestone. They successfully spotted the top quark pairs in this environment for the first time ever.
• The Finding: They saw the signal clearly (with a statistical certainty of 5 standard deviations, which in science means "we are almost 100% sure this isn't a fluke"). They measured how often these pairs appeared and found it matched predictions based on how the "soup" should behave.

3. The "Detective Work"

How did they find these invisible particles?

• Top quarks decay (break apart) almost instantly.
• The scientists acted like detectives looking for specific clues left behind: electrons, muons (heavy cousins of electrons), and jets of particles.
• They built six different "search zones" (signal regions) in their data, looking for specific combinations of these clues.
• They used powerful computer models to predict what the background noise (random particle collisions) would look like and subtracted it to find the "signal" (the top quarks).

4. The Bottom Line

• In Proton-Lead collisions: They confirmed that top quarks are produced at the expected rate, giving them a new tool to understand the internal structure of heavy atomic nuclei.
• In Lead-Lead collisions: They achieved a historic "first observation." They proved that top quarks can be created and detected even in the extreme environment of the quark-gluon plasma.

Why does this matter?
The paper concludes that because top quarks are so heavy and short-lived, they act like perfect "time capsules." By studying how they behave in these collisions, scientists can learn new things about the "soup" (QGP) that existed just after the Big Bang and how the building blocks of matter are arranged inside heavy atoms.

In short: The ATLAS team successfully found the universe's heaviest particles in two different types of heavy collisions, proving they can be used as powerful tools to study the fundamental nature of matter.

Technical Summary

Technical Summary: Top-Quark Pair Production in Heavy-Ion Collisions in the ATLAS Experiment

Problem and Motivation
Top-quark pair ($t\bar{t}$) production in heavy-ion collisions serves as a critical probe for two distinct areas of nuclear physics: the characterization of nuclear parton distribution functions (nPDFs) in kinematic regimes that are currently poorly constrained, and the investigation of the time evolution of strongly interacting matter, specifically the quark-gluon plasma (QGP). While top quarks are the heaviest particles carrying color charge, their production in heavy-ion environments (proton–lead and lead–lead) had not been fully established or measured with high precision prior to this work. The study aims to observe $t\bar{t}$ production in both proton–lead (p+Pb) and lead–lead (Pb+Pb) collisions and to quantify deviations from proton–proton (pp) expectations using the nuclear modification factor ($R_{pA}$).

Methodology
The analysis utilizes data recorded by the ATLAS detector at the Large Hadron Collider (LHC) during Run 2.

• p+Pb Collisions: Data collected in 2016 at a nucleon–nucleon center-of-mass energy of $\sqrt{s_{NN}} = 8.16$ TeV, corresponding to an integrated luminosity of 165 nb$^{-1}$. The analysis reconstructs $t\bar{t}$ events in the lepton+jets and dilepton channels.

  • Selection: Events require single-electron or single-muon triggers ($p_T > 15$ GeV). Leptons must satisfy $p_T > 18$ GeV and specific pseudorapidity ($|\eta|$) and identification/isolation criteria. Jets are reconstructed using the anti-$k_t$ algorithm ($R=0.4$) with $p_T > 20$ GeV, and $b$-jets are identified using the DL1r algorithm.
  • Background Estimation: Fake-lepton backgrounds are estimated using the Matrix Method.
  • Signal Regions: Six signal regions are defined based on the scalar sum of lepton and jet transverse momenta ($H_T^{\ell j}$) and the number of $b$-tagged jets: four in the lepton+jets channel ($1\ell1b$ and $1\ell2b_{incl}$ for electrons and muons) and two in the dilepton channel ($2\ell1b$ and $2\ell2b_{incl}$).
  • Analysis Technique: A profile-likelihood fit is employed to extract the signal strength ($\mu_{t\bar{t}}$), defined as the ratio of the observed cross-section to the Standard Model expectation.

• Pb+Pb Collisions: Data recorded in 2015 and 2018 at $\sqrt{s_{NN}} = 5.02$ TeV, with an integrated luminosity of 1.9 nb$^{-1}$.

  • Selection: The analysis focuses on the electron–muon ($e\mu$) final state within the 0–80% centrality interval. Triggers require $p_T > 15$ GeV (electrons) and $p_T > 8$ GeV (muons). Candidates must satisfy $p_T > 18$ GeV (electrons) and $p_T > 15$ GeV (muons) with 'Loose' identification.
  • Background Handling: Underlying event contributions are subtracted on an event-by-event basis, and fake-lepton contributions are estimated via data-driven methods.
  • Signal Regions: Two regions are defined based on the transverse momentum of the $e\mu$ pair ($p_T^{e\mu}$): SR1 ($p_T^{e\mu} > 40$ GeV) and SR2 ($p_T^{e\mu} \le 40$ GeV).
  • Analysis Technique: A profile-likelihood fit is performed on the $e\mu$ invariant mass ($m_{e\mu}$) distributions in both signal regions.

Key Contributions and Results

1. p+Pb Measurements:

   • Observation: $t\bar{t}$ production is observed in both the lepton+jets and dilepton channels with significances exceeding 5 standard deviations in each.
   • Cross-Section: The inclusive $t\bar{t}$ production cross-section is measured as $\sigma_{t\bar{t}}^{p+Pb} = 58.1^{+5.2}_{-4.9}$ nb. This represents the most precise $t\bar{t}$ cross-section measurement in heavy-ion collisions to date, with a total relative uncertainty of 9%.
   • Nuclear Modification Factor: For the first time, the nuclear modification factor $R_{pA}$ is measured for this process. The result is $R_{pA} = 1.090 \pm 0.100$. This value is consistent with the expectation from scaled pp collisions within one standard deviation.
   • Theoretical Comparison: Both the cross-section and $R_{pA}$ are consistent with predictions from four nPDF sets: TUJU21, nNNPDF3.0, nCTEQ15HQ, and EPPS21.

2. Pb+Pb Measurements:

   • Observation: This work presents the first observation of $t\bar{t}$ production in Pb+Pb collisions, achieving a significance of 5.0 standard deviations.
   • Cross-Section: The inclusive $t\bar{t}$ production cross-section in the 0–100% centrality class is determined to be $\sigma_{t\bar{t}}^{Pb+Pb} = 3.6^{+1.2}_{-1.0}$ $\mu$b. The total relative uncertainty is 31%, dominated by statistical contributions (26%).
   • Theoretical Comparison: The measured cross-section agrees with CMS measurements in Pb+Pb collisions and ATLAS pp measurements at $\sqrt{s}=5.02$ TeV (scaled by $A_{Pb}^2$). It is also consistent with calculations using the same four nPDF sets used for the p+Pb analysis.

Significance
The paper establishes top-quark pairs as valuable tools for investigating heavy-ion collisions. The precision of the p+Pb measurement provides new constraints on nPDFs at high Bjorken-$x$. Furthermore, the first observation of $t\bar{t}$ in Pb+Pb collisions opens new opportunities to explore the time evolution of the QGP, specifically through the study of hadronic W-boson decays originating from top quarks. These results pave the way for future studies of nPDFs and QGP properties at the LHC.