⚛️ high-energy experiments

Observation of top-quark pair production in lead-lead collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV with the ATLAS detector

Using the ATLAS detector, researchers observed top-quark pair production in lead-lead collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV with a significance of 5.0 standard deviations, measuring a cross-section consistent with theoretical predictions and confirming the existence of all quark flavors in the high-energy-density environment of the quark-gluon plasma.

Original authors: ATLAS Collaboration

Published 2026-10-14

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: ATLAS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the earliest moments of the universe, just fractions of a second after the Big Bang, matter did not exist as the solid atoms we know today. Instead, it was a seething, super-hot soup of fundamental particles called quarks and gluons, free from the bonds that usually hold them together. Physicists call this state of matter the quark-gluon plasma. To understand how the universe evolved from this chaotic beginning into the structured cosmos we see now, scientists recreate these extreme conditions in the laboratory. They do this by smashing heavy atomic nuclei together at nearly the speed of light, generating temperatures trillions of degrees hotter than the center of the sun. For decades, researchers have studied how this plasma behaves, but a specific question remained unanswered: did the heaviest known particles, the top quarks, exist within this primordial soup? Because top quarks are so massive and decay so quickly, they were thought to be too rare to form in the brief, chaotic lifetime of the plasma created in these collisions.

A team of physicists using the ATLAS detector at the Large Hadron Collider has now answered that question with a definitive observation. By analyzing data from lead-ion collisions recorded in 2015 and 2018, the researchers found clear evidence that top quark pairs are indeed produced in these heavy-ion collisions. The team sifted through billions of collision events to find a very specific signature: a single electron and a single muon, accompanied by at least two jets of particles. This particular combination of debris is the telltale fingerprint of a top quark and its antimatter partner, the anti-top quark, being created and then decaying almost instantly. The researchers identified 32 such events in their data sample, a number large enough to rule out random chance. The statistical certainty of this finding is extremely high, equivalent to a five-sigma result, which in the language of particle physics means the observation is virtually certain to be real and not a fluke.

The significance of this discovery extends beyond simply counting particles. The top quark is unique because it is so heavy that it decays before it can form the composite particles, like protons or neutrons, that make up ordinary matter. This means the top quark exists as a free particle for a fleeting moment, offering a rare, unfiltered glimpse into the environment of the quark-gluon plasma. Because these particles appear and disappear so quickly, they act as probes that can reveal the time structure of the plasma itself, showing how the medium evolves in the very first instants after the collision. The researchers measured the rate at which these top quark pairs are produced and found it to be consistent with theoretical predictions that account for the complex behavior of particles inside a heavy nucleus. This agreement suggests that our current understanding of how matter behaves under these extreme conditions is on the right track.

This observation also confirms that all flavors of quarks, including the heaviest ones, are present in the pre-equilibrium stage of the quark-gluon plasma. This stage represents the moment just after the collision when the plasma is forming but has not yet settled into a uniform state. Finding top quarks here reinforces the idea that the conditions in these collisions mirror the extreme energy densities present in the early universe. While the measurement still carries a degree of uncertainty due to the limited amount of data available, the result is robust enough to be considered a solid observation rather than a mere hint. It opens a new window for studying the fundamental forces of nature, allowing scientists to use these heavy particles to test the limits of their theories about how the universe began and how matter is constructed at its most fundamental level.

Technical Summary: Observation of $t\bar{t}$ production in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV with the ATLAS detector

Problem and Motivation
Heavy-ion (HI) collisions at the Large Hadron Collider (LHC) recreate the quark-gluon plasma (QGP), a deconfined state of quarks and gluons believed to have existed in the early universe. While the QGP lifetime is extremely short ( $\approx 10^{-23}$ s), preventing direct measurement, particles produced in collisions serve as probes. Top quarks are of particular interest because their decay timescale is shorter than the hadronization timescale, meaning they decay before interacting strongly with the medium. This allows for the study of the time structure of the QGP via the subsequent decay of $W$ bosons from top quarks. Furthermore, top-quark measurements in HI collisions can constrain nuclear modifications to parton distribution functions (nPDFs) in kinematic regions poorly constrained by other data. While $t\bar{t}$ production has been observed in proton-lead ( $p$ +Pb) collisions and evidence has been found in lead-lead (Pb+Pb) collisions by the CMS collaboration, a definitive observation in Pb+Pb collisions by ATLAS was pending.

Methodology
The analysis utilizes data collected by the ATLAS detector during the 2015 and 2018 Pb+Pb run periods at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV, corresponding to an integrated luminosity of 1.9 nb $^{-1}$ .

Event Selection: The analysis targets the dilepton decay channel $t\bar{t} \to e\nu_e \mu\nu_\mu b\bar{b}$ $t \overset{ˉ}{t} \to e ν_{e} μ ν_{μ} b \overset{ˉ}{b}$ , which offers the lowest background contamination compared to same-flavor channels dominated by $Z \to \ell\ell$ $Z \to ℓℓ$ . Events are required to have exactly one electron and one muon with opposite electric charges, and at least two jets.
- Leptons: Electrons must have $p_T > 18$ GeV and $|\eta| < 2.47$ ; muons must have $p_T > 15$ GeV and $|\eta| < 2.5$ . Both must satisfy "Loose" identification criteria and isolation requirements.
- Jets: Reconstructed using the anti- $k_t$ algorithm with radius parameter $R=0.4$ , requiring $p_T > 35$ GeV and $|\eta| < 2.5$ . No $b$ -tagging is imposed to allow for the calibration of heavy-flavor jets in HI collisions.
- Centrality: Events are selected within the 0–80% centrality interval to minimize contributions from photon-induced processes.
Signal Regions: The sample is split into two signal regions (SR1 and SR2) based on the dilepton transverse momentum ( $p_T^{e\mu}$ ), with SR1 defined by $p_T^{e\mu} > 40$ GeV (signal-dominated) and SR2 by $p_T^{e\mu} \le 40$ GeV.
Background Estimation:
- Fake Leptons: Estimated using the $ABCD$ method in data, utilizing same-sign lepton pairs and inverted isolation criteria to define control regions.
- Physics Backgrounds: $Z$ +jets, single-top ( $tW$ ), and diboson ( $VV$ ) processes are modeled using Monte Carlo (MC) simulations (Powheg, Sherpa) normalized to theoretical cross-sections and validated in control regions.
Systematics and Corrections: Detector response corrections for leptons and jets are derived from $Z \to \ell\ell$ events and validated in data. Jet quenching effects, which are not explicitly modeled in the MC, are addressed by optimizing the jet selection (relying on jet multiplicity rather than kinematics) and deriving effective correction factors for the jet energy scale in central and peripheral collisions. Systematic uncertainties include signal modeling, jet energy scale, lepton reconstruction, and luminosity (1.5%).

Key Results

Observation: The analysis observes 22 events in SR1 and 10 events in SR2. The observed significance of the $t\bar{t}$ signal is 5.0 standard deviations, exceeding the expected significance of 4.1 standard deviations. This constitutes the first observation of $t\bar{t}$ production in Pb+Pb collisions by the ATLAS experiment.
Cross-Section Measurement: The measured inclusive $t\bar{t}$ production cross-section is:
$\sigma_{t\bar{t}} = 3.6^{+1.0}_{-0.9} \text{ (stat.)} ^{+0.8}_{-0.5} \text{ (syst.) } \mu\text{b}$
The total relative uncertainty is 31%, dominated by statistical uncertainties due to the limited data sample.
Comparison with Theory: The measured cross-section is consistent with Standard Model predictions calculated at next-to-next-to-leading-order (NNLO) and next-to-next-to-leading-logarithmic (NNLL) precision, scaled by the square of the lead mass number ( $A_{Pb}^2$ ). The results agree with predictions using various state-of-the-art nPDF sets (EPPS21, nCTEQ15HQ, nNNPDF30, TUJU21).

Significance and Claims
The paper claims that this observation consolidates the evidence for the existence of all quark flavors, including the top quark, in the pre-equilibrium stage of the QGP at very high energy densities, conditions analogous to the early universe. By successfully reconstructing top-quark pairs in heavy-ion collisions without $b$ -tagging requirements, this work establishes a foundation for future studies of hadronically decaying top quarks within the QGP. The author notes that while the current measurement agrees with theoretical predictions, further improvements in precision are required to effectively constrain nuclear parton distribution functions.

More like this