TOP2024: an overview of experimental results

Imagine the top quark as the "heavyweight champion" of the particle world. Discovered in 1995, it is so massive that it lives for only a split second before vanishing. Because it is so heavy, it interacts strongly with the mechanism that gives particles mass (the Higgs field), making it a perfect "test subject" for scientists trying to understand the fundamental rules of the universe.

This paper is a report card from a recent gathering of scientists called TOP2024. It summarizes what the ATLAS and CMS experiments (two giant detectors at the Large Hadron Collider, or LHC) have learned about this heavyweight champion and where they plan to go next.

Here is a breakdown of the key points using everyday analogies:

1. The Big Picture: A Well-Stocked Library

Think of the LHC as a massive library where scientists are reading books about how the universe works. For the last 20 years, they have been reading the "Top Quark" section.

What they found: They have confirmed that top quarks are created in pairs (like twins) very often, and sometimes alone. They have measured how often this happens with incredible precision.
The Goal: By measuring these twins, scientists can check if the "rulebook" of physics (the Standard Model) is correct or if there are hidden chapters (New Physics) that we haven't discovered yet.

2. The New Tool: Machine Learning as a Super-Filter

In the past, sorting through the debris of particle collisions was like trying to find a specific needle in a haystack by hand. Now, scientists are using Machine Learning (ML), which acts like a super-smart, automated filter.

The Analogy: Imagine a bouncer at a club who can instantly tell the difference between a VIP guest (a top quark), a regular guest (a light quark), and a staff member (a gluon). The new AI "bouncers" are so good they can reject the wrong guests 2–3 times better than before while still letting the VIPs in.
Why it matters: This helps scientists measure the top quark's properties more accurately and saves them from needing to generate billions of computer simulations to fill in the gaps.

3. Looking at the Details: The "Slope" and the "Shadow"

Scientists don't just count how many top quarks they find; they look at how they are moving (their momentum).

The Mystery Slope: In the past, scientists noticed the top quarks were moving in a pattern that didn't quite match the predictions (a "slope" that was off). This led to a chain reaction of studies to figure out why.
The "Toponium" Ghost: Scientists are looking for a rare event where a top quark and an anti-top quark stick together briefly to form a "ghostly" pair called toponium. The CMS experiment saw a "bump" in the data that might be this ghost, but they need more data and better theory to confirm it's not just a trick of the light.
Quantum Entanglement: Even when two top quarks are far apart, they seem to be "linked" in a spooky way (quantum entanglement). The experiments confirmed this link exists even when the particles are moving so fast they are technically outside each other's "communication range." It's like two dice rolling in different rooms that always land on the same number, no matter how far apart they are.

4. The "Plus One" Events: Top Quarks with Friends

Sometimes, top quarks don't just appear in pairs; they show up with other particles, like a top quark bringing a friend (a bottom quark, a charm quark, or a vector boson).

The Challenge: Comparing results between the two big detectors (ATLAS and CMS) is like trying to compare two photos taken with different cameras and different lenses. The paper suggests they need to use the same "settings" (definitions and computer simulations) to make a fair comparison.
The Future: They are now looking for even rarer "party" combinations, like a top quark bringing a photon (light particle) or a Z boson.

5. Hunting for New Physics: The "EFT" Map

Scientists are using top quark measurements to hunt for "Beyond the Standard Model" (BSM) physics—things that shouldn't exist if our current rulebook is complete.

The Analogy: Think of the Standard Model as a map of a known city. Scientists are using the top quark as a drone to fly over the edges of the map to see if there are new continents (Dark Matter, new particles) hiding just beyond the horizon.
The Strategy: They are using a mathematical framework called Effective Field Theory (EFT) to organize all their measurements. It's like creating a giant spreadsheet where every measurement helps fill in a blank cell, narrowing down exactly where the "new physics" might be hiding.

6. The Road Ahead

The paper concludes that the field is moving fast. By the end of 2025, the LHC will have collected all its data for this run, and by the mid-2030s, a "High-Luminosity" upgrade will provide a flood of new data.

The Takeaway: The scientists are ready. They have the tools (AI), the data (from the LHC), and the teamwork (between ATLAS and CMS) to explore uncharted territory in the world of top quarks.

In short: This paper is a status update saying, "We have the best tools and the most data ever. We are measuring the heaviest particle with extreme precision, using AI to clean up the noise, and we are ready to find out if there is something new hiding in the shadows of our current understanding."

Technical Summary: TOP2024 Overview of Experimental Results

Problem and Context
The top quark, discovered in 1995, remains a central focus of particle physics due to its large mass, short lifetime, and Yukawa coupling near unity. These properties make it an ideal probe for studying bare quark properties, electroweak symmetry breaking, and potential new phenomena beyond the Standard Model (BSM). At the Large Hadron Collider (LHC), top quarks are produced primarily in pairs ( $t\bar{t}$ ) via strong interactions and, to a lesser extent, as single top quarks via electroweak processes. While most production modes are observed and measured, the field faces challenges in managing systematic uncertainties, handling the vast data samples expected from the High-Luminosity LHC (HL-LHC), and interpreting results within the context of BSM theories and Effective Field Theory (EFT). The 17th International Workshop on Top Quark Physics (TOP2024) aimed to summarize recent experimental results from the ATLAS and CMS collaborations and identify pathways for future investigations.

Methodology
The paper synthesizes experimental results presented at TOP2024, drawing on data collected by ATLAS and CMS. The methodology relies on:

High-Precision Measurements: Utilizing state-of-the-art analysis techniques and particle identification to measure inclusive and differential cross-sections for various top quark production modes ( $t\bar{t}$ , single-top, $t\bar{t}V$ , $t\bar{t}H$ , etc.).
Machine Learning (ML) Integration: Employing modern ML techniques, such as graph neural networks and advanced tagging algorithms (e.g., UPART, s-tagging), to improve jet flavor identification (specifically $b$ -tagging and $c$ -tagging) and to estimate systematic uncertainties without relying on prohibitively large Monte Carlo (MC) samples.
Differential and Multidifferential Analyses: Analyzing distributions of observables (e.g., transverse momentum $p_T$ , spin correlations, angular separations) to localize discrepancies with theory and extract Standard Model (SM) parameters like the strong coupling constant ( $\alpha_S$ ) and top quark mass.
Global Fits and EFT Interpretations: Combining likelihood functions from diverse measurements to constrain Wilson coefficients in the EFT Lagrangian, comparing experimental fits with theoretical global fits.

Key Contributions and Results
The paper outlines several key experimental achievements and observations presented at the workshop:

Machine Learning Advances:
- The ATLAS jet flavor tagger, using graph neural networks, achieves a rejection rate for $c$ -quark and light-quark/gluon jets that is 2–3 times higher than previous methods while maintaining 70% efficiency for $b$ -jets.
- The CMS UPART algorithm introduces a strange-tagging node, potentially enabling the identification of rare decays like $t \to Ws$ and probing the $V_{ts}$ CKM matrix element.
- ML techniques are utilized to estimate the effects of varied parameters in phase space, mitigating the need for large MC productions, a crucial capability for the HL-LHC era.
Differential Measurements and New Physics Searches:
- Toponium Search: CMS observed an excess with a significance $>5\sigma$ in the $t\bar{t}$ invariant mass ( $m_{t\bar{t}}$ ) distribution near the production threshold. This excess is compatible with a pseudoscalar toponium signal (though color-octet states are not accounted for in current calculations).
- Quantum Entanglement: Both ATLAS and CMS observed quantum entanglement between top quarks near the $t\bar{t}$ threshold. CMS further observed entanglement in regions of high $m_{t\bar{t}}$ and low scattering angles, where top quarks are outside each other's light cones, ruling out hidden variable transmission.
- Associated Production:
  - $t\bar{t}b\bar{b}$ and $t\bar{t}c\bar{c}$ measurements show trends consistent with theory but highlight the need for harmonized fiducial phase space definitions and common MC samples between experiments.
  - $t\bar{t}t\bar{t}$ production was observed in Run 2, with prospects for higher precision in Run 3. Off-shell $t\bar{t}t\bar{t}$ production provides constraints on the Higgs boson width and the top-Higgs Yukawa coupling ( $\kappa_t$ ).
  - Measurements of $t\bar{t}\gamma$ and $t\bar{t}Z$ are advancing, with CMS reporting a simultaneous differential measurement of $t\bar{t}Z$ and $tZq$ to better constrain electroweak interactions in the EFT context. Evidence for $tWZ$ production has been found, though dedicated $tW\gamma$ measurements are pending.
BSM and EFT Interpretations:
- CMS released a global EFT fit based on likelihoods from a wide range of measurements. This experimental fit is intended to be compared with theoretical global fits (e.g., constraining 50 Wilson coefficients using 455 data points).
- The paper notes that future experimental fits will expand to include more event signatures, updated process modeling, and ML techniques, as well as searches interpreted within the EFT framework.

Significance and Future Outlook
The paper claims that the top quark physics program at the LHC is advancing through the synergy of theoretical and experimental efforts. The significance of the TOP2024 results lies in:

Precision: Achieving high-precision measurements that constrain SM parameters and rule out various BSM possibilities.
Technique: Demonstrating the critical role of ML in handling systematic uncertainties and improving reconstruction, which is essential for the massive data volumes expected at the HL-LHC (starting mid-2030).
New Frontiers: Opening new avenues of inquiry, such as the study of quantum entanglement in top quark pairs and the search for bound states (toponium).
Collaboration: Emphasizing the need for harmonized definitions, common MC samples, and cross-experiment comparisons (e.g., between ATLAS and CMS) to facilitate robust physics interpretations.

The author concludes that while the full LHC data at $\sqrt{s} = 13.6$ TeV will be available by the end of 2025, the existing expertise and the upcoming HL-LHC upgrades provide a unique opportunity to explore uncharted territories in top quark physics. The paper serves as a summary of current capabilities and a modest suggestion for expanding research through better standardization and the integration of advanced computational methods.