⚛️ high-energy experiments

Differential cross-section measurements of coherent production of singly and doubly resonant top-quark in $WWbb$ events with one lepton at $\sqrt{s}$ = 13 TeV with the ATLAS detector

Using 140 fb $^{-1}$ of 13 TeV proton-proton collision data collected by the ATLAS detector, this paper presents differential cross-section measurements of $WWbb$ events that reveal significant discrepancies between data and theoretical predictions in regions sensitive to the interference between singly and doubly resonant top-quark production, thereby providing new insights into top-quark production mechanisms and off-shell effects.

Original authors: ATLAS Collaboration

Published 2026-09-18

📖 5 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

At the heart of the Large Hadron Collider, a massive ring of magnets buried beneath the border of France and Switzerland, scientists smash protons together at nearly the speed of light. The goal is to recreate the conditions of the universe just moments after the Big Bang, allowing particles to emerge that are too heavy or unstable to exist in our everyday world. Among these fleeting visitors, the top quark stands out as the heaviest known elementary particle. It is so massive that it weighs roughly as much as a gold atom, yet it decays almost instantly into other particles. Because it is so heavy, the top quark is a unique laboratory for testing the Standard Model, the theory that describes how the fundamental building blocks of the universe interact. Physicists are particularly interested in how top quarks are created. They can appear in pairs, a process that happens frequently, or they can appear alone, a rarer event. While these two ways of creating top quarks have been studied separately for years, a lingering question remains: do they interfere with each other? In the quantum world, particles can exist in multiple states at once, and when different ways of creating the same final result overlap, they can cancel each other out or amplify each other, much like two sound waves meeting to create a louder or quieter noise. Understanding this interference is crucial because it affects how accurately we can predict what happens in these high-energy collisions, which in turn is essential for spotting new, unknown physics hiding in the data.

A researcher using the ATLAS detector, one of the giant instruments at the collider, has now taken a fresh look at this problem. They analyzed a vast collection of collision data recorded between 2015 and 2018, corresponding to 140 inverse femtobarns of proton-proton collisions at an energy of 13 tera-electronvolts. Instead of looking at the messy, raw data directly, the team focused on a specific signature: events where a single charged particle, either an electron or a muon, flies out alongside missing energy and a cluster of jets, some of which are known to come from bottom quarks. This specific combination of debris is the hallmark of a top quark decaying, but it can arise from two different sources: a pair of top quarks or a single top quark produced alongside a W boson. The researcher wanted to see if the current computer simulations, which are used to predict what these collisions should look like, could accurately describe the data when both sources are present and interfering.

To do this, the researcher did not just count how many events they found. They measured the "differential cross-section," a sophisticated way of mapping out how often these events occur across different ranges of energy and momentum. They broke the data down into three distinct regions to probe different aspects of the physics. The first was a broad, inclusive region designed to capture the widest possible range of events. The second focused specifically on cases where the W boson decayed into jets, allowing them to study the properties of that particle in detail. The third was a "search-like" region, tuned to look at the most extreme corners of the data where the interference between the single and double top quark production is expected to be strongest. By carefully reconstructing the original particles from the detector signals and correcting for the limitations of the machine, they produced a clean, particle-level view of the events.

The results revealed that the current best theories are not telling the whole story. When the researcher compared their measurements against the most advanced computer simulations available, they found that none of the models could simultaneously describe all the different ways the particles moved and interacted. The discrepancy was most noticeable in the regions where the interference between the two types of top quark production is significant. In these areas, the simulations either overestimated or underestimated the number of events, or they got the shape of the energy distributions wrong. Specifically, the models struggled to accurately predict the behavior of the particles in the high-energy tails of the distributions and in the specific kinematic region designed to highlight the interference effects. The uncertainty in these measurements is dominated by how well the simulations model the interference itself, suggesting that the theoretical tools used to calculate these complex quantum interactions need refinement.

This work does not claim to have discovered a new particle or a breakdown of the laws of physics, but it does highlight a gap in our understanding of how the Standard Model operates in these complex scenarios. The findings provide a new set of precise measurements that future computer programs must be able to reproduce. By showing exactly where the current predictions fail, this study gives theorists a clear target for improvement. It suggests that the way we currently separate the calculation of single top quarks from double top quarks, or how we handle the quantum interference between them, is not quite right. As the Large Hadron Collider continues to operate and collect more data, these refined measurements will serve as a critical benchmark, ensuring that the next generation of simulations can accurately model the quantum dance of the heaviest known particle, paving the way for the discovery of physics beyond our current understanding.

Technical Summary: Differential Cross-Section Measurements of Coherent Top-Quark Production in $W^+W^-b\bar{b}$ Events

Problem and Motivation
The study of top-quark production is central to the Large Hadron Collider (LHC) physics programme. Traditionally, top-quark production has been categorized into two distinct processes: doubly resonant top-quark pair production ( $t\bar{t}$ ) and singly resonant top-quark production ( $tW$ , $t$ -channel, $s$ -channel). However, at Next-to-Leading Order (NLO) in Quantum Chromodynamics (QCD), the associated production of a single top quark and a $W$ boson ( $tWb$ ) shares the same final state as $t\bar{t}$ production ( $W^+W^-b\bar{b}$ ). The separation of these processes is arbitrary and relies on theoretical approximations that treat interferences and off-shell (finite-width) effects separately. This separation introduces significant uncertainties in Standard Model (SM) measurements and searches for physics beyond the Standard Model (BSM).

To address this, a coherent treatment of the complete process $pp \to W^+W^-b\bar{b}$ is required, encompassing doubly resonant, singly resonant, non-resonant diagrams, and their interferences. While theoretical calculations for this process exist up to NLO, experimental validation has been limited, particularly in final states with a single lepton. Previous measurements assessing these predictions were largely restricted to dilepton final states. This paper aims to provide the first differential cross-section measurements of coherent singly and doubly resonant top-quark production in the single-lepton channel, specifically targeting kinematic regions where interference effects are enhanced.

Methodology
The analysis utilizes 140 fb $^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV collected by the ATLAS detector during Run 2 (2015–2018). The target topology consists of events with one charged lepton ( $e$ or $\mu$ ), missing transverse momentum ( $E_T^{\text{miss}}$ ), and at least four jets, two of which are identified as $b$ -jets.

The analysis defines three distinct kinematic regions to probe different aspects of the $W^+W^-b\bar{b}$ phase space:

Inclusive Region (IR): Designed to study both single and double top-quark processes and their interference across the broadest phase space.
$W$ -Hadronic Region (WR): Optimized to directly probe the $W^+W^-b\bar{b}$ process by reconstructing the hadronically decaying $W$ boson ( $W_{\text{had.}}$ ) and studying its kinematic properties alongside $b$ -jet observables. This region constrains higher-order corrections and real emission modeling.
Search-Like Region (SLR): Inspired by BSM searches, this region targets extreme phase space corners where interference effects between $t\bar{t}$ and $tWb$ are enhanced. Selection criteria include high transverse momenta for $b$ -jets and specific requirements on the invariant mass of the lepton- $b$ system ( $m_{b\ell}^{\text{min}} \ge 150$ GeV) to suppress on-shell $t\bar{t}$ events.

Unfolding and Simulation
Differential cross-sections are extracted at the particle level using an unfolding procedure (TUnfold) to correct for detector effects. The fiducial volumes and observables are defined at the particle level to match the detector-level selections.

Signal Modeling: The signal processes ( $t\bar{t}$ and $tWb$ ) are modeled using the Powheg Box v2 generator interfaced with Pythia 8.2. The analysis employs the Diagram Removal (DR) scheme as the nominal prediction for combining $t\bar{t}$ and $tW$ processes at NLO, while the Diagram Subtraction (DS) scheme is used as an alternative to estimate modeling uncertainties related to double-counting removal.
Background Estimation: Standard Model backgrounds (e.g., $W$ +jets, dibosons, single top) are estimated using Monte Carlo simulations, with the exception of QCD multi-jet backgrounds, which are estimated using a data-driven fake-factor method. The normalization of the $W$ +jets background is constrained via a dedicated control region.

Key Contributions and Results
The paper presents unfolded differential cross-sections for a range of kinematic observables, including:

The invariant mass of the lepton and $b$ -jet system ( $m_{b\ell}^{\text{min}}$ ).
Transverse momenta of the lepton and $b$ -jets.
Transverse mass variables ( $m_T^\ell$ , $m_T^{\text{min}}(b\ell)$ ).
Kinematic properties of the hadronic $W$ candidate in the WR region ( $p_T(W_{\text{had.}})$ , $y(W_{\text{had.}})$ ).
Composite variables like $m_{bW}^{\text{minimax}}$ and the invariant mass of the $Wb\bar{b}\ell$ system.

The results are compared against several state-of-the-art theoretical predictions, including:

Powheg Box v2 + Pythia 8.2 (DR and DS schemes).
MiNNLOPS (incorporating NNLO QCD corrections for $t\bar{t}$ ).
Powheg Box v2 + Herwig 7 (alternative shower model).

Observations:

Discrepancies in Interference Regions: Significant deviations are observed between the unfolded data and theoretical predictions in regions sensitive to the interference between singly and doubly resonant top-quark production. The disagreement is most pronounced in the SLR and in the tails of the $m_{b\ell}^{\text{min}}$ distribution.
Modeling Limitations: No single prediction simultaneously describes all observables across all kinematic regions. For instance, while the MiNNLOPS prediction improves the description of $p_T$ spectra in the IR where $t\bar{t}$ dominates, it fails to describe the data in the WR and SLR regions as well as other models in specific kinematic regimes.
Uncertainty Sources: The dominant systematic uncertainties arise from the modeling of the interference between $t\bar{t}$ and $tWb$ (specifically the choice between DR and DS schemes) and detector-related uncertainties (jet energy scale and $b$ -tagging). In the SLR, the uncertainty related to the interference modeling is particularly large, leading to discrepancies of approximately one standard deviation when included, and increasing to two or three standard deviations when excluded.

Significance and Claims
The author states that this measurement provides the first differential cross-section results for coherent singly and doubly resonant top-quark production in the single-lepton channel. The unfolded results offer new information regarding the modeling of interference and off-shell effects in the $W^+W^-b\bar{b}$ final state.

The paper claims that these results:

Improve the understanding of top-quark production mechanisms, particularly the interplay between resonant and non-resonant contributions.
Provide critical inputs for the development of future Monte Carlo generators, highlighting areas where current theoretical approximations (such as the separation of $t\bar{t}$ and $tW$ ) are insufficient.
Demonstrate that current state-of-the-art simulations struggle to simultaneously describe the full kinematic phase space, particularly in regions dominated by interference effects.

The author concludes that while the findings are consistent with other recent analyses in the dilepton channel, the single-lepton channel offers access to a larger number of observables sensitive to interference effects and provides a complementary perspective on the modeling of the $W^+W^-b\bar{b}$ process. The results are intended to serve as a benchmark for refining theoretical predictions and reducing uncertainties in future top-quark physics and BSM searches.

More like this