ATLAS and CMS measurements of the $t\bar{t}$ cross section, including off-shell and near threshold

Abstract: Recent measurements of the $t\bar{t}$ cross section, performed both inclusively and differentially by the ATLAS and CMS Collaborations, are reported. In particular, off-shell effects are probed in the $pp\to W^+bW^-\bar{b}$ and $pp\to e^\pmμ^\mp +b\bar{b}$ processes, and modelling aspects of the POWHEG bb4$\ell$ Monte Carlo generator are discussed. Cross section and properties measurements are also performed at the threshold: we review an indirect extraction of the top quark Yukawa coupling, as well as the recent observations by both experiments of an excess of events near the top pair production threshold that is consistent with the formation of quasi-bound states.

• The paper presents precision measurements of t-tbar production using both low-pileup and high-luminosity data, comparing results with NNLO+NNLL predictions.
• It details advanced Monte Carlo modeling improvements that address off-shell effects, interference, and residual mismodeling in differential observables.
• It reveals near-threshold enhancements consistent with toponium signatures and uses these findings to indirectly constrain the top quark Yukawa coupling.

Review of "ATLAS and CMS measurements of the $t\bar{t}$ cross section, including off-shell and near threshold" (2604.01984)

Introduction

This paper provides a comprehensive status report on precision measurements of the top-antitop ($t\bar{t}$) production cross section at the LHC, based on the high-luminosity Run 2 data collected by ATLAS and CMS. The work emphasizes inclusive and differential measurements, addresses off-shell and near-threshold effects, and investigates phenomenology beyond the Standard Model (SM) via quasi-bound state searches and indirect Yukawa coupling extractions. The study systematically compares results from different Monte Carlo (MC) implementations, scrutinizing the sources and limitations of current theoretical and experimental systematics.

Precision $t\bar{t}$ Cross Section Measurements

The analysis documents two strategies for cross section measurement. In a low-pileup run at $\sqrt{s}=5.02$ TeV, CMS achieves $$\sigma(t\bar{t}) = 62.5 \pm 1.6\ (\text{stat.})^{+2.6}_{-2.5}\ (\text{syst.}) \pm 1.2\ (\text{lumi.})$$ pb [CMS:2024ghc], consistent with NNLO+NNLL predictions but limited by matrix element/parton shower matching, $b$-tagging, and trigger uncertainties. In contrast, a high-luminosity analysis by ATLAS at $\sqrt{s}=13$ TeV focuses on the clean $e\mu+b\bar{b}$ final state, utilizing $140$ fb$^{-1}$ to extract $t\bar{t}$0 pb [ATLAS:2025iza], in excellent agreement with NNLO+NNLL calculations.

The study critically examines MC modeling: standard NLO-accurate \textsc{Powheg} hvq + \textsc{Pythia} or \textsc{Herwig} significantly underperforms for $t\bar{t}$1-sensitive observables, whereas kinematic reweighting to higher-order QCD+EW corrections or usage of MiNNLOps leads to superior description of lepton-based distributions. The $t\bar{t}$2 observable, sensitive to near-threshold and off-shell topologies, exhibits residual mismodeling even with advanced MC tools, highlighting a limitation of current generator accuracy.

Off-shell and Interference Effects in $t\bar{t}$3 Production

A substantial portion of the analysis is devoted to the modeling of off-shell and non-resonant contributions. At leading order, $t\bar{t}$4 final states receive contributions from doubly-resonant $t\bar{t}$5, single-resonant $t\bar{t}$6, and non-resonant $t\bar{t}$7 diagrams. Interference effects at NLO are not fully captured by the widely used Diagram Removal (DR) or Diagram Subtraction (DS) schemes, which are not gauge invariant and lead to significant theoretical uncertainties in the tails of kinematic distributions.

The \textsc{Powheg} bb4 approach provides a unified $t\bar{t}$8 matrix element description including full off-shell and interference contributions at NLO [ATL-PHYS-PUB-2025-036]. Its improved treatment mitigates ad hoc "lineshape" and DR/DS uncertainties and more accurately describes leptonic and inclusive differential observables. Nevertheless, this gain is accompanied by heightened sensitivity to variations in $t\bar{t}$9 during Sudakov evolution, and the lack of complete NNLO corrections to bb4 is highlighted as a current theoretical bottleneck.

Experimentally, precision unfolded measurements in the semi-leptonic channel by ATLAS confront the modeling limitations of DR/DS and narrow-width approximations, with significant discrepancies emerging in high-invariant-mass regimes [ATLAS:2025fhs]. The direct confrontation of these measurements with bb4 and MiNNLOps MC tools underscores the necessity for improved treatment of both production and decay matrix elements.

Near-Threshold Dynamics and Fundamental Parameter Extraction

Toponium Formation

Both ATLAS and CMS have recently reported statistically significant excesses ($t\bar{t}$0) near the $t\bar{t}$1 production threshold, interpreted as evidence for quasi-bound color-singlet, spin-singlet $t\bar{t}$2 (toponium) states [CMS:2025kzt, ATLAS:2026dbe]. These resonant enhancements, absent in pQCD matrix elements, are modeled using NRQCD-inspired Green’s functions or explicit pseudo-scalar $t\bar{t}$3 hypotheses. The measured inclusive cross sections ($t\bar{t}$4 pb) slightly overshoot NRQCD predictions ($t\bar{t}$5 pb), with systematic uncertainties dominated by the pQCD $t\bar{t}$6 background and details of the signal modeling.

These measurements utilize spin-sensitive observables to distinguish toponium from the SM continuum. The emergence of toponium signatures—the first robust evidence for such near-threshold enhancements at hadron colliders—demonstrates the capacity of the LHC Run 2 dataset for probing subtle, non-perturbative QCD phenomena in the heaviest SM sector.

Indirect Measurement of the Top Quark Yukawa Coupling

A novel analysis by ATLAS leverages virtual Higgs exchange effects in the near-threshold $t\bar{t}$7 region as a probe of the top Yukawa coupling $t\bar{t}$8 [ATLAS:2025ciy]. By fitting the $t\bar{t}$9 distribution to a parameterization sensitive to $\sqrt{s}=5.02$0, and incorporating toponium effects as well as all relevant experimental uncertainties, the analysis yields $\sqrt{s}=5.02$1 ($\sqrt{s}=5.02$2) and sets an upper limit $\sqrt{s}=5.02$3 at 95% CL. This agrees with SM predictions but remains limited by systematics associated with $\sqrt{s}=5.02$4 modeling and the jet energy scale.

Implications and Outlook

The reported suite of measurements illustrates that the LHC's available data and advancing experimental techniques enable differential top quark studies extending into both off-shell and near-threshold kinematic regimes. The precise measurements yield crucial information for validating and constraining QCD and EW theory at high accuracy, and for probing small couplings and rare QCD phenomena such as formation of quasi-bound states.

Theoretical progress remains crucial, particularly in removing the remaining modeling uncertainties in off-shell and decay matrix element treatments (NNLO corrections to bb4), and in refining non-perturbative calculations for threshold phenomena. Extensions to higher-order corrections and more sophisticated MC reweighting schemes are expected to enhance the reach and systematic precision in future analyses, especially as the larger Run 3 datasets become available.

These studies have immediate impact for SM parameter extractions, indirect BSM searches through the Yukawa sector, and the refinement of the MC tools upon which both precision tests and new physics searches rely.

Conclusion

This work exemplifies the maturation of precision top quark physics at the LHC, leveraging large datasets and improved MC tools to probe SM dynamics up to and beyond the current theoretical modeling frontier. The robust observation of threshold enhancements consistent with toponium, and the ability to set limits on the top Yukawa via indirect means, solidify the LHC as a unique facility for both QCD and EW precision studies. Continued theoretical development and experimental scrutiny, particularly at the level of off-shell and non-resonant contributions, are necessary to fully exploit future datasets and sharpen indirect probes of fundamental SM couplings.