ATLAS has measured the production rate for the tZq process at 95.2 fb, with a total uncertainty of 10.2%, and found it compatible with the cited Standard Model prediction. In particle physics, a cross-section is a way of expressing how often a process is produced. The study also reports differential cross-sections, the top-quark spin asymmetry and limits on dimension-six interactions involving the top quark.
The result uses ATLAS proton-proton collisions collected from 2015 to 2018 at 13 TeV, corresponding to 140 fb−1 of integrated luminosity. The selected events contain three light leptons and two to four jets, one or two of them initiated by b quarks, placing the measurement in a defined trilepton channel.
The main rate is close to prediction
To obtain the inclusive number, the collaboration used a binned profile-likelihood fit with Poisson yield terms and nuisance parameters. In practical terms, the event yields were compared with the model while uncertainty sources were allowed for in the fit. The reported goodness-of-fit value was 0.9.
The measured 95.2 fb was compared with an 84.0 fb five-flavour-scheme next-to-leading-order QCD prediction. The statistical uncertainty was +8.5/−7.9 fb and the systematic uncertainty was +5.5/−5.3 fb. Statistical uncertainty was the larger part of the total: 8.5%, against 5.6% systematic and 10.2% overall.
Among the listed systematic sources, tZq modelling made the largest contribution at 4.3%.
Top and antitop rates measured separately
The collaboration separated production of top quarks from production of top antiquarks. It measured a top-quark cross-section of 56.0 fb and an antitop cross-section of 39.3 fb, giving a top-to-antitop ratio of 1.42. The nominal prediction for that ratio was 1.79, while the ratio’s uncertainty was +0.27/−0.22 from statistics and ±0.05 from systematic effects.
Measurements tied to defined phase spaces
ATLAS also reported particle-level and parton-level fiducial cross-sections. Fiducial measurements are defined within specified phase spaces, so they provide a controlled way to compare the selected data with predictions. The combined particle-level measurement was 5.36 fb, close to the 5.41 fb Monte Carlo prediction. At parton level, the corresponding values were 12.60 fb and 12.92 fb.
The combined particle-level result carried statistical uncertainty of +0.49/−0.47 fb and systematic uncertainty of +0.27/−0.28 fb. At parton level, the statistical uncertainty was +1.15/−1.07 fb and the systematic uncertainty was +0.65/−0.64 fb.
The distributions tell the same story
To measure how tZq production is distributed across phase space, the analysis used profile-likelihood unfolding at both particle and parton level. The method uses response matrices plus simulation-based acceptance and selection-efficiency corrections. The unfolded spectra were generally described by five-flavour-scheme predictions; all reported compatibility p-values were greater than 0.19, and statistical uncertainty dominated every bin.
The study also measured the top quark’s spin asymmetry from an unfolded polarization-angle distribution. The asymmetry was 0.34, with an uncertainty of ±0.14 from statistics and ±0.06 from systematic effects. A simultaneous fit gave a parton-level fiducial cross-section of 13.4 fb, with statistical and systematic uncertainties of ±1.5 fb and ±0.7 fb, respectively.
Nine coefficients constrained
The authors also used SMEFT to test whether additional dimension-six interaction terms are compatible with the data. The interpretation considered seven such operators producing nine Wilson coefficients. It used second-order polynomial detector-level parameterizations and fitted each coefficient individually, keeping the others at zero.
Individual limits were reported for all nine Wilson coefficients, and ATLAS found no significant deviation from the Standard Model. One coefficient, C_Ht, had a best fit and 68% interval displaced from zero, but zero remained within its 95% confidence interval. Because the coefficients were varied one at a time, these are individual limits rather than the result of a simultaneous fit.
Precision still depends on more data and modelling
The analysis is restricted to the trilepton channel, selected jet and b-jet multiplicities, and defined dilepton or fiducial phase spaces. The inclusive and differential results remain statistically limited, while the unfolding depends on simulation-derived response matrices, acceptance corrections and selection efficiencies.
A broader global SMEFT fit would be needed to combine these limits with other processes and address possible operator degeneracies. The current result is therefore a set of one-coefficient-at-a-time constraints, not a freely varying multi-coefficient interpretation.
The document is an arXiv preprint, version 1 of arXiv:2608.25476, dated 26 August 2026 and submitted to the European Physical Journal C on 27 August. The authors describe it as the most precise ATLAS tZq study to date.
Paper data and sources
Original title: Measurement of $tZq$ inclusive and differential cross-sections in $pp$ collisions at $\sqrt{s} = 13~\text{TeV}$ with the ATLAS detector
Authors: ATLAS Collaboration
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text