Inclusive top cross sections in ATLAS

The ATLAS collaboration at the LHC has published inclusive cross-section measurements for the single-top and $t\overline{t}$ production modes at center-of-mass energies of $\sqrt{s} = 5.02, 8.16$, $13$, and $13.6$ TeV. Single-top measurements are conducted in the $t$-channel and $tW$ channel. In addition to the nominal cross-section measurements, various measurements of other interesting observables such as the $V_{tb}$ element of the Cabibbo Kobayashi Maskawa (CKM) quark-mixing matrix, the ratio of the inclusive cross-sections between $tq$ and $t\overline{q}$, the ratio of inclusive cross-sections between $t\overline{t}$ and $Z\rightarrow \ell\ell$, and the nuclear modification factor (defined as the ratio of the inclusive $t\overline{t}$ cross section in heavy-ion collisions to the inclusive $t\overline{t}$ cross-section in $pp$ collisions) are also reported. These results are compared to their corresponding SM predictions, calculated at (N)NLO in QCD. All results are in good agreement with SM predictions.

💡 Research Summary

The ATLAS Collaboration has produced a comprehensive set of inclusive cross‑section measurements for top‑quark production at the LHC, covering both single‑top (t‑channel and tW) and top‑antitop (tt̄) processes across a wide range of centre‑of‑mass energies (√s = 5.02, 8.16, 13, 13.6 TeV). The paper summarises results obtained with the full Run 2 dataset (up to 140 fb⁻¹) and the early Run 3 dataset (29 fb⁻¹), as well as a dedicated proton‑lead (p‑Pb) data sample (165 nb⁻¹).

For the tW associated production at √s = 13 TeV, a dileptonic (e±μ∓) final state with at least one b‑tagged jet was selected. Events were split into three orthogonal signal regions (1j1b, 2j1b, 2j2b) and a Boosted Decision Tree (BDT) was trained to separate tW from the dominant tt̄ background. A binned profile‑likelihood fit to the BDT output yielded σ(tW) = 75 ± 1(stat) + 15/−14(syst) ± 1(lumi) pb, in excellent agreement with the NNLO+NNLL SM prediction of 79.3 pb. The dominant systematic uncertainties stem from tt̄ modelling (13.2 %), jet‑energy scale (12 %) and missing‑transverse‑energy reconstruction (11 %). From the measured cross‑section the effective left‑handed coupling |fLV Vtb| was extracted as 0.97 ± 0.10, consistent with the SM expectation of unity.

The t‑channel single‑top measurement was performed at both 13 TeV (140 fb⁻¹) and 5.02 TeV (255 pb⁻¹). At 13 TeV, events with exactly one lepton, two jets (one b‑tagged) were selected, and the lepton charge defined two signal regions (SR⁺, SR⁻). A neural‑network discriminant (Dnn) was built from several kinematic variables; a profile‑likelihood fit to Dnn in each region gave σ(tq) = 137 ± 8 pb and σ(t̄q) = 84 + 6/−5 pb, matching NNLO+NNLL predictions of 134.2 pb and 80.0 pb respectively. The ratio R_t = σ(tq)/σ(t̄q) = 1.636 + 0.036/−0.034 provides a high‑precision observable that can be used to constrain parton‑distribution functions (PDFs). The extracted |fLV Vtb| = 1.015 ± 0.031 again agrees with the SM.

At the lower energy of 5.02 TeV, the analysis faced a low‑pile‑up environment (⟨μ⟩≈2) and consequently larger statistical uncertainties (≈16 %). The measured cross‑sections were σ(tq) = 19.8 + 3.9/−3.1(stat) + 2.9/−2.2(syst) pb and σ(t̄q) = 7.3 + 3.2/−2.1(stat) + 2.8/−1.5(syst) pb, compatible with SM expectations but with a relative uncertainty of about 58 %. The ratio R_t = 2.73 + 1.43/−0.82(stat) + 1.01/−0.29(syst) carries a large error, reflecting the limited dataset.

The inclusive tt̄ cross‑section at √s = 13.6 TeV was measured using 29 fb⁻¹ of Run 3 data. The analysis required two opposite‑flavour leptons (eμ) and one or two b‑jets. Four signal regions were defined based on jet and b‑jet multiplicities, and a simultaneous profile‑likelihood fit to the event yields in these regions gave σ(tt̄) = 850 ± 3(stat) ± 18(syst) ± 20(lumi) pb. This result is consistent within one standard deviation with the NNLO+NNLL prediction of 924 + 32/−40 pb. In the same dataset, the Z→ℓℓ cross‑section was measured (σ_Z = 744 ± 11(stat+syst) ± 16(lumi) pb) and the ratio R_tt̄/Z = 1.145 ± 0.003(stat) ± 0.021(syst) ± 0.002(lumi) was derived. The ratio is particularly sensitive to the gluon‑to‑quark PDF ratio, offering a powerful constraint for future PDF fits.

Top‑pair production was also studied in proton‑lead collisions at √s_NN = 8.16 TeV using 165 nb⁻¹ of data. Both dileptonic (2ℓ + b‑jets) and semileptonic (ℓ + jets) final states were examined. The scalar sum of transverse momenta (H_T) of leptons and jets served as the discriminating observable in a profile‑likelihood fit across six signal regions. The measured inclusive tt̄ cross‑section is σ(tt̄) = 58.1 ± 2.0(stat) + 4.8/−4.4(syst) nb, compatible with predictions from various nuclear‑PDF sets. The nuclear modification factor, defined as R_pA = σ(pPb tt̄)/(A·σ(pp tt̄)) with A = 208, was found to be 1.090 ± 0.039(stat) + 0.094/−0.087(syst), consistent with the SM expectation of unity. The dominant systematic uncertainties arise from the jet‑energy scale (4.6 %) and the tt̄ generator modelling (4.5 %).

Across all measurements, the extracted values of the CKM matrix element |Vtb| (through the effective coupling |fLV Vtb|) are compatible with the SM value of one, and the various cross‑section ratios (σ(tq)/σ(t̄q), σ(tt̄)/σ(Z), and R_pA) all agree with theoretical predictions within their uncertainties. The precision achieved for the tt̄ cross‑section at 13.6 TeV (≈3 % relative uncertainty) represents one of the most accurate top‑quark measurements to date, while the p‑Pb result opens a new avenue for probing nuclear PDFs with top quarks. Overall, the ATLAS inclusive top‑quark measurements provide stringent tests of perturbative QCD at NNLO+NNLL accuracy and furnish essential inputs for PDF determinations and searches for physics beyond the Standard Model.