Measurements of $tar{t}$ in association with charm quarks at 13 TeV with the ATLAS experiment

This talk presents the ATLAS Collaboration’s first measurement of the inclusive cross-section for top-quark pair production in association with charm quarks. Using the full Run 2 proton-proton collision data sample at $\sqrt{s}$ = 13 TeV, collected with the ATLAS experiment at the LHC between 2015 and 2018, the measurement selects $t\bar{t}$ events with one or two charged leptons and at least one additional jet in the final state. A custom flavour-tagging algorithm is employed to simultaneously identify $b$-jets and $c$-jets. The fiducial cross-sections for $t\bar{t}+{\geq}2c$ and $t\bar{t}+1c$ production are found to largely agree with predictions from various $t\bar{t}$ simulations, though all underpredict the observed values.

💡 Research Summary

The ATLAS Collaboration has performed the first inclusive measurement of top‑quark pair production in association with charm quarks ($t\bar t + c$) using the full Run 2 proton–proton data set at $\sqrt{s}=13,$TeV, corresponding to an integrated luminosity of 140 fb$^{-1}$ collected between 2015 and 2018. The analysis selects events with one or two charged leptons (electrons or muons) and at least one additional jet. A dedicated flavour‑tagging algorithm, the $b/c$‑tagger, is introduced to simultaneously identify $b$‑jets and $c$‑jets. The tagger re‑optimises the standard DL1r $b$‑tagger output into a two‑dimensional discriminant, defining five working points: two for $c$‑jets (11 % and 22 % efficiencies), two for $b$‑jets (60 % and 70 % efficiencies), and an untagged region.

Events are divided into single‑lepton and dilepton channels, further split by jet multiplicity, yielding 19 orthogonal regions (12 control regions and 7 signal regions). Control regions either veto $c$‑tagged jets or require exactly one $c$‑tag, thereby constraining the dominant backgrounds ($t\bar t + \ge 1b$, $t\bar t$ + light‑flavour jets, single‑top, $W/Z$+jets, diboson). Signal regions require at least two $c$‑tagged jets (single‑lepton) or at least one $c$‑tagged jet (dilepton), providing sensitivity to $t\bar t + \ge 2c$ and $t\bar t + 1c$ respectively.

Monte‑Carlo simulations model the signal and backgrounds. Inclusive $t\bar t+$jets are generated in the five‑flavour scheme with POWHEG‑BOX 2 interfaced to PYTHIA 8 (NNPDF3.0 NLO PDFs). The $t\bar t+b\bar b$ component is modelled in the four‑flavour scheme using POWHEG‑BOX RES, OpenLoops, and either PYTHIA 8 or HERWIG 7. Additional samples (MadGraph5_aMC@NLO + HERWIG 7) are used for cross‑checks. Since dedicated $t\bar t + c\bar c$ calculations are not available, charm jets arise from gluon splitting $g\to c\bar c$ in the parton‑shower. Events are categorised at particle level into $t\bar t + \ge 2c$, $t\bar t + 1c$, $t\bar t + \ge 2b$, and $t\bar t$ + light, and the corresponding samples are combined while avoiding double‑counting.

A profile likelihood fit simultaneously extracts the signal strengths for $t\bar t + \ge 2c$ and $t\bar t + 1c$, together with normalisation factors for the $t\bar t + \ge 1b$ and $t\bar t$ + light backgrounds. Systematic uncertainties (modelling of $t\bar t + \ge 1c$, $t\bar t + \ge 1b$, $t\bar t$ + light, parton‑shower matching, $b/c$‑tagger efficiencies and mis‑tag rates, jet energy scale, lepton identification, and luminosity) are incorporated as nuisance parameters with Gaussian constraints. The fit achieves a goodness‑of‑fit of 98 % (saturated model).

The measured fiducial cross‑sections are
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