This paper reports a search by the ATLAS experiment for a new heavy particle that would couple mainly to the top quark. The hypothetical particle, called Z′ (Z-prime), would be a colour‑singlet vector boson that can be produced together with one or two top quarks and then decay into a top–antitop pair. The search used proton–proton collision data recorded at centre‑of‑mass energies of 13 TeV and 13.6 TeV, corresponding to data samples of 140 inverse femtobarns and 56 inverse femtobarns respectively (a measure of the total amount of collisions collected).

The analysis looks for events with unusual lepton patterns that are rare in the Standard Model. Specifically, it selects events with exactly two leptons of the same electric charge, or at least three leptons, where “leptons” means electrons or muons (including those from tau decays). These final states reduce ordinary background from common processes and are sensitive to events with multiple top quarks. To separate any potential signal from remaining background, the team trained a transformer‑based machine‑learning classifier that was parameterised as a function of the assumed Z′ mass.

The results are interpreted in a simplified theory where the Z′ couples only to top quarks with a coupling strength c_t and a chirality angle θ that controls whether the coupling prefers left‑ or right‑handed top quarks. The search includes two main production mechanisms: production with a top–antitop pair (tt̄) and single‑top‑like electroweak production. At leading order the tt̄‑associated production rate does not depend on the chirality angle, while the single‑top‑like mode does and can be much smaller for some chirality choices. Signal events were simulated for Z′ masses between 400 GeV and 3000 GeV; the relative width of the resonance grows with mass, from about 4% at 400 GeV to about 35% at 3000 GeV. Standard Model backgrounds considered include processes that produce top quarks together with W, Z or Higgs bosons and other multi‑top final states.