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Abstract

A measurement of the top-quark pole mass m pole t is presented in tt¯ events with an additional jet, tt¯+ 1-jet, produced in pp collisions at √ s = 13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb−1 . Events with one electron and one muon of opposite electric charge in the final state are selected to measure the tt¯+ 1-jet differential cross-section as a function of the inverse of the invariant mass of the tt¯+ 1-jet system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp → ttj¯ (2 → 3), where top quarks are taken as stable particles, and the process pp → b ¯bl+νl−νj¯ (2 → 7), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ 2 fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the 2 → 3 measurement: m pole t = 170.7±0.3 (stat.)±1.4 (syst.)±0.3 (scale)±0.2 (PDF⊕αS) GeV, which is in good agreement with other top-quark mass results.