Measurement of the top-quark mass using decays with a J/ψ meson at $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

A bstract The top-quark mass is measured using top-quark decays producing an isolated lepton and J / ψ meson reconstructed in its μ + μ − decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $$ \sqrt{s}=13 $$ s = 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb − 1 . The measurement is based on the invariant mass m ( ℓμ + μ − ) of the system made of the isolated lepton ℓ from the W boson decay and the non-isolated μ + μ − pair from a J / ψ decay of a b -hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the m ( ℓμ + μ − ) distribution is performed to extract the top-quark mass. The top-quark mass is measured to be m top = 172.17 ± 0.80 (stat) ± 0.81 (syst) ± 1.07 (recoil) GeV, with a total uncertainty of 1.56 GeV. The third uncertainty arises from changing the dipole parton shower gluon-recoil scheme used in top-quark decays.