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

The top-quark mass is measured using top-quark decays producing an isolated lepton and $J\mathrm{/}\psi$ meson reconstructed in its ${\mu }^{+}{\mu }^{-}$ decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ 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 $m(\mathrm{\ell }{\mu }^{+}{\mu }^{-})$ of the system made of the isolated lepton $\mathrm{\ell }$ from the $W$ boson decay and the non-isolated ${\mu }^{+}{\mu }^{-}$ pair from a $J\mathrm{/}\psi$ decay of a $b$-hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the $m(\mathrm{\ell }{\mu }^{+}{\mu }^{-})$ distribution is performed to extract the top-quark mass. The top-quark mass is measured to be $m_{top}=172.17\pm 0.80(stat)\pm 0.81(syst)\pm 1.07(recoil)$ 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.