Flavor-changing neutral currents are not present in the Standard Model at tree level and are suppressed in loop processes by the unitarity of the Cabibbo-Kobayashi-Maskawa matrix; the corresponding rates for top quark decay processes are experimentally unobservable. Extensions of the Standard Model can generate new flavor-changing neutral current processes, leading to signals which, if observed, would be unambiguous evidence of new interactions. A data set corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider is used to search for top quarks decaying to up or charm quarks with the emission of a Higgs boson, with subsequent Higgs boson decay to final states with at least one electron or muon. No signal is observed and limits on the branching fractions $\mathcal{B}(t\to Hc)<0.16\%$ and $\mathcal{B}(t\to Hu)<0.19\%$ at 95% confidence level are obtained (with expected limits of 0.15% in both cases).

• Received 10 May 2018

DOI:https://doi.org/10.1103/PhysRevD.98.032002

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© 2018 CERN, for the ATLAS Collaboration