Search for flavor-changing neutral currents in top quark decays $t\to Hc$ and $t\to Hu$ in multilepton final states in proton-proton collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

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 $36.1\text{}\text{}{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ 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 $B\left(t\to Hc\right)<0.16%$ and $B\left(t\to Hu\right)<0.19%$ at 95% confidence level are obtained (with expected limits of 0.15% in both cases).