Search for flavor-changing neutral currents in top quark decays t -> Hc and t -> Hu in multilepton final states in proton-proton collisions at root s=13 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  fb−1 of 𝑝⁢𝑝 collisions at a center-of-mass energy of √𝑠 =13  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⁡(𝑡 →𝐻⁡𝑐) <0.16% and ℬ⁡(𝑡 →𝐻⁡𝑢) <0.19% at 95% confidence level are obtained (with expected limits of 0.15% in both cases)