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 pp 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→Hc)<0.16% and B(t→Hu)<0.19% at 95% confidence level are obtained (with expected limits of 0.15% in both cases).

Aaboud M., A.G. (2018). Search for flavor-changing neutral currents in top quark decays t →hc and t →hu in multilepton final states in proton-proton collisions at s =13 TeV with the ATLAS detector. PHYSICAL REVIEW D, 98(3), 032002-1-032002-25 [10.1103/PhysRevD.98.032002].

Search for flavor-changing neutral currents in top quark decays t →hc and t →hu in multilepton final states in proton-proton collisions at s =13 TeV with the ATLAS detector

Alberghi G. L.;Bellagamba L.;Bindi M.;Boscherini D.;Cabras G.;Caforio D.;De Castro S.;Di Sipio R.;Fabbri L.;Franchini M.;Gabrielli A.;Giacobbe B.;Lasagni Manghi F.;Macchiolo A.;Massa L.;Mengarelli A.;Monzani S.;