Single-top associated production with a Z or H boson at the LHC: the SMEFT interpretation

At the LHC, top quarks can be produced singly with a sizeable rate via electroweak interactions. This process probes a limited set of top-quark electroweak couplings, i.e. , the same entering the top-quark decay, yet at higher scales and with a different sensitivity. Requiring the production of a $Z$ or $H$ boson in association with single-top significantly extends the sensitivity of this process to new physics, opening up the unique possibility of testing top-Higgs, top-gauge, triple gauge, gauge-Higgs interactions without being dominated by QCD interactions. We consider $tZj$ and $tHj$ production at the LHC, providing predictions at next-to-leading accuracy in QCD in the framework of the standard model effective field theory, including all relevant operators up to dimension six. We perform the first complete study of the sensitivity to new interactions of these processes, highlighting the interplay and complementarity among $tj$, $tZj$ and $tHj$ in simultaneously constraining top-quark, triple gauge, and gauge-Higgs interactions in the current and future runs at the LHC.