Speakers
Description
Flavour-changing neutral currents in the top-quark sector are heavily suppressed in the Standard Model. However, several theories beyond the Standard Model predict significant enhancements for these transitions, motivating searches for them at current and future collider experiments.
In this talk, studies for the sensitivity to a left-handed $tZc$ coupling $g$ are presented. In contrast to traditional searches that focus on leptonic $Z$ boson decays, an approach based on the interference of diagrams containing the $t \to b\overline{b}c$ transition in the decay of top-quark pairs is chosen. In the Standard Model, this transition occurs via the CKM-suppressed $W^{+} \to \overline{b}c$ decay, while the $Z \to b \overline{b}$ decay contributes in the case of a non-vanishing coupling $g$.
The signal topology comprises four $b$-tagged jets, one lepton, and missing transverse energy. The expected $95\%$ CLs upper limit on the coupling $g$ is obtained using the output distributions of a parametrised multiclass neural network. The interference-based approach is projected to be slightly less sensitive than the traditional approach at the HL-LHC, while the limits become comparable for an FCC-hh scenario, highlighting the advantage of the presented method when probing increasingly smaller couplings with large statistical power.
