Tracing the anomalous $tqg$ and $tq\gamma$ flavor changing interactions at the FCC-he

TL;DR

This paper explores the potential of the FCC-he collider to detect and distinguish flavor-changing neutral current couplings of the top quark with gluon and photon, using angular distributions and electron polarization.

Contribution

It introduces a method to identify the nature of FCNC couplings of the top quark at FCC-he using electron and top polarization, and estimates the sensitivity to these couplings.

Findings

Couplings can be probed down to 10^{-2} levels.

Branching ratios for top decays to uγ and cγ are constrained to 4-7×10^{-6} and 1-2×10^{-5}.

Limits on gluon couplings lead to BR(t→ug) ≤ 1.7×10^{-5}.

Abstract

We investigate the possible presence of Flavor Changing Neutral Current (FCNC) couplings of the top quark with gluon and photon through $e^-p\to e^-tj$ process at the Future Circular Collider in the proton-electron mode (FCC-he). Focusing on disentangling the effects of different couplings that could be present, we exploit the presence of the scattered electron, the angular distribution of which is sensitive to the type of coupling involved. Top quark polarisation accessed through the angular distribution of the decay lepton provides additional handle in identifying the nature of the couplings. Further, we demonstrate the potential of electron beam polarisation in distinguishing the left-handed and right-handed couplings of both gluon and photon separately. Considering an $e^-p$ collider of beam energies of $E_{e(p)} = 60~(50000)$~GeV at 2~ab$^{-1}$ integrated luminosity, couplings can be probed at the level of $10^{-2}$ with the corresponding branching fractions of ${\rm BR}(t\to u\gamma)\le 4 - 7 \times 10^{-6}$ and ${\rm BR}(t\to c\gamma) \le 1-2 \times 10^{-5}$, depending on if the coupling is right-handed or left-handed. The corresponding limits on the gluon couplings lead to ${\rm BR}(t\to ug)\le 1.7 \times 10^{-5}$ and ${\rm BR}(t\to cg) \le 3-4 \times 10^{-5}$.