Flavour-Changing Neutral Current Top Decays in the Three Higgs Doublet Model

TL;DR

This paper investigates rare top quark decays mediated by flavor-changing neutral currents within a Three Higgs Doublet Model, revealing potential enhancements over Standard Model predictions and prospects for detection at the High-Luminosity LHC.

Contribution

It provides the first detailed calculation of one-loop induced top FCNC decays in the Three Higgs Doublet Model with specific scalar potential and flavor conservation assumptions.

Findings

Branching ratios can significantly exceed Standard Model expectations.

Certain parameter regions predict observable rates at the High-Luminosity LHC.

Light non-standard scalars can enhance rare top decay signals.

Abstract

We study flavour-changing neutral current decays of the top quark in the democratic Three Higgs Doublet Model featuring a $Z_3$-symmetric scalar potential and Natural Flavour Conservation. In this framework, while such processes are absent at tree-level, the extended scalar sector induces new one-loop contributions to rare top decays. We compute the branching ratios for processes of the form $t \to q X$ (with $q = u, c$ and $X$ denoting a boson of the model), and explore the viable regions of the parameter space under theoretical consistency conditions and current experimental constraints. Several alignment-limit scenarios corresponding to different hierarchies among the CP-even Higgs states are analysed, and we find that the predicted branching ratios can significantly exceed their Standard Model expectations while remaining consistent with existing limits. In particular, we identify scenarios with light non-standard scalars that can lead to rates within the projected sensitivity of the High-Luminosity LHC. Our results therefore highlight rare top decays as a promising probe of the extended scalar sector of the Three Higgs Doublet Model.