# Search for the $t\to ch$ decay at hadron colliders

## TL;DR

This study evaluates the potential to observe the rare top quark decay $t o ch$ at current and future hadron colliders, analyzing different Higgs decay channels and collider energies within the Type-III Two-Higgs Doublet Model framework.

## Contribution

It provides a comprehensive Monte Carlo analysis of $t o ch$ decay observability across multiple collider scenarios and constrains the decay within the THDM-III model.

## Key findings

- $t o ch$ decay is undetectable at current LHC luminosities.
- Future colliders like HL-LHC, HE-LHC, and FCC-hh could potentially discover the decay.
- Constraints on the THDM-III parameter space are derived from Higgs coupling measurements.

## Abstract

We study the observability for the flavor-changing decay of a top quark $t\to ch$ at the LHC and future hadron colliders, namely, High-Luminosity LHC (HL-LHC), High-Energy LHC (HE-LHC) and Future Circular hadron-hadron Collider (FCC-hh). Two scenarios in which the Higgs boson could decay: into a quark bottom pair $(\textit{bb-channel})$ and two photons $(\textit{$\gamma\gamma$-channel})$ are analyzed. A Monte Carlo analysis of the signal and the Standard Model (SM) background is computed. Center-of-mass energies of $\sqrt{s}=14,\;27\; $\text{and}$\; 100$ TeV and integrated luminosities from $0.3$ to $30$ ab$^{-1}$ are explored. The theoretical framework adopted in this work is the Type-III Two-Higgs Doublet Model (THDM-III) for which, constraints on the parameter space from the Higgs boson coupling modifiers $\kappa_i$ are presented and used in order to evaluate the branching ratio of the $t\to ch$ decay and the ($pp\to t\bar{t},\,t\to ch$) production cross section. We find that with the integrated luminosity achieved at the LHC, the $t\to ch$ decay is out of the reach of detection. More promising results emerge for the HL-LHC, HE-LHC and FCC-hh in which potential discoveries could be claimed.