# Revisiting Vector-like Quark Model with Enhanced Top Yukawa Coupling

## TL;DR

This paper explores a vector-like quark model with an enhanced top Yukawa coupling, showing that such scenarios are compatible with LHC and precision data and revealing a relation between loop contributions in Higgs pair production.

## Contribution

It demonstrates the viability of models with large Yukawa couplings and negative VLQ Yukawa, and uncovers a specific relation between triangle and box diagram contributions in gg → hh.

## Key findings

- Parameter space consistent with LHC bounds is wide.
- Enhanced top Yukawa can be realized in strongly interacting theories.
- Relation between triangle and box diagram contributions in Higgs pair production.

## Abstract

We revisit a scenario with an enhanced top yukawa coupling in vector-like quark (VLQ) models, where the top yukawa coupling is larger than the standard model value and the lightest VLQ has a negative yukawa coupling. We find that the parameter space satisfying the LHC bounds of the Higgs signal strengths consistently with the precision measurements is rather wide. Because the Lagrangian parameters of the yukawa couplings are large, such scenario can be realized in some strongly interacting theories. It also turns out that there is a noticeable relation between the contributions of the triangle and box diagrams in the $gg \to hh$ process by using the lowest order of the $1/M$ expansion where $M$ is the heavy mass running in the loops.