# Double Higgs boson production at $e^+e^-$ colliders in the two-Higgs-doublet model

## TL;DR

This paper investigates how double Higgs production at electron-positron colliders can be significantly enhanced in the two-Higgs-doublet model due to heavy Higgs resonances, especially outside the alignment limit, with implications for collider experiments.

## Contribution

It demonstrates the potential for large cross section enhancements in double Higgs production within the 2HDM, linking these effects to Higgs coupling modifications and experimental constraints.

## Key findings

- Cross section can be enhanced by hundreds of percent due to heavy Higgs resonances.
- Strong correlation between cross section enhancement and Higgs coupling scaling factors.
- Enhancements are significant outside the alignment limit, constrained by current data.

## Abstract

We study the double Higgs boson production processes $e^+e^- \to hh f\bar{f}$ ($f\neq t$) with $h$ being the 125 GeV Higgs boson in the two-Higgs-doublet model with a softly-broken $Z_2$ symmetry. The cross section can be significantly enhanced, typically a few hundreds percent, as compared to the standard model prediction due to resonant effects of heavy neutral Higgs bosons, which becomes important in the case without the alignment limit. We find a strong correlation between the enhancement factor of the cross section and the scaling factor of the $hf\bar{f}$ couplings under constraints from perturbative unitarity, vacuum stability and current experimental data at the LHC as well as the electroweak precision data.