Large triple Higgs couplings in the 2HDM at $e^+e^-$ colliders

Abstract: Within the framework of the $\cal CP$ conserving Two Higgs Doublet Models (2HDM) type I and II we investigate the di-Higgs production at future $e^+e^-$ colliders in order to find effects coming from triple Higgs couplings. We define and explore some benchmark planes that show large values of these couplings, still in agreement with all the relevant theoretical and experimental constraints. Within those planes two production channels are considered: $e^+e^-\to h_ih_jZ$ and $e^+e^-\to h_ih_j\nu\bar{\nu}$, with $h_ih_j=hh,\ HH\ \mathrm{and}\ AA$. We discuss on the sensitivity to $\kappa_\lambda:=\lambda_{hhh}/\lambda_{hhh}^\mathrm{SM}$ and $\lambda_{hhH}$ in the $hh\nu\bar{\nu}$ production and to $\lambda_{hHH}$ ($\lambda_{hAA}$) in the $HH\nu\bar{\nu}$ ($AA\nu\bar{\nu}$) production at CLIC 3 TeV via the cross section distribution on the invariant mass of the final-state Higgs-pair.