# High-scale validity of a two Higgs doublet scenario: predicting collider signals

## TL;DR

This paper explores the collider signals of a two Higgs doublet model that maintains vacuum stability up to the Planck scale, focusing on the detectability of near-degenerate heavy scalars at future colliders.

## Contribution

It provides a detailed analysis of collider signatures within a constrained 2HDM parameter space, emphasizing the potential to distinguish heavy scalar states at high-luminosity colliders.

## Key findings

- LHC can distinguish neutral heavy scalar and pseudoscalar with >3σ significance.
- Muon colliders can probe scalar masses up to about 1 TeV.
- Parameter space consistent with stability, unitarity, and collider constraints is identified.

## Abstract

It is possible to ameliorate the Higgs vacuum stability problem by switching over to two Higgs doublet models (2HDM), ensuring a stable electroweak vacuum up to the Planck scale, even though the top quark mass may be on the high side. However, the simultaneous requirements of perturbative unitarity, and also compatibility with collider and flavour data, constrain the parameter space severely. We investigate the collider signals answering to the regions allowed by such constraints. In particular, the near degeneracy of the neutral heavy scalar and the pseudoscalar is a feature that is probed. The LHC allows distinguishability of these two states, together with signal significance of at least 3$\sigma$, in its high-luminosity run. While $e^+ e^-$ colliders may have rather low event rates, muon colliders, cashing on the principle of radiative return, can probe 2HDM scenarios with (pseudo)scalar masses up to a TeV or so, though with the price of losing distinction between the CP-even and odd states.