# On the validity of perturbative studies of the electroweak phase transition in the Two Higgs Doublet model

## TL;DR

This study uses nonperturbative lattice simulations within an effective theory to evaluate the electroweak phase transition in the Two Higgs Doublet model, highlighting limitations of perturbative methods especially with large scalar couplings.

## Contribution

It demonstrates that perturbative approaches can be improved with effective theory techniques, but face limitations with very large scalar couplings in the Two Higgs Doublet model.

## Key findings

- Perturbative methods have shortcomings in infrared resummation.
- Effective theory improves perturbative accuracy for moderate couplings.
- Strong phase transitions with large couplings are not reliably studied with current methods.

## Abstract

Making use of a dimensionally-reduced effective theory at high temperature, we perform a nonperturbative study of the electroweak phase transition in the Two Higgs Doublet model. We focus on two phenomenologically allowed points in the parameter space, carrying out dynamical lattice simulations to determine the equilibrium properties of the transition. We discuss the shortcomings of conventional perturbative approaches based on the resummed effective potential - regarding the insufficient handling of infrared resummation but also the need to account for corrections beyond 1-loop order in the presence of large scalar couplings - and demonstrate that greater accuracy can be achieved with perturbative methods within the effective theory. We find that in the presence of very large scalar couplings, strong phase transitions cannot be reliably studied with any of the methods.