Long-lived oscillons and domain walls in the $$\mathbb {Z}_{2}$$-symmetric two-Higgs-doublet model

Abstract

Domain walls arising from spontaneous symmetry breaking in the early universe are typically unstable and decay rapidly. However, numerical simulations of the

-symmetric two-Higgs-doublet model (2HDM) reveal an unexpected long-lived closed domain wall structure that emerges at late stages of decay, which we identify as a high-amplitude oscillon. Their longevity stems from a potential with two distinct vacua: an interior oscillating in one vacuum and an exterior static in the other. Within a specific parameter regime, radiation is highly suppressed and the lifetime can exceed cosmological timescales. To gain analytical insight into the mechanism, we first consider reduced models with fewer fields (2 and 4 real scalars). These are not approximations to the 2HDM but rather toy models that capture the essential feature of a

$$\mathbb {Z}_{2}$$ Z 2

-symmetric potential with two vacua. Floquet analysis further confirms the stability of these structures under perturbations in both the 4-field toy model and the full 2HDM.