Vacuum Instability and False Vacuum Decay Induced by Domain Walls in the N2HDM

TL;DR

This paper investigates how domain walls in the N2HDM can induce vacuum decay by eliminating the barrier between minima, potentially ruling out parameter points previously considered long-lived or stable.

Contribution

It demonstrates that domain walls can cause classical roll-over to deeper minima in the N2HDM, altering vacuum stability assessments.

Findings

Domain walls can eliminate the barrier between vacua.

Vacuum decay can occur inside domain walls even for long-lived minima.

Some parameter points are ruled out due to domain wall effects.

Abstract

The Next-to-Two-Higgs-Doublet model (N2HDM) has a rich vacuum structure where multiple electroweak (EW) breaking minima, as well as CP and electric-charge breaking minima, can coexist. These minima can be deeper than the electroweak vacuum $v_{ew} \approx 246\text{ GeV}$ of our universe, making our vacuum metastable. In such a case, one needs to calculate the tunneling rate from the EW vacuum to the deeper minimum. If the lifetime of the EW vacuum is longer than the universe's age, our vacuum is deemed long-lived, and the parameter point is, in principle, allowed. If the decay rate is smaller than the universe's age, then our vacuum is unstable and the parameter point is ruled out. However, domain walls (DW) in the N2HDM can substantially alter this picture. We show in this work that inside the DW, the barrier between our electroweak minimum and the deeper minimum can disappear, leading the scalar fields to classically roll over to the deeper minimum that nucleates inside the DW and then expands outside of it everywhere in the universe. We show that such behavior can happen to parameter points where the lifetime of our minimum is even several orders of magnitude larger than the age of the universe. Such parameter points with a metastable EW minimum are ruled out.