Domain walls in the Two-Higgs-Doublet Model and their charge and CP-violating interactions with Standard Model fermions

TL;DR

This paper explores the formation and properties of domain walls in the Two-Higgs-Doublet Model, focusing on their charge and CP-violating interactions with Standard Model fermions, revealing potential exotic phenomena.

Contribution

It introduces a detailed analysis of various classes of domain wall solutions in the 2HDM and their interactions with SM fermions, highlighting novel charge and CP-violating effects.

Findings

Multiple classes of domain walls with distinct properties identified.

Interactions lead to phenomena like fermion transmission and reflection.

Charge and CP violation effects within the domain walls analyzed.

Abstract

Discrete symmetries play an important role in several extensions of the Standard Model (SM) of particle physics. For instance, in order to avoid flavor changing neutral currents, a discrete $Z_2$ symmetry is imposed on the Two-Higgs-Doublet Model (2HDM). This can lead to the formation of domain walls (DW) as the $Z_2$ symmetry gets spontaneously broken during electroweak symmetry breaking in the early universe and domain walls form between regions whose vacua are related by the discrete symmetry. Due to this simultaneous spontaneous breaking of both the discrete symmetry and the electroweak symmetry, the vacuum manifold consists of two disconnected 3-spheres. Such a non-trivial disconnected vacuum manifold leads to several choices for the vacua at two adjacent regions, in contrast to models where only the discrete symmetry gets spontaneously broken and the vacuum manifold consists of several disconnected points. Due to this, we end up with several classes of DW solutions having different properties localized inside the wall, such as charge and/or CP violating vacua. We discuss the properties of these different classes of DW solutions as well as the interaction of SM fermions with such topological defects leading to different exotic phenomena such as, for example, the top quark being transmitted or reflected off the wall as a bottom quark.