Spontaneous dark matter stability from a fermiophobic U(1)' gauge symmetry

Abstract

In model building, discrete symmetries cannot only play an important role in preserving the structure of Yukawa interactions but also provide a pathway to stabilise dark matter candidates. However, such discrete symmetries are not necessarily be imposed directly by hand. Instead, they can arise from the spontaneous symmetry breaking (SSB) of continuous symmetries. As an example, we study the type Ib seesaw model, where the effective neutrino mass operator involves two different Higgs doublets and two right-handed (RH) neutrinos forming a single heavy Dirac pair. The heavy neutrino, together with the Higgs doublets, is charged under a $U(1)'$ gauge symmetry which helps to preserve the special structure of the type Ib seesaw mechanism. After the SSB, the $U(1)'$ symmetry turns into a $Z_2$ symmetry which stabilises a dark matter candidate. The dark matter candidate interacts with the other particles in the thermal bath through the massive boson resulting from the $U(1)'$ symmetry breaking. We explore how the correct dark matter relic abundance can be produced thermally in both a low energy effective model and a renormalisable model with a complete fourth family of vector-like fermions.