Dark Matter Bound State Formation in Fermionic $Z_2$ DM model with Light Dark Photon and Dark Higgs Boson

TL;DR

This paper investigates dark matter bound state formation in a fermionic Z2 model with light dark photon and Higgs, highlighting the importance of Goldstone bosons and their impact on relic abundance and constraints.

Contribution

It introduces a detailed analysis of bound state formation including excited states and Goldstone boson effects in a Z2 dark matter model with light mediators.

Findings

Goldstone boson contributions significantly affect the potential matrix.

Longitudinal vector boson emissions are crucial for reannihilation.

Most existing constraints are evaded in this scenario.

Abstract

If fermionic dark matter (DM) is stabilized by dark $U(1)$ gauge symmetry that is spontaneously broken into its subgroup $Z_2$, the particle contents of the model becomes very rich: DM and excited DM, both of them are Majorana fermions, as well as two dark force mediators, dark photon and dark Higgs boson are naturally present due to the underlying dark gauge symmetry. In this paper, we study the DM bound state formation processes within this scenario, assuming both dark photon and dark Higgs are light mediators and including the effects of excited DM. The Goldstone boson contributions to the potential matrix in the Schr\"{o}dinger equations are found to be important. The emissions of a longitudinal vector boson (or somehow equivalently a Goldstone boson) during the DM bound state formations are crucial to induce a significant reannihilation process, reducing the dark matter relic abundance. Most of the stringent constraints for this kind of dark matter considered in the literature are simply evaded.