Dark matter from a complex scalar singlet: The role of dark CP and other discrete symmetries

TL;DR

This paper explores a complex scalar singlet model with dark CP symmetry, analyzing its dark matter phenomenology, stability conditions, and potential for sub-GeV candidates within various symmetry breaking scenarios and effective field theories.

Contribution

It introduces a detailed study of a pseudo-scalar dark matter candidate from a complex scalar singlet, highlighting the role of dark CP symmetry and symmetry breaking patterns in its stability and phenomenology.

Findings

Several parameter regions reproduce observed dark matter abundance.

Dark CP symmetry can be spontaneously broken, affecting stability.

Light sub-GeV dark matter candidates are possible.

Abstract

We study the case of a pseudo-scalar dark matter candidate which emerges from a complex scalar singlet, charged under a global U(1) symmetry, which is broken both explicitly and spontaneously. The pseudo-scalar is naturally stabilized by the presence of a remnant discrete symmetry: dark CP. We study and compare the phenomenology of several simplified models with only one explicit symmetry breaking term. We find that several regions of the parameter space are able to reproduce the observed dark matter abundance while respecting direct detection and invisible Higgs decay limits: in the resonances of the two scalars, featuring the known as forbidden or secluded dark matter, and through non-resonant Higgs-mediated annihilations. In some cases, combining different measurements would allow one to distinguish the breaking pattern of the symmetry. Moreover, this setup admits a light DM candidate at the sub-GeV scale. We also discuss the situation where more than one symmetry breaking term is present. In that case, the dark CP symmetry may be spontaneously broken, thus spoiling the stability of the dark matter candidate. Requiring that this does not happen imposes a constraint on the allowed parameter space. Finally, we consider an effective field theory approach valid in the pseudo-Nambu-Goldstone boson limit and when the U(1) breaking scale is much larger than the electroweak scale.