The Z-Z' Mass Hierarchy in a Supersymmetric Model with a Secluded U(1)'-Breaking Sector

TL;DR

This paper tackles the Z–Z′ mass hierarchy in a SUSY model featuring a secluded U(1)′-breaking sector. It introduces a two-tier structure with an ordinary sector (H1,H2,S) that generates an effective μ-term and a secluded sector (S1,S2,S3) that drives a large Z′ mass while remaining weakly coupled to the SM fields; a small Yukawa coupling lifts an almost flat direction to yield a large Z′ mass but a small Z–Z′ mixing angle. Electroweak symmetry breaking is driven by large A-terms, yielding tanβ ≈ 1 and s_i ≫ v_i, with μ ≈ h s and h allowed up to ~0.5–0.8 thanks to U(1)′ running effects. The resulting spectrum is rich, featuring strong doublet–singlet mixing in Higgses and neutralinos, a heavy Z′ with α_{Z-Z′} ∼ 10^{-3}–10^{-3}, and a lightest neutralino that is often singlino-dominated. Although the full collider and cosmological implications require further study, the framework demonstrates a viable path to a sizeable Z′ mass without drastically altering MSSM-like sparticle spectra, and predicts distinctive Higgs and neutralino signatures from the secluded-sector dynamics.

Abstract

We consider the Z'/Z mass hierarchy in a supersymmetric model in which the U(1)' is broken in a secluded sector coupled to the ordinary sector only by gauge and possibly soft terms. A large mass hierarchy can be achieved while maintaining the normal sparticle spectra if there is a direction in which the tree level potential becomes flat when a particular Yukawa coupling vanishes. We describe the conditions needed for the desired breaking pattern, to avoid unwanted global symmetries, and for an acceptable effective mu parameter. The electroweak breaking is dominated by A terms rather than scalar masses, leading to tan beta ~ 1. The spectrum of the symmetry breaking sector is displayed. There is significant mixing between the MSSM particles and new standard model singlets, for both the Higgs scalars and the neutralinos. A larger Yukawa coupling for the effective mu parameter is allowed than in the NMSSM because of the U(1)' contribution to the running from a high scale. The upper bound on the tree-level mass of the lightest CP even Higgs doublet mass is about c x 174 GeV, where c is of order unity, but the actual mass eigenvalues are generally smaller because of singlet mixing.