Natural Heavy Supersymmetry

TL;DR

The paper presents a minimal, monodromy-based cosmological relaxion mechanism embedded in a supersymmetric framework, where a single relaxion superfield scans SUSY-breaking during inflation and is halted by QCD instanton barriers. The resulting spectrum resembles Split Mini-Split scenarios, with gauginos lighter than scalars by a loop factor and the relaxino/gravitino as the LSP, allowing distinctive collider signatures such as displaced NLSP decays. A central tension is the strong-CP problem, which the authors address via three potential remedies—inflaton-assisted slope suppression, inflaton-assisted barrier dynamics, or a non-QCD relaxion sector—each with its own model-building challenges. The mechanism relies on a small shift-symmetry-breaking parameter and monodromic dynamics to yield a very large field excursion while maintaining control over UV and gravitational corrections, and it predicts testable connections between high-scale SUSY-breaking and low-energy axion-like phenomenology. The work highlights a viable path to realizations of Split SUSY free from naturalness concerns, with concrete phenomenological footprints for future colliders and axion experiments.

Abstract

We study how, as a result of the scanning of supersymmetry breaking during the cosmological evolution, a relaxation mechanism can naturally determine a hierarchy between the weak scale and the masses of supersymmetric particles. Supersymmetry breaking is determined by QCD instanton effects, in an extremely minimal setup in which a single field drives the relaxation and breaks supersymmetry. Since gauginos are lighter than the other supersymmetric particles by a one-loop factor, the theory is a realisation of Split Supersymmetry free from the naturalness problem.

Figures (3)

• Figure 1: The Feynman diagrams that generate the four-fermion operator $(q^cq)\, (\tilde{a} \tilde{a})$, after integrating out the squarks and the srelaxion $s$.
• Figure 2: The suppression of the axion potential at finite temperature, calculated using the approximate expression from ref. Wantz:2009it.
• Figure 3: A schematic illustration of the resolution of the relaxion strong-CP problem with inflaton-dependent instanton barriers. During inflation the axion-like potential is suppressed (blue dashed line). At late times the axion-like potential is unsuppressed and has grown relative to its value during inflation (solid black line). This change shifts the final relaxion minimum closer to a value in which the effective $\theta$ is much smaller, as shown by the example red minima.