Naturalness of the relaxion mechanism

Abstract

The relaxion mechanism is a novel solution to the hierarchy problem. In this first statistical analysis of the relaxion mechanism, we quantify the relative plausibility of a QCD and a non-QCD relaxion model versus the Standard Model with Bayesian statistics, which includes an automatic penalty for fine-tuning. We find that in light of the hierarchy between the weak and Planck scales, relaxion models are favoured by colossal Bayes-factors. Constraints upon \eg the vacuum energy during relaxation, however, shrink the Bayes-factors such that relaxion models are only slightly favoured. Including the bounds on $\left|{\theta }_{\text{QCD}}\right|$ shatters the plausibility of the QCD relaxion model as it typically yields $\left|{\theta }_{\text{QCD}}\right|\gg 0$. Finally, we augment our models with scalar-field inflation and consider measurements of inflationary observables from BICEP/Planck. We find that, all told, the Standard Model is favoured by huge Bayes-factors as the relaxion models require fine-tuning such that the Hubble parameter is less than the height of the periodic barriers. Thus, whilst we confirm that relaxion models could solve the hierarchy problem, we find that their unconventional cosmology demolishes their plausibility.

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Cited by in corpus (20)

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• Constraints on Relaxion Windows
• Dynamics of the cosmological relaxation after reheating
• Naturalizing Supersymmetry with a Two-Field Relaxion Mechanism
• Relaxion: A Landscape Without Anthropics
• Low-Scale D-term Inflation and the Relaxion
• Constraints on the relaxion mechanism with strongly interacting vector-fermions
• Relaxion window
• Fast-Rolling Relaxion
• Naturalness made easy: two-loop naturalness bounds on minimal SM extensions
• Attracting the Electroweak Scale to a Tachyonic Trap
• Cosmological Relaxation from Dark Fermion Production
• Expectations for Inflationary Observables: Simple or Natural?
• Bayesian naturalness, simplicity, and testability applied to the $B-L$ MSSM GUT
• Light Higgs Boson from a Pole Attractor
• Testing Higgs boson scenarios in the phenomenological NMSSM
• Cosmological relaxation and high scale inflation
• Dynamical relaxation in 2HDM models
• Gravitational tests of electroweak relaxation
• Precise interpretations of traditional fine-tuning measures