[Paper Review] Naturalness of supersymmetric models

Алессандро Струмиа|ArXiv.org|Apr 6, 1999

Black Holes and Theoretical Physics7 citations

TL;DR

This paper evaluates the naturalness of supersymmetric models using a Bayesian-like procedure, finding that LEP2 data exclude about 95% of the minimal supergravity MSSM parameter space. It concludes that naturalness is severely challenged in both minimal supergravity and gauge mediation models, especially with light messengers, suggesting that supersymmetry's absence in experiments may not be due to chance but to deeper theoretical constraints.

ABSTRACT

After presenting a simple procedure for testing naturalness (similar to Bayesian inference and not more subjective than it) we show that LEP2 experiments pose a naturalness problem for `conventional' supersymmetric models. About 95% of the parameter space of minimal supergravity MSSM is excluded by LEP2 experiments. Moreover in this model electroweak baryogenesis, or detectable supersymmetric corrections to mixing of K and B mesons, are possible only in very small corners of the parameter space. The naturalness problem is stronger in gauge mediation models, expecially with light messengers. We recall some possible explanations (different from an improbable numerical accident) of why supersymmetry has not (yet?) been found.

Motivation & Objective

• To assess the naturalness of supersymmetric models using a quantitative, non-subjective procedure similar to Bayesian inference.
• To determine the impact of LEP2 experimental results on the viable parameter space of minimal supergravity and gauge mediation supersymmetry models.
• To explore why supersymmetry has not yet been observed, excluding the possibility of a mere numerical coincidence.
• To identify the regions of parameter space where electroweak baryogenesis or measurable B-meson mixing corrections could still occur.
• To evaluate the robustness of naturalness as a guiding principle in supersymmetric model building.

Proposed method

• Adopts a naturalness criterion based on a procedure analogous to Bayesian inference, minimizing subjectivity.
• Applies the naturalness criterion to the minimal supergravity (mSUGRA) framework, using constraints from LEP2 on the chargino mass.
• Evaluates the viability of electroweak baryogenesis and supersymmetric contributions to K-B mixing in the remaining parameter space.
• Extends the analysis to gauge mediation models, particularly focusing on scenarios with light messengers.
• Uses experimental bounds to map the fraction of parameter space consistent with both naturalness and LEP2 results.
• Compares the size of allowed regions in different models to assess the severity of the naturalness problem.

Experimental results

Research questions

• RQ1 To what extent does the LEP2 chargino mass limit constrain the natural parameter space of the minimal supergravity model?
• RQ2 In which regions of the mSUGRA parameter space can electroweak baryogenesis or measurable supersymmetric corrections to K-B mixing occur, and how large are these regions?
• RQ3 How does the naturalness problem manifest in gauge mediation models, especially when messengers are light?
• RQ4 Could the non-observation of supersymmetry be explained by a naturalness-based selection rather than a statistical fluke?
• RQ5 What fraction of the mSUGRA parameter space remains viable under both naturalness and LEP2 constraints?

Key findings

• Approximately 95% of the mSUGRA parameter space is excluded by LEP2 chargino mass bounds, indicating a severe naturalness problem.
• Electroweak baryogenesis or detectable supersymmetric corrections to K-B mixing are possible only in very small, highly constrained regions of the mSUGRA parameter space.
• The naturalness problem is even more acute in gauge mediation models, particularly when messengers are light, due to tighter constraints on the supersymmetry-breaking scale.
• The analysis suggests that the non-observation of supersymmetry is unlikely to be due to a random numerical accident, implying deeper theoretical issues.
• The remaining viable regions for naturalness and experimental consistency are so small that they challenge the plausibility of conventional supersymmetric models.
• The study reinforces the idea that naturalness, when rigorously applied, places strong and potentially falsifiable constraints on supersymmetric model building.