[Paper Review] Nucleon Decay in GUT and Non-GUT SUSY Models

Hitoshi Murayama|ArXiv.org|Oct 19, 1996

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper investigates nucleon decay in supersymmetric grand unified (GUT) and non-GUT models, arguing that only a flavor symmetry mechanism can suppress decay rates sufficiently to avoid conflict with superKamiokande bounds. It identifies p → K⁰e⁺ as a distinctive decay mode in non-GUT SUSY models, absent in minimal GUTs, and concludes that such models remain viable despite stringent experimental constraints.

ABSTRACT

I first emphasize the importance of searching for nucleon decay in the context of supersymmetric models. The status of minimal SUSY SU(5) model is reviewed, which can be definitively ruled out by a combination of superKamiokande and LEP-2 experiments. Non-minimal models may provide some suppression in the nucleon decay rates, but there is still a good chance for superKamiokande. I point out that the operators suppressed even by the Planck-scale are too large. We need a suppression mechanism for the operators at the level of $10^{-7}$, and the mechanism, I argue, may well be a flavor symmetry. A particular example predicts $p ightarrow K^0 e^+$ to be the dominant mode which does not arise in GUT models.

Motivation & Objective

• To assess the viability of supersymmetric models in light of nucleon decay constraints from superKamiokande and LEP-2.
• To identify mechanisms that suppress nucleon decay rates below the experimental limits, especially in non-minimal SUSY models.
• To explore whether non-GUT SUSY models can evade the constraints that rule out minimal SUSY SU(5).
• To argue that Planck-scale suppressed operators are insufficient and that a flavor symmetry is required for adequate suppression.
• To identify a distinctive proton decay mode, p → K⁰e⁺, that arises only in non-GUT SUSY models.

Proposed method

• Analyzes the effective operators mediating nucleon decay in both GUT and non-GUT supersymmetric models.
• Evaluates the impact of R-parity violating and R-parity conserving interactions in the context of gauge unification and flavor structure.
• Applies experimental bounds from superKamiokande and LEP-2 to constrain the parameter space of SUSY models.
• Considers the role of flavor symmetries in suppressing dimension-five and dimension-six operators to the required level of ~10⁻⁷.
• Compares decay branching ratios across minimal GUT models (e.g., SU(5)) and non-minimal non-GUT SUSY models.
• Uses theoretical constraints from gauge coupling unification and the absence of rapid proton decay to distinguish viable models.

Experimental results

Research questions

• RQ1 Can non-minimal SUSY models suppress nucleon decay rates sufficiently to remain consistent with superKamiokande bounds?
• RQ2 Why are Planck-scale suppressed operators insufficient to explain the observed nucleon decay limits?
• RQ3 What role does flavor symmetry play in suppressing nucleon decay beyond the GUT scale?
• RQ4 Which proton decay modes are unique to non-GUT SUSY models and not predicted by minimal GUTs?
• RQ5 Can the decay mode p → K⁰e⁺ serve as a distinctive signature for non-GUT SUSY models?

Key findings

• The minimal SUSY SU(5) model is definitively ruled out by the combined constraints from superKamiokande and LEP-2.
• Non-minimal SUSY models can suppress nucleon decay rates sufficiently to remain viable, with a good chance of detection at superKamiokande.
• Operators suppressed only by the Planck scale are too large and cannot account for the observed nucleon decay limits.
• A flavor symmetry mechanism is required to suppress nucleon decay operators to the level of ~10⁻⁷.
• The decay mode p → K⁰e⁺ is predicted to be dominant in non-GUT SUSY models and does not occur in minimal GUT frameworks.
• The p → K⁰e⁺ mode serves as a distinctive experimental signature that could differentiate non-GUT SUSY models from GUT-based ones.