A Bayesian Analysis of the Credence in Low Energy Supersymmetry

Core Concepts

Despite the lack of empirical evidence for low energy supersymmetry (SUSY) at the LHC, a Bayesian analysis, incorporating various theoretical and empirical arguments, suggests that a substantial degree of credence in the theory remains justifiable.

Abstract

• Bibliographic Information: Dawid, R., & Wells, J. D. (2024). A Bayesian Model of Credence in Low Energy Supersymmetry. arXiv preprint arXiv:2411.03232v1.
• Research Objective: This paper investigates the evolution of credences in low energy supersymmetry (SUSY) in light of empirical data, particularly from the LHC, using a Bayesian framework.
• Methodology: The authors employ Bayesian reasoning to analyze the impact of various arguments, both theoretical and empirical, on the plausibility of low energy SUSY. They consider factors such as gauge coupling unification, the Higgs mass, the hierarchy problem, and the lack of observed proton decay and supersymmetric particles at the LHC.
• Key Findings: The analysis reveals that while LHC data has led to a decrease in credence for low energy SUSY, the decrease is moderate, and substantial credence remains, especially for those who held optimistic priors based on theoretical arguments. The authors demonstrate that even with a high degree of pessimism towards theory building, significant credence in SUSY can be maintained.
• Main Conclusions: The paper concludes that the rational basis for belief in low energy SUSY, while weakened by LHC results, remains strong. The authors emphasize that the Bayesian analysis provides a framework for understanding the evolution of scientific beliefs in the face of empirical evidence.
• Significance: This research contributes to the philosophy of science by providing a quantitative framework for assessing the impact of empirical data on theoretical beliefs in high-energy physics. It highlights the role of both theoretical and empirical arguments in shaping scientific credence.
• Limitations and Future Research: The authors acknowledge the inherent subjectivity in assigning prior probabilities and the limitations of their simplified model. Future research could explore more sophisticated Bayesian models incorporating a wider range of theoretical and empirical considerations.

Stats

The maximum ∆λij needed for gauge coupling unification within supersymmetric MSSM theory is less than 10.
The maximum ∆λij required for the Standard Model is about 200.
The probability of three randomly selected gauge couplings unifying is estimated to be less than 0.5%.
Randomly selecting hypercharge values for fermions, only 3.2% of anomaly-free spectra were "GUT-able," suggesting the uniqueness of the Standard Model's fermion arrangement.
The separation of scales between the Planck scale and the electroweak scale is 10^-16.
The Higgs boson was discovered at a mass value less than 150GeV.
The LHC has set bounds on the masses of electroweak superpartner masses of approximately a few TeV.

Quotes

"...it would be a cruel joke of nature if there is no grand unification given how unified the couplings appear to be."
"...a supersymmetry theory is fine-tuned to at least one part in a hundred."

Key Insights Distilled From

A Bayesian Model of Credence in Low Energy Supersymmetry

by Richard Dawi... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.03232.pdf

Deeper Inquiries

How might advancements in string theory or alternative theories of quantum gravity influence the credence in supersymmetry?

Advancements in string theory or alternative theories of quantum gravity could significantly influence the credence in supersymmetry in several ways:
Strengthening the Case for Supersymmetry:

Verification of String Theory Predictions: If string theory, which often implies supersymmetry, finds empirical support through testable predictions (e.g., regarding the properties of dark matter or the early universe), the credence in supersymmetry would be significantly boosted. This would be a form of meta-empirical confirmation, where the success of a theory in one domain lends credence to its predictions in another.
Specific String Models Favoring Low-Energy SUSY: The discovery of specific string theory models that naturally give rise to low-energy supersymmetry, as opposed to supersymmetry at scales inaccessible to current experiments, would provide strong theoretical motivation for the latter.
Emergence of Supersymmetry in Other Quantum Gravity Theories: If supersymmetry emerges as a necessary ingredient in other promising approaches to quantum gravity, independent of string theory, it would further solidify the theoretical foundation of supersymmetry and increase its perceived plausibility.
Weakening the Case for Supersymmetry:

String Theory Moving Away from Low-Energy SUSY: Conversely, if string theory research increasingly favors scenarios where supersymmetry is broken at very high energies, far beyond the reach of current or foreseeable colliders, it would weaken the motivation for low-energy SUSY.
Success of Alternative Quantum Gravity Theories Without SUSY: The successful development and empirical support for alternative quantum gravity theories that do not require supersymmetry would diminish the perceived need for SUSY as a solution to problems like the hierarchy problem.
Overall, the relationship between supersymmetry and quantum gravity is complex and intertwined. Advancements in our understanding of quantum gravity, particularly in string theory, have the potential to either significantly bolster or undermine the case for supersymmetry, depending on the nature of those advancements.

Could the apparent fine-tuning problem in supersymmetry be a hint towards a deeper, yet undiscovered, principle governing the universe?

The apparent fine-tuning problem in supersymmetry, where parameters need to be precisely adjusted to produce the observed electroweak scale, could indeed be a hint towards a deeper principle at play. Here are some possibilities:
1. Anthropic Selection:

The vast landscape of string theory: String theory suggests the existence of a vast landscape of possible universes, each with different physical constants and laws.
Fine-tuning for observers: It's possible that only a tiny fraction of these universes have the right conditions for life, including a finely-tuned electroweak scale.
Our existence as evidence: Our very existence in such a universe would then be a selection effect, not requiring any further explanation for the fine-tuning.
2. New Symmetries or Mechanisms:

Beyond the MSSM: The Minimal Supersymmetric Standard Model (MSSM) might be too simplistic. More complex supersymmetric models, with additional particles and interactions, could potentially alleviate the fine-tuning problem.
Dynamical solutions: Mechanisms like dimensional transmutation or strong dynamics might naturally generate the electroweak scale without requiring extreme fine-tuning of fundamental parameters.
3. Modified Understanding of Naturalness:

Rethinking "technical naturalness": The traditional notion of "technical naturalness" might be too restrictive. Perhaps there are other principles or criteria for assessing the naturalness of a theory that we haven't fully grasped yet.
Emergent naturalness: The electroweak scale could be an emergent property of a more fundamental theory, where the apparent fine-tuning arises from our limited low-energy perspective.
4. Multiverse and Cosmological Considerations:

Eternal inflation: Scenarios like eternal inflation, where our universe is just one of many bubbles in a larger multiverse, could provide a statistical context for understanding fine-tuning.
Cosmological evolution: The value of the electroweak scale might be a consequence of cosmological evolution, potentially related to phase transitions or other dynamical processes in the early universe.
It's crucial to remember that the fine-tuning problem is not a logical inconsistency within supersymmetry. It's a philosophical puzzle that highlights our incomplete understanding of fundamental physics. Whether it points towards anthropic selection, new physics, or a revised understanding of naturalness remains an open and exciting question.

If the concept of "naturalness" in physics is ultimately abandoned, what implications would this have for our understanding of fundamental physics and the search for new theories?

Abandoning the concept of "naturalness" in physics would represent a significant shift in our approach to understanding fundamental physics and evaluating new theories. Here are some potential implications:
1. Reevaluating Existing Theories:

Reduced pressure on SUSY and other BSM scenarios: Theories like supersymmetry, motivated in part by addressing the hierarchy problem and the perceived unnaturalness of fine-tuning, would lose some of their theoretical appeal.
Greater openness to a wider range of models: Theories that were previously considered less favorable due to fine-tuning issues might become more viable contenders, as the criteria for naturalness would no longer be a primary concern.
2. Shifting Focus in Theory Development:

Emphasis on empirical testability: With naturalness no longer a guiding principle, the emphasis in theory development would likely shift more strongly towards empirical testability and the ability to make concrete predictions that can be verified or falsified by experiments.
Exploration of alternative guiding principles: Physicists would need to explore and potentially embrace alternative theoretical principles or criteria for evaluating the plausibility and promise of new theories.
3. Impact on the Scientific Method:

Rethinking the role of aesthetics and intuition: Naturalness, while not a rigorous scientific principle, has often served as a guiding intuition in physics. Abandoning it might lead to a reassessment of the role of aesthetics and intuition in scientific theory development.
Greater reliance on empirical data: The absence of naturalness as a guiding principle could lead to a greater reliance on empirical data and a more data-driven approach to fundamental physics.
4. Philosophical Implications:

The nature of fine-tuning: Abandoning naturalness would require grappling with the philosophical implications of fine-tuning in fundamental physics. Is it a sign of deeper principles at work, or simply a brute fact about our universe?
The limits of human understanding: It could highlight the limits of human intuition and our ability to discern what counts as a "natural" or "elegant" explanation for the universe.
In conclusion, abandoning the concept of naturalness would have profound implications for both the practice and the philosophy of physics. It would necessitate a reevaluation of existing theories, a shift in the focus of theory development, and a deeper reflection on the role of human intuition and the nature of scientific explanation.