Study Constrains Chromodynamics Theta Angle to One Part in Ten Billion, Resolving the Strong Problem

The enduring mystery of why the universe appears remarkably stable, despite theoretical predictions of instability, presents a significant challenge to physicists, known as the Strong problem. Joshua Benabou from Lawrence Berkeley National Laboratory, Anson Hook from the Maryland Center for Fundamental Physics, and Claudio Andrea Manzari from the Institute for Advanced Study, alongside Hitoshi Murayama and Benjamin Safdi, investigate potential resolutions to this puzzle. Their work directly addresses recent claims that the Strong problem is not genuine, demonstrating that established principles of quantum chromodynamics predict a measurable instability. Furthermore, the team clarifies that certain theoretical solutions, involving gauged discrete symmetries, remain viable, despite earlier concerns, and do not inherently conflict with experimental observations. This research reinforces the importance of the Strong problem and provides a clearer path toward understanding the fundamental forces governing the universe.

Parameter and Strong CP Problem Consistency

This collection of appendices presents a rigorous exploration of symmetries and gauge theories, particularly as they relate to the consistency of quantum chromodynamics (QCD). Researchers delve into the subtle constraints that emerge when attempting to create consistent quantum field theories, focusing on the implications of the θ-parameter in QCD and the conditions under which symmetries can be consistently gauged. The work employs advanced mathematical techniques from group theory and functional analysis to support its claims. The appendices examine the mass of the η’ meson, a particle composed of quarks and gluons, using the large N limit, a simplification technique in QCD. This analysis connects the η’ meson’s mass to the topological susceptibility, a measure of vacuum fluctuations, and instantons, quantum tunneling effects within QCD. Researchers also investigate the physical viability of superpositions of different θ-vacua, demonstrating that such superpositions would violate the cluster decomposition principle, a fundamental requirement for a consistent quantum field theory.

Neutron Electric Dipole Moment Constrains Strong CP Problem

Scientists have rigorously examined the long-standing Strong CP problem in particle physics, confirming its continued validity and exploring potential solutions rooted in discrete symmetries. The work confirms that the absence of an electric dipole moment in the neutron constrains a parameter of quantum chromodynamics, denoted as θ, to an extremely small value, presenting a significant fine-tuning challenge. Researchers addressed recent claims suggesting the problem is illusory, definitively showing that a non-zero neutron electric dipole moment arises directly from well-understood QCD dynamics when θ is non-zero. The study highlights the viability of solutions based on spontaneously broken P or CP symmetry, demonstrating that gauged discrete symmetries can preserve CP in the vacuum.

Researchers showed that while model-building presents challenges, such as avoiding unwanted contributions to the neutron electric dipole moment after symmetry breaking, gauged discrete models possess no fundamental obstructions. Furthermore, the research establishes that the Strong CP problem requires P/CP symmetry to hold across all quantum correlation functions. Through analysis, including the Witten-Veneziano relation, scientists confirmed the existence of the problem and identified flaws in arguments claiming its absence. The team proposes that theories emerging from quantum gravity dynamically determine the neutron electric dipole moment, eliminating dependence on an unknowable parameter of the QCD vacuum.

Gauged Symmetries Resolve Strong CP Problem

Scientists have revisited long-standing questions surrounding the strong CP problem in particle physics, specifically the unexpectedly small value of the neutron’s electric dipole moment. Their work reinforces the validity of this problem and investigates potential solutions involving discrete symmetries. Researchers demonstrated that models employing gauged discrete symmetries, while presenting model-building challenges, do not face fundamental theoretical obstructions, contrary to recent suggestions. They showed that these symmetries can naturally restrict the possible values of a key parameter, known as the theta angle, to a very narrow range. The authors acknowledge that models involving these symmetries may require careful construction to avoid cosmological issues related to the formation of stable domain walls. Future research could focus on addressing these challenges and further refining models that incorporate gauged discrete symmetries as a means of resolving the strong CP problem and explaining the observed properties of the neutron.

Gauged Symmetries and the Neutron EDM

This research investigates potential solutions to the strong CP problem, focusing on the theoretical constraints and possibilities surrounding the neutron electric dipole moment (EDM). Researchers critically analyzed gauged discrete symmetries as a means of resolving the problem, demonstrating that these symmetries, when realized in specific ways, preserve the necessary conditions for a solution, contrary to previous assertions. This involved rigorous mathematical modeling to demonstrate compatibility with observed physical constraints. Scientists also re-evaluated arguments suggesting the strong CP problem is illusory, employing well-established principles of quantum chromodynamics (QCD) to demonstrate that a non-zero neutron EDM would naturally arise at finite energy scales, directly contradicting the claim that the problem lacks a fundamental basis. The research team leveraged generalized global symmetries and modular invariance to explore potential solutions, investigating their implications for string theory compactifications and the resulting constraints on the QCD theta angle.