Challenges for the Nelson-Barr Mechanism

Understand

Solutions to strong CP based on chiral structure have been subject to the most careful scrutiny and critique.

• Basic theoretical issues include hierarchy and fine-tuning problems, quality and genericity of symmetries, and compatibility with solutions to the electroweak hierarchy problem.
• We study the similar set of challenges for solutions to strong CP based on spontaneous CP violation and the Nelson-Barr mechanism.
• Some of our observations have appeared in the literature previously, and others are new; our purpose is to collect and analyze the issues as a whole and provide an assessment of the most plausible settings for the Nelson-Barr solution.