Pheno Journal Club: ‘Is the Strong CP Problem Really a Problem?’

Speaker: Giulio Barni (IFT)

Venue & Time: 14:30 – 15:30

Abstract: The vanishingly small value of the QCD θ-angle inferred from neutron EDM bounds is usually taken as evidence for the Strong CP Problem, prompting a wide range of proposed solutions. In this talk, I will first review what the Strong CP Problem is: why a generic θ term in QCD leads to large CP violation, and how current experiments force θ ≲ 10e-10. I will then summarize the main solution strategies, including the Peccei–Quinn axion, parity- or CP-based models, and other less conventional ideas. The second part of the talk will focus on a more conceptual question raised in recent years: does the Strong CP Problem actually exist, or could θ be unphysical once confinement and the full structure of QCD are properly taken into account? Building on the recent analysis of Benabou, Hook, Manzari, Murayama and Safdi (arXiv:2510.18951), I will discuss this debate, highlighting what lattice QCD teaches us about θ-dependence, topological sectors, and the neutron EDM.