On the Origins of the Strong CP Problem

Abstract

The conventional strong $CP$ problem arises from the apparent tension between the existence of a $CP$-violating $θ$ term in the conventional topological formulation of QCD and the experimental constraint that the corresponding parameter satisfies $θ\lesssim 10^{-10}$. The standard formulation assumes a globally classified topological structure of gauge-field configuration space, leading to topological sectors and $θ$-vacua. Within this framework one investigates the resulting physical consequences, including possible resolutions of the strong $CP$ problem. A logically distinct and prior question is whether known physical principles or observables require the additional assumptions leading to such a global topological classification of gauge fields in the first place. In this work, we address the latter question. We distinguish between those aspects of QCD that follow directly from local gauge invariance and causal locality, and those that rely on additional assumptions leading to a global topological classification of gauge-field configurations. The former include the local topological charge density, the topological susceptibility, the anomalous Ward identity, the 't~Hooft vertex, the Witten--Veneziano relation, the Leutwyler--Smilga relation, and the corresponding nonperturbative results obtained in lattice QCD. We are not aware of any established physical observable that requires the additional assumptions leading to a global topological classification of gauge-field configurations. From this perspective, the conventional strong $CP$ problem is contingent upon the adoption of additional global structure rather than being a consequence of presently known QCD dynamics. This observation does not question the mathematical consistency or phenomenological success of the conventional framework, but instead clarifies the logical assumptions underlying its formulation.