Related papers: Reply to "Clearing up the Strong $CP$ problem"
Recent studies have claimed that the strong $CP$ problem does not occur in QCD, proposing a new order of limits in volume and topological sectors when studying observables on the lattice. In order to shed light on this issue, we study the…
Recently, Ai, Cruz, Garbrecht, and Tamarit (arXiv:2001.07152, arXiv:2404.16026, arXiv:2511.04216) claimed that the strong CP problem can be avoided by adopting a particular order of limits in the Euclidean path integral, in which the…
On the basis of allowed local gauge symmetries, the QCD Lagrangian admits a CP-violating term proportional to the topological charge density, commonly referred to as the $\theta$ term. A priori, any value of $\theta$ is consistent with the…
We discuss matters related to the point that topological quantization in the strong interaction is a consequence of an infinite spacetime volume. Because of the ensuing order of limits, i.e. infinite volume prior to summing over topological…
While $CP$ violation has never been observed in the strong interactions, the QCD Lagrangian admits a $CP$-odd topological interaction proportional to the so called $\theta$ angle, which weighs the contributions to the partition function…
Recent studies have claimed that the strong $CP$ problem does not occur in QCD, proposing a new order of limits in volume and topological sectors when studying observables on the lattice. We study the effect of the topological term on a…
The absence of a neutron electric dipole moment (EDM) constrains the quantum chromodynamics (QCD) theta angle to be less than one part in ten billion, posing the Strong $CP$ problem. We revisit two classes of proposed solutions. First, we…
The strong CP problem is inseparably connected with the topology of gauge fields and the mechanism of color confinement, which requires nonperturbative tools to solve it. In this talk I present results of a recent lattice investigation of…
The conventional view is that a solution of the strong CP problem lies beyond QCD. A strong argument supporting this view is that the chiral expansion shows that observables depend on theta (unless a quark mass is zero); this eliminates the…
Quantum Chromodynamics admits a CP-violating contribution to the action, the $\theta$ term, which is expected to give rise to a nonvanishing electric dipole moment of the neutron. Despite intensive search, no CP violations have been found…
It is shown that the quark mass aligns QCD $\theta$ vacuum in such a way that the strong CP is conserved, resolving the strong CP problem.
Solutions of the Strong CP Problem based on the spontaneous breaking of CP must feature a non-generic structure and simultaneously explain a coincidence between a priori unrelated CP-even and CP-odd mass scales. We show that these…
Chiral perturbation theory predicts that in quantum chromodynamics (QCD), light dynamical quarks suppress the gauge-field topological susceptibility of the vacuum. The degree of suppression depends on quark multiplicity and masses. It…
Motivated by recent claims questioning the existence of strong CP violation, we present a pedagogical review of CP violation in Quantum Chromodynamics (QCD). Using fundamental properties of the QCD partition function, we analyze the…
I describe how the QCD vacuum structure, necessary to resolve the $U(1)_A$ problem, predicts the presence of a P, T and CP violating term proportional to the vacuum angle $\bar{\theta}$. To agree with experimental bounds, however, this…
Quantum Chromodynamics (QCD) admits a topological $\bar{\theta}$ term that violates charge-parity ($CP$) symmetry, yet experiments indicate that $\bar{\theta}$ is extremely small. To investigate this problem in a controlled setting, we…
We show that the strong CP problem can, in principle, be solved dynamically by adding extra-dimensions with compact topology. To this aim we consider a toy model for QCD, which contains a vacuum angle and a strong CP like problem. We…
The QCD axion fails to solve the strong CP problem unless all explicit PQ violating, Planck-suppressed, dimension n<10 operators are forbidden or have exponentially small coefficients. We show that all theories with a QCD axion contain an…
In this chapter we introduce the $\theta$-dependence and the topological properties of QCD, features of the strongly interacting sector which give rise to the strong CP problem in the more general context of the Standard Model of particle…
Three possible strategies have been advocated to solve the strong CP problem. The first is the axion, a dynamical mechanism that relaxes any initial value of the CP violating angle $\bar{\theta}$ to zero. The second is the imposition of new…