The strong CP problem is a puzzle in theoretical physics related to the behavior of the strong force (or strong nuclear force), which is one of the four fundamental forces of nature. It concerns a parameter in the theory of quantum chromodynamics (QCD), the theory that describes the strong force and the interactions of quarks and gluons.

In QCD, there is a term called the "theta term" in the Lagrangian, which is a mathematical expression describing the dynamics of the theory. This term involves a parameter denoted as θ (theta), which measures the strength of a certain type of CP (charge-parity) violation in the strong interactions.

The strong CP problem arises from the fact that experimental observations indicate that this parameter θ is very close to zero, but there is no obvious reason from the theory why it should be exactly zero. According to QCD, θ could take any value between 0 and 2π, leading to potentially observable effects of CP violation. However, experiments have shown that these effects are extremely small, suggesting that θ is indeed very close to zero, but why it is so close to zero remains a puzzle.

Solving the strong CP problem is a significant challenge in theoretical physics, and several proposed solutions have been suggested over the years. One well-known solution is the Peccei-Quinn mechanism, which introduces a new symmetry into the theory called the Peccei-Quinn symmetry and predicts the existence of a new particle called the axion. If the axion exists, it could resolve the strong CP problem by dynamically adjusting the value of θ to zero without the need for fine-tuning.

Despite many theoretical efforts, the exact solution to the strong CP problem remains an open question in particle physics.