the strong CP problem
/ see-pee /
Run a film of a physical process backward and reflect it in a mirror, and you get its 'CP mirror image.' Most of nature looks almost the same in this CP mirror, but the weak force is a known exception — it behaves slightly differently for matter and antimatter. The strong force, by contrast, appears to be perfectly even-handed: experiments find no sign of CP violation in it at all. That sounds tidy, but here is the catch — the theory of the strong force allows a term that would break CP, and there is no obvious reason that term should be zero. Why does the strong force bother to be so perfectly fair?
The culprit is a single number, usually written as the angle theta, that sits inside the equations of quantum chromodynamics, the theory of quarks and gluons. If theta were anything but tiny, it would make the neutron behave like a little electric compass needle — it would have a permanent separation of positive and negative charge, called an electric dipole moment, which is exactly the kind of thing CP violation produces. Experiments have searched for the neutron's electric dipole moment with breathtaking precision and found nothing, which forces theta to be smaller than about one part in ten billion. Nothing in the Standard Model demands such a tiny value; theta could have been anything, yet nature seems to have set it to essentially zero.
This is a naturalness puzzle in the same spirit as the hierarchy problem: a number that 'ought' to be of order one is instead astonishingly small, with no explanation. The most elegant proposed solution introduces a new symmetry that automatically relaxes theta to zero, and a side effect of that mechanism would be a brand-new, extremely light particle called the axion. The axion is now also a leading candidate for dark matter, which is part of why the strong CP problem has stayed an active and unusually rewarding frontier — solving one mystery might solve two.
If the strong force violated CP even a little, the neutron would act like a tiny battery with a measurable electric dipole moment. Physicists have searched for that dipole for over half a century and found nothing, pinning the theta angle to below about a ten-billionth.
No neutron electric dipole has ever been found — forcing the strong force's CP-breaking angle to near zero.
The strong CP problem is not that the strong force violates CP — experiment says it does not. The puzzle is the opposite: the theory permits CP violation, yet nature has switched it off to fantastic precision, and we do not know why.