the strong CP problem
/ CP = see-pee /
Symmetries in physics are like rules nature seems to honor. One such rule is CP symmetry: roughly, the laws of physics should look the same if you swap every particle for its antiparticle (C) and reflect everything in a mirror (P). The weak force is known to break this rule a little. The puzzle of the strong CP problem is the reverse: the equations of QCD are allowed to break CP, yet measurements show the strong force does not break it at all — or if it does, only by an absurdly tiny amount. Why is the strong force so exactly even-handed when nothing forces it to be?
In detail, QCD's equations contain a free number, usually called theta, that controls how much the strong force violates CP symmetry. Theta could in principle be anything between 0 and about 6 (one full turn). If theta were even moderately large, the neutron would have a measurable electric lopsidedness — a so-called electric dipole moment — yet experiments have searched and found none, pinning theta below about a billionth. A parameter that could be anything, but turns out to be essentially zero for no apparent reason, is exactly the kind of unexplained fine-tuning that makes physicists suspicious.
The strong CP problem is a teaser pointing beyond the Standard Model, because the model offers no reason for theta to be so small. The most popular proposed fix invents a brand-new particle, the axion, whose dynamics would automatically drive theta to nearly zero. That axion has never been detected, but it is doubly attractive: it would solve the strong CP problem and, as a bonus, it is a leading candidate for the universe's dark matter. Several experiments are now hunting for it.
If theta were of order 1, the neutron would have an electric dipole moment vastly larger than experiments allow; the measured limit forces theta below about 10^-10.
Why theta is so near zero, with nothing in QCD requiring it, is the strong CP problem.
This is an open problem, not a settled result. The axion is the favored solution but remains hypothetical; no axion, and no neutron electric dipole moment, has yet been observed.