# The strong CP problem

- kind: problem
- status: open
- year: 1977

**Statement.** Why does the strong force appear to respect CP symmetry when the theory allows it to be broken?

Quantum chromodynamics has a parameter θ that could take any value, and the neutron's electric dipole moment bounds it below about 10^-10. The Peccei–Quinn mechanism explains the small value with a new particle, the axion, which is also a dark matter candidate. Axion searches are running now.

## Related problems

- The original curation links this problem to [What dark matter is](/wiki/Problem:dark-matter.md); the argument for the link has not been written yet.

## Sources

- [Wikipedia: Strong CP problem](https://en.wikipedia.org/wiki/Strong_CP_problem)

[The Standard Model's loose ends](/wiki/Space:standard-model.md) · [Physics](/wiki/Continent:physics.md)

## Relations

- related to [dark-matter](/wiki/Problem:dark-matter.md) — The original curation links this problem to What dark matter is; the argument for the link has not been written yet.

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