The neutron CP problem is a disparity between the Standard Model (SM) view of particle physics, and theoretical aspects of Quantum Chromo Dynamics (QCD).

The neutron (as the name implies) has a neutral electrical 'charge', because the threeQuarksplugin-autotooltip__plain plugin-autotooltip_bigQuarks

Following theoretical calculations, particle accelerator experiments in the 1970s confirmed that quarks are responsible for the internal structure of protons and neutrons. ( 3 quarks each, in different configurations, see :Electron / Quark charge balance).

Again, as calculations predicted, six 'flavours' of quark have now been experimentally identified (ref.from which it is formed have fractional positive and negative charges which add to exactly zero. No experimental measurements to-date have shown anything other than a non-zero charge. This is in conflict with the highly-robust QCD theory, which insists that Charge and Parity (CP) would not be zero under all conditions.

A theoretical particle, the Axion, was proposed in 1977 as a possible way of resolving the problem. To date, it has not been detected in any experiment.

Further reading MIT

Also see : Electron / Quark charge balanceplugin-autotooltip__plain plugin-autotooltip_bigElectron / Quark charge balance

inexplicable

It's known that all atoms are based on a structure where the 'negative' electric charge of the electron(s) (conventionally known as '-1') exactly balances the 'positive' charge of the corresponding proton(s) in the atom's nucleus.