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).

Agai…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

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.

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