The 'textbook' explanation for the structure of the proton is that it's a 'state' of two up quarks and one down quark bound by gluons, But it has been known for 40 years or so that this is an over-simplification. According to accepted quantum theory, there should also be infinite varying probabilities for the inclusion of quark–antiquark pairs.

A 2022 paper in Nature, from the NNPDF Collaboration, reports on experimental data which suggests the possibility that protons might also routinely feature a 'charm' quark and its antimatter equivalent, an 'anticharm'.

The research team suggest that around 0.5% of the proton's momentum (and by implication, its mass) might be coming from the charm / anticharm pair.

If confirmed, it would have broad-ranging and profound implications for nuclear physics research - as many other experiments heavily rely on an accurate model of the proton's internal structure.

See : Evidence for intrinsic charm quarks in the proton Nature, volume 608, pages 483–487 (2022)

Also see : Proton Mass Calculationsplugin-autotooltip__plain plugin-autotooltip_bigProton Mass Calculations

The mass of a proton has only been calculated to an accuracy of around 4% - (roughly 938 MeV/c2 or 1.672 × 10-27 kg). The constituent particles - quarks and gluons - which make up the proton, have individual masses that add up to only around 1% or so of its measured mass (which can be determined accurately with specialised devices called

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