'Spin' is an intrinsic property of many subatomic particles - it's been likened in some ways to the 'angular momentum' of a spinning ball, - but more accurately is a mathematical 'vector' or 'property'.

After the discovery that protons are composed of 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…and their associated 'gluons' (holding them together via the Strong Force) it was assumed that the proton's spin property must be derived from the 'spin' of its constituent quarks.

A 1987 experiment at CERN (which has subsequently been replicated by other experiments) showed that this is not the case. It's now thought that the quarks only contribute between 4% and 24%.

This surprising and puzzling result was termed the “proton spin crisis”. The problem is considered one of the important unsolved problems in physics.

Source : Wikipedia

Note that the as yet unexplained anomaly also applies to the Neutron, which is also built from three quarks.

For a technical description, see The Spin Structure of the Nucleon Reports on Progress in Physics Volume 82, Number 7,at arXiv.org

Also see :Proton radius puzzleplugin-autotooltip__plain plugin-autotooltip_bigProton radius puzzle

Until 2010, measurements (backed up by theory) had put the radius of the proton at about 0.87 femtometres, but in that year, experiments at the Max Planck Institute of Quantum Optics in Germany found a 4% discrepancy.

And in 2016, new experiments … and 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%…

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