As shown below the proton spin puzzle is just an important proof of quark compositeness.
The proton spin puzzle started with the paper by the European Muon Collaboration (EMC) [Phys. Lett. B Vol. 206(2), 1988] which found for the proton spin the result (1±12±24)% of the total spin.
The solution is, actually, very simple and is directly linked to quark compositeness. As shown in the papers The Higgs boson and quark compositeness (published in Moriond 2014 proceedings) and The Higgs-like Boson and Quark Compositeness, and also in the presentation at Moriond 2014, the proton has two layers of primons (prequarks) whose spin components in the Z direction are ± 1/4, although they are fermions of spin 1/2. Thus, each layer of 3 primons in the proton has a total spin of +1/4 in the +Z direction. Therefore, the two layers yield +1/4 + (+1/4) = +1/2, but deep inelastic experiments (that is, very short Compton wavelength) probe only the inner layer that yields only + 1/4 which agrees with the above result by EMC.
As shown in the paper The Ultimate Division of Matter, since primons are 1/2 spin fermions, there is an intrinsic spin asymmetry in the proton (and also in the neutron) because as <Sx>^2 + <Sy>^2 + <Sz>^2 = 1/2(1/2+1) and <Sx>^2 = <Sy>^2, <Sz>^2=1/16=0.063. And 1/16 is just 25% of (1/2)^2=0.25. This intrinsic asymmetry should be very import for experiments with polarized proton beams, such as the RHIC’ s spin experiment.
The proton structure shown in the papers above mentioned was found out a long time ago at SLAC by the great physicist Robert Hofstadter (Nobel Prize of 1961 with R. L. Mössbauer). For details take a look at the post
in this web page.
In RHIC experiments polarized protons beams hit each other, so that it is expected the results to be completely different from DIS results. Over all effects are measured in this case, but I really do not know anything about this experiment. I hope that RHIC scientists will take a look at their results considering my model for the proton. I am sure that their understanding will be improved.
Please, take also a look at the post BIASED NUCLEON STRUCTURE.