The proton spin crisis (sometimes called the "proton spin puzzle") is a theoretical crisis precipitated by an experiment in 1987 which tried to determine the spin configuration of the proton. The experiment was carried out by the European Muon Collaboration (EMC).
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Physicists expected that the quarks carry all the proton spin. However, not only was the total proton spin carried by quarks far smaller than 100%, these results were consistent with almost zero (4%-24%) proton spin being carried by quarks. This surprising and puzzling result was termed the "proton spin crisis". The problem is considered one of the important unsolved problems in physics.
Background
A key question is how the nucleon's spin is distributed amongst its constituent partons (quarks and gluons). Physicists originally expected that quarks carry all of the nucleon spin.
A proton is built from two up quarks, one down quark, and gluons. The ruling assumption was that since the proton is stable, then it exists in the lowest possible energy level. Therefore, it was expected that the quark's wave function is the spherically symmetric s-wave with no spatial contribution to angular momentum. The proton is, like each of its quarks, a spin-1/2 particle. Therefore, it was assumed that two of the quarks have opposite spins and the spin of the third quark is parallel to the proton spin.
The experiment
In this EMC experiment, a quark of a polarized proton target was hit by a polarized muon beam, and the quark's instantaneous spin was measured. In a polarized proton target, all the protons' spin take the same direction, and therefore it was expected that the spin of two out of the three quarks cancels out and the spin of the third quark is polarized in the direction of the proton's spin. Thus, the sum of the quarks' spin was expected to be equal to the proton's spin.
However, it was found in this EMC experiment that the number of quarks with spin in the proton's spin direction was almost the same as the number of quarks whose spin was in the opposite direction. This is the proton spin crisis. Similar results have been obtained in later experiments.
Recent work
A 2008 work shows that more than half of the spin of the proton comes from the spin of its quarks, and that the missing spin is produced by the quarks' orbital angular momentum. This work uses relativistic effects together with other quantum chromodynamic properties and explains how they boil down to an overall spatial angular momentum that is consistent with the experimental data. 2016 results from the RHIC indicate that gluons may carry even more of protons' spin than quarks do.