proton spin puzzle

A proton has spin one-half, a basic and well-measured fact. The obvious explanation, from the simple quark model, is that the proton's three quarks each carry spin one-half, and their spins add up to give the proton its spin. For years this seemed too simple to be wrong. Then experiments looked closely at where the spin actually comes from, and the simple answer collapsed.

In the late 1980s, experiments that scattered polarized particles off polarized protons found that the spins of the three quarks account for only about a quarter to a third of the proton's total spin — not nearly all of it. The rest had to come from somewhere else. This shortfall is the proton spin puzzle (sometimes called the spin crisis). The missing spin is now understood to be shared among the gluons, which carry spin of their own, and the orbital motion of the quarks and gluons swirling inside the proton, like planets orbiting as well as spinning.

The puzzle matters because it shows how far a proton is from a simple bag of three quarks. Pinning down each contribution — quark spin, gluon spin, orbital motion — is hard experimental and theoretical work that is still ongoing, and it is one of the main scientific goals of a future machine, the Electron-Ion Collider. Understanding the proton's spin in full is, in effect, understanding how the strong force assembles a particle out of restless, interacting pieces.