Atom Smasher Sheds New Light On Proton Spin Mystery

Spin Quarks Proton

Protons have a constant spin that is an intrinsic particle property like mass or charge. Yet where this spin comes from is such a mystery it’s dubbed the “proton spin crisis.” Initially physicists thought a proton’s spin was the sum of the spins of its three constituent quarks. But a 1987 experiment showed that quarks can account for only a small portion of a proton’s spin, raising the question of where the rest arises. The quarks inside a proton are held together by gluons, so scientists suggested perhaps they contribute spin. That idea now has support from a pair of studies analyzing the results of proton collisions inside the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, N.Y.   Physicists often explain spin as a particle’s rotation, but that description is more metaphorical than literal. In fact, spin is a quantum quantity that cannot be described in classical terms. Just as a proton is not really a tiny marble but rather a jumble of phantom particles appearing and disappearing continuously, its spin is a complex probabilistic property. Yet it is always equal to one half.

  Quarks also have a spin of one half. Physicists originally assumed that two of the proton’s three quarks were always spinning in opposite directions, canceling one another out, leaving the remaining one half as the proton’s total spin. “That was the naïve idea 25 years ago,” says Daniel de Florian of the University of Buenos Aires, leader of one of the new papers, which was published July 2 in Physical Review Letters. “By the end of the ‘80s it was possible to measure the contribution of the spin of the quarks to the spin of the proton, and the first measurement showed it was 0 percent. That was a very big surprise.” Later measurements actually suggested quarks can contribute up to 25 percent of the proton’s total spin, but that still leaves the lion’s share unaccounted for.  

Gluons are also present inside protons as the representatives of the strong nuclear force, a fundamental interaction that binds the quarks together. Gluons each have a spin of 1, and depending on which direction it is they could add up to make most of rest of the proton’s spin. Measuring their contribution is a tricky task. RHIC is the only experiment that can address the question, because it is the only particle accelerator built to collide “spin-polarized” protons, meaning that the particles are all spinning in a certain direction when they crash. (At the more powerful Large Hadron Collider in Switzerland, the particles’ spins are not aligned.)   When two protons slam together, their interaction is controlled by the strong force, so gluons are intimately involved ( via ).

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