July 23, 2014 - 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‚Äôs dubbed the ‚Äúproton
spin crisis.‚ÄĚ Initially physicists thought a proton‚Äôs 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‚Äôs 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‚Äôs 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‚Äôs three quarks were always spinning in opposite directions, canceling one another out, leaving the remaining one half as the proton‚Äôs total spin. ‚ÄúThat was the na√Įve idea 25 years ago,‚ÄĚ 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. ‚ÄúBy the end of the ‚Äė80s 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.‚ÄĚ Later measurements actually suggested quarks can contribute up to 25 percent of the proton‚Äôs total spin, but that still leaves the lion‚Äôs 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‚Äôs 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 ‚Äúspin-polarized‚ÄĚ 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‚Äô spins are not aligned.) ¬† When two protons slam together, their interaction is controlled by the strong force, so gluons are intimately involved.