Quarks in a Surprising Spin

Strange spin. Down quarks spin in the opposite direction as their proton, contrary to predictions.

Quarks spin in an unexpected way, say physicists who have completed the first measurements of how quarks spin inside protons and neutrons. The finding may shed light on a basic question in subatomic physics--what makes protons and neutrons spin the way they do.

The protons and neutrons that make up the nucleus of an atom are in turn made up of smaller particles called quarks. Protons have two "up" quarks and one "down" quark; neutrons have one "up" and two "downs." Until the late 1980s, physicists though that the spins of protons and neutrons--which physicists collectively call "nucleons"--came mostly from their quarks' intrinsic angular momentum, analogous to the momentum of a planet spinning about its axis. But experiments proved otherwise: The quarks' spin contributed only 20% to 30%. The rest seemed to be coming from gluons, particles that carry the attraction between quarks, and from the additional angular momentum of the quarks as they trace tiny orbits inside the nucleon.

To get a better sense of how quark and nucleon spin relate, researchers measured the direction of quark spin at different energies. The favored theory predicted that as a single up or down quark acquired more of the nucleon's total energy, its spin direction would be increasingly likely to match that of the nucleon. But together with earlier work, the new study, co-led by Jian-Ping Chen at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, and Zein-Eddine Meziani of Temple University in Philadelphia, indicates that only up quarks behave as expected. Down quarks tend to spin against the grain, the team will report in an upcoming issue of Physical Review Letters. "It's telling us orbital angular momentum is probably very important" in determining a quark's direction of spin, as well as other nucleon properties, says Chen.

Theorist Stan Brodsky of the Stanford Linear Accelerator Center in California says that interpretation is plausible. "It's always been a fiction to ignore orbital angular momentum," as some physicists tend to do when thinking about nucleons, he says. The theory behind quarks isn't understood well enough to say why up and down quarks behave differently, says Xiangdong Ji of the University of Maryland, College Park. He suspects that's not because the theory is wrong; rather researchers may be misinterpreting it due to the approximations they are forced to use.

Related sites
Jefferson Lab
Background on the "spin crisis"

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