Researchers at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility said it had been thought the spin of the proton would come from its quarks, but experiments established the quarks' spin accounts for only one-third of the proton's spin. Researchers thus began investigating other sources of the proton's spin.
The new U.S. study concerns a theoretical model proposed by Jefferson Lab Chief Scientist Tony Thomas and University of South Carolina Professor Fred Myhrer. They suggested some of the proton's spin is generated as orbital angular momentum by its quarks.
"Rather than the way the quarks are spinning, it's the way they're moving in orbital motion," said Thomas. "In fact, more than half of the spin of the proton is orbital motion of the quarks. That's a really fascinating thing."
The study is reported in the Sept. 5 issue of the journal Physical Review Letters.