Surprising result shocks scientists studying spin

Imagine playing a game of billiards, putting a bit of counter-clockwise spin on the cue ball and watching it deflect to the right as it strikes its target ball. With luck, or skill, the target ball sinks into the corner pocket while the rightward-deflected cue ball narrowly misses a side-pocket scratch. Now imagine your counter-clockwise spinning cue ball striking a bowling ball instead, and deflecting even more strongly—but to the left—when it strikes the larger mass.

That's similar to the shocking situation scientists found themselves in when analyzing results of spinning protons striking different sized atomic nuclei at the Relativistic Heavy Ion Collider (RHIC)—a U.S. Department of Energy (DOE) Office of Science User Facility for nuclear physics research at DOE's Brookhaven National Laboratory. Neutrons produced when a spinning proton collides with another proton come out with a slight rightward-skew preference. But when the spinning proton collides with a much larger gold nucleus, the neutrons' directional preference becomes larger and switches to the left.

"What we observed was totally amazing," said Brookhaven physicist Alexander Bazilevsky, a deputy spokesperson for the PHENIX collaboration at RHIC, which is reporting these results in a new paper just published in Physical Review Letters. "Our findings may mean that the mechanisms producing particles along the direction in which the spinning proton is traveling may be very different in proton-proton collisions compared with proton-nucleus collisions."

Understanding different particle production mechanisms could have big implications for interpreting other high-energy particle collisions, including the interactions of ultra-high-energy cosmic rays with particles in the Earth's atmosphere, Bazilevsky said.