

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  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  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.

Detecting particles' directional preferences


"We anticipated something similar to the proton-proton effect, because we couldn't think of any reasons why the asymmetry could be different," said Itaru Nakagawa, a physicist from Japan's RIKEN laboratory, who served as PHENIX's deputy run coordinator for spin measurements in 2015. "Can you imagine why a bowling ball would scatter a cue ball in the opposite direction compared with a target billiard ball?"


"This was truly a collaborative effort between experimentalists and accelerator physicists who could tune such a huge and complicated accelerator facility on the fly to meet our experimental needs," Bazilevsky said, expressing gratitude for those efforts and admiration for the versatility and flexibility of RHIC.


"So we have three sets of data—colliding polarized protons with protons, aluminum, and gold," Bazilevsky said. "The asymmetry gradually increases from negative in proton-proton—with more neutrons scattering to the right—to nearly zero asymmetry in proton-aluminum, to a large positive asymmetry in proton-gold collisions—with many more scatterings to the left."

Particle production mechanisms


"In the particle world, things are much more complicated than the simple case of (spinning) billiard balls colliding," Bazilevsky said. "There are a number of different processes involved in particle scattering, and these processes themselves can interact or interfere with one another."

"The measured asymmetry is the sum of these interactions or interferences of different processes," said Kim.




The scientists will continue to analyze the 2015 data in different ways to see how the effect depends on other variables, such as the momentum of the particles in various directions. They'll also look at how preferences of  other than neutrons are affected, and work with theorists to better understand their results.


"If we observe exactly the asymmetry we predict based on the electromagnetic interaction, then this becomes very strong evidence to support our hypothesis," Nakagawa said.

In addition to providing a unique way to understand different particle production mechanisms, this new result adds to the puzzling story of what causes the transverse spin asymmetry in the first place—an open question for physicists since the 1970s. These and other results from RHIC's polarized proton collisions will eventually contribute to solving this question.