Physics: Spinning Protons Change Direction When They Collide With Larger Particles, ‘Shocking’ Scientists

Physicists studying atomic spin surprised themselves by discovering that spinning protons bizarrely change direction when they crash into larger particles, like the nuclei of gold atoms.

On a pool table, you’d expect a left-spinning cue ball to deflect off to the right after it hits another ball. What the physicists observed would be the equivalent of a left-spinning cue ball acting normally when striking another billiard ball, but deflecting to the left—and way more forcefully—after striking a bowling ball, according to a press release from the Brookhaven National Laboratory.

Brookhaven National Laboratory is one of 10 labs across the United States funded by the Office of Science of the U.S. Department of Energy. It contains the Relativistic Heavy Ion Collider (RHIC), the only collider for spin-polarized protons in the world, which was previously responsible for the discovery that antiprotons (antimatter protons) are capable of behaving like regular protons, according to Scientific American. The RHIC became the first instrument to cause a high-energy collision between polarized protons and gold nuclei in 2015. 

A research team used the RHIC to study what happens when polarized protons collided with particles representing a range of different sizes—protons the same size as each other as well as larger aluminum nuclei and still larger gold nuclei. A paper describing the research was published in the scientific journal Physical Review Letters.

“What we observed was totally amazing,” said Brookhaven physicist Alexander Bazilevsky in the press release. “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.”

Proton-proton collisions usually produce right-skewing effects. But large nuclei, like gold, have large positive electric charges, and because of that, the electromagnetic interaction between the two colliding particles becomes a much more dynamic force than it is for two protons, which aside from being smaller also have equal charges, according to Brookhaven National Laboratory. That unequal electromagnetic charge is what causes the incoming proton to switch preference and rocket off to the left.

“We anticipated something similar to the proton-proton effect, because we couldn't think of any reasons why the asymmetry could be different,” Itaru Nakagawa, a physicist from Japan’s RIKEN laboratory, said in the press release. “Can you imagine why a bowling ball would scatter a cue ball in the opposite direction compared with a target billiard ball?"

This is a big step forward in RHIC's ongoing quest to solve the mystery of atomic spin. Each proton contains three smaller quarks that account for roughly one-fifth of its spin, but the particulars of the other four-fifths are still something of a mystery, according to Quanta Magazine. And the more physicists learn about the nuances of how particles are created, the closer we get to understanding other kinds of high-energy particle collisions.


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