Antiprotons match protons in response to strong nuclear force

Tightly bunched antiprotons stick together, just like their proton cousins.

Physicists sifting through subatomic shrapnel inside a particle accelerator have made the first analysis of the interaction between antiprotons, particles of antimatter that are negatively charged but otherwise nearly identical to protons. The findings, published online November 4 in Nature, reveal that the strong nuclear force securely binds antiprotons in close proximity with the same intensity that it does for protons inside the nuclei of atoms.

The study provides insight into the structure of antimatter nuclei, which consist of bound antiprotons and antineutrons. It also adds to the tally of papers finding no differences in the behavior of antimatter and ordinary matter. Any discrepancy could help scientists determine why matter, and not antimatter, dominates the universe.

Physicists studying how protons interact have it easy: Just fire a proton beam at a target made of proton-filled nuclei and see what happens. But antiprotons are hard to make and, like all antimatter, get destroyed when they touch matter.

So Michael Lisa, a particle physicist at Ohio State University in Columbus, and hundreds of colleagues tracked antiprotons inside the Relativistic Heavy Ion Collider, a 3.8-kilometer-around particle accelerator at Brookhaven National Laboratory in Upton, N.Y. Antiprotons are among the scores of particles produced when gold nuclei collide inside the machine at nearly the speed of light. By measuring the energies, trajectories and speeds of various particles created in about 500 million gold collisions, the researchers identified antiprotons and flagged pairs that came into close contact. “The detector is pretty good at digging out antiprotons,” Lisa says.

The physicists found that the attractive strong nuclear force between antiprotons, which kicks in when particles are within a few millionths of a billionth of a meter of each other, overcomes the particles’ repulsion due to their like charge. Measurements of two key aspects of the strong force between antiprotons match those for the proton, the researchers report. “They’ve accomplished a very difficult measurement,” says William Gibbs, a particle physicist at New Mexico State University in Las Cruces.

Recent work has revealed that the charge and mass of protons and antiprotons are indistinguishable from each other (SN: 9/19/15, p. 8); now it seems the particles’ behavior in close quarters is also strikingly similar. A team at the world’s largest particle accelerator, the Large Hadron Collider near Geneva, plans to publish an analysis of antiproton interactions soon.