A new measurement by RHIC’s STAR collaboration reveals that the force between antiprotons (p with bar above) is attractive and strong—just like the force that holds ordinary protons together within the nuclei of atoms.
Antimatter is extremely rare, but can be created in relative abundance in particle colliders such as the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Smashing together atomic nuclei traveling at nearly the speed of light creates fireballs of intense energy that produce particles of matter and antimatter in equal amounts, just like the Big Bang that gave birth to the universe. Searching for instances of antiprotons emitted close to one another within these collisions, scientists from RHIC’s STAR collaboration have—for the first time—measured the strength and range of their interactions. They found that this force is strongly attractive and short-ranged, just like the strong nuclear force that holds ordinary atomic nuclei together. This work looked at the question of matter-antimatter asymmetry in a new way, and it provided critical ingredients for understanding more complex antinuclei and their properties.
Understanding the behavior of antimatter is key to unlocking one of the biggest mysteries in science: why the universe today consists mainly of ordinary matter with virtually no antimatter to be found. This result provides the first measurement of the strong force between antiprotons, and shows no difference in the way the strong nuclear force operates on the antimatter counterparts of ordinary protons.
Antimatter nuclei as large as antihelium-4—containing two antiprotons and two antineutrons—have been observed in heavy-ion collisions at the RHIC. To obtain a “clean” measurement of the force that binds antiprotons and antineutrons together into antinuclei, scientists in the STAR Collaboration at RHIC searched data from gold-gold collisions to identify neighboring pairs of unbound antiprotons. They studied correlation properties of antiprotons to infer the strength and range of their interaction. The measurements showed that antiprotons experience a strong, short-range attractive force similar to the strong nuclear force that binds protons and neutrons to form ordinary atomic nuclei. Within the error of the measurement, they could not identify any differences between the antiproton-antiproton interactions and proton-proton interactions. The successful demonstration of this analysis technique also paves the way for future detailed studies into the strong interaction between other species that are created in relative abundance in particle colliders.
The STAR Collaboration, “Measurement of interaction between antiprotons.” Nature (2015). [DOI: 10.1038/nature15724].