The first measurement of the interaction between antiprotons – the antimatter equivalent of protons – has been completed by an international team of physicists working at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York.
The team, part of the STAR (Solenoidal Tracker at RHIC) collaboration, has discovered that antiproton-antiproton interactions are consistent with proton-proton ones; this is the first study to show that antiproton interactions are attractive. Antiprotons have the same mass but different electric charge to regular protons, and understanding the subtlety of how fundamental particles interact is a central goal of nuclear physics.
The RHIC experiment uses collisions between two beams of gold ions that travel at the speed of light to produce new particles. The collision breaks the protons and neutrons from the gold into quarks, and it produces thousands more particles, including antiprotons.
Protons are fundamental particles that reside in the nucleus, at the center of atoms. They have positive electric charge and they are surrounded by neutrons, which have no electric charge. Both protons and neutrons, collectively called nucleons, are made of quarks which interact using the strong nuclear force, gluing the particles together and making (most) atoms perfectly stable.
The strong nuclear force is attractive around 1 femtometer (10-15 meters), but it becomes repulsive for smaller values and quickly decreases to insignificance for higher values. As the strong force is about 60 times stronger than the electromagnetic force at those scales, it is possible to have an attractive proton-proton interaction.
Antimatter is very difficult to study, as it will annihilate at the first interaction with matter. So far anti-hydrogen, anti-helium, and their isotopes have been observed but this study was the first time the actual forces between nucleons were measured.
Matter and antimatter possess a high degree of symmetry and, in cases like this, they are the exact mirror of one another. The symmetry is not perfect and the laws of physics at the beginning of the universe favored matter slightly more than antimatter, which explains why we only observe matter around us and we have to create antimatter in laboratories. Understanding the potential and subtle difference in the behavior of antimatter could give us important information about the nature of reality.
The paper is published in Nature.