According to a large international team of physicists, the attractive force between antiprotons is similar to that between protons.

The team, called the STAR collaboration, measured two key important parameters: the scattering length and the effective range of interaction between two antiprotons. This gave physicists a new way to understand the force that holds together the nuclei in antimatter and how this compares to matter.

“This is about the subtle difference in the way matter and antimatter interact with each other,” explained team member Dr Frank Geurts, a physicist at Rice University in Houston, Texas.

The scattering length is a measurement of how particles deviate as they travel from source to destination; their paths are visible as 3D traces. The effective range indicates how close particles need to be for their charges to influence each other, like magnets.

For antiprotons measured by the team, the scattering length was roughly 7.41 femtometers, and the effective range was 2.14 femtometers, nearly equivalent to their proton counterparts.

The physicists have previous experience detecting and studying rare forms of antimatter-including anti-alpha particles, the largest antimatter nuclei ever created in a laboratory, each made of two antiprotons and two antineutrons.

Those experiments gave the scientists some insight into how the antiprotons interact within these larger composite objects.

“But in that case, the force between the antiprotons is a convolution of the interactions with all the other particles. We wanted to study the simple interaction of unbound antiprotons to get a cleaner view of this force,” said team member Dr Aihong Tang, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory.

To do that, the scientists searched the data – collected by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) – from gold-gold collisions for pairs of antiprotons that were close enough to interact as they emerged from the fireball of the original collision.

“We see lots of protons, the basic building blocks of conventional atoms, coming out, and we see almost equal numbers of antiprotons,” said team member Zhengqiao Zhang of the Shanghai Institute of Applied Physics, China.

“The antiprotons look just like familiar protons, but because they are antimatter, they have a negative charge instead of positive, so they curve the opposite way in the magnetic field of the detector,” he explained.

“By looking at those that strike near one another on the detector, we can measure correlations in certain properties that give us insight into the force between pairs of antiprotons, including its strength and the range over which it acts.”

The findings were published this week in the journal Nature.

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The STAR Collaboration. Measurement of interaction between antiprotons. Nature, published online November 04, 2015; doi: 10.1038/nature15724