Attractive forces unlikely to be source of matter – antimatter difference

The attractive force between antiprotons – the antiparticle of proton – has been measured  for the first time.  

A research team at Rice University measured the length and range of interaction between two antiprotons – stable short-lived antiparticles – and succeeded in quantifying the force holding together the nuclei in antimatter. These findings support previous studies showing that antimatter and matter forces are similar.

PhD student Kefeng Xin said: “This discovery isn’t a surprise. We’ve been studying the interaction between particles that make up an atom’s nucleus for decades, and we’ve always thought the forces between antimatter particles are the same as for matter. But this is the first time we’ve been able to quantify it.”

Antiprotons carry the opposite electrical charge and spin that protons do. Like all matter, antimatter was also created at the instant of the Big Bang.

In the study, published in the journal Nature, the scientists used the STAR experiment at the Relativistic Heavy Ion Collider to smash gold ions and produce hundreds of millions of particles. This allowed them to measure the scattering length of antiprotons – a measurement of how particles deviate as they travel from source to destination – and their effective range that showed how close particles need to be for their charges to influence each other.

They found that the scattering length was roughly 7.41femtometers (1x10^?15 metre), and the effective range was 2.14femtometers, nearly equivalent to their proton counterparts.

Physicists had thought that the differences in attractive forces could explain why they see so few antiparticles in nature even though particles and antiparticles were produced in equal amounts.

“It could have been that antimatter didn't have the same attractive force as matter and would have helped explain how these differences, during the initial part of the Big Bang, might have resulted in antimatter not having survived in the shape of stars and planets, as matter did,” said Professor Frank Geurts at Rice University

However, these recent measurements suggest the mechanism was more complex. “The interactions between two antimatter particles turn out to be quite similar to matter particles. It may not give us a solution to the bigger problem, but we most definitely removed one option,” he said.

Paper: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature15724.html