The nuclear strong force binds the smallest bits of matter together to form atoms, thereby making our material world possible. Now physicists at Brookhaven National Laboratory have made the first-ever measurement of a similar strong force for antimatter — the mirror image of regular matter that lies at the heart of one of our biggest cosmological mysteries.
The experiments were conducted by the STAR collaboration at Brookhaven's Relativistic Heavy Ion Collider (RHIC); the results appeared last week in Nature. RHIC recreates conditions similar to those of the universe just after the Big Bang by accelerating heavy atoms (like gold) to speeds approaching the speed of light, then slamming them into each other. The resulting fireball creates a gooey plasma of quarks and gluons — the most fundamental building blocks — as well as tons of matter and antimatter particles.
These particles don't hang around for very long because matter and antimatter are polar opposites: antimatter has a negative charge to counter matter's positive charge, and they annihilate into energy when they collide. There should have been equal amounts of matter and antimatter at the birth of the universe, too, but for some reason, matter gained the tiniest bit of an edge. And good thing, too, otherwise our beautiful material world wouldn't be here. Today, matter dominates our universe, while antimatter is extremely rare.
Physicists would love to understand why this imbalance came about in the first place. "It's a huge mystery," Brookhaven physicist Aihong Tang told Gizmodo. "Anything we learn about the nature of antimatter can help us solve this puzzle."
RHIC is one of the leading facilities probing this particular mystery, because it is capable of producing large amounts of antiprotons, the better to study how they interact.
Previously, the STAR collaboration succeeded in creating the largest antimatter nuclei yet achieved: two antiprotons and two antineutrons, joined together to make an anti-alpha particle. This was clear evidence that something akin to the strong force applied to antimatter.
But Tang and his fellow physicists wanted to get a better look at this force that governs how unbound antiprotons interact. They combed through hundreds of millions of gold-on-gold collisions in the STAR data, to find those that produced pairs of antiprotons sufficiently close to interact with each other. Then they used statistical analysis to determine the strength of the force between them, as well as the distance over which it could act.
It turns out that the antimatter strong force that created those antimatter nuclei looks a lot like the regular strong force that binds matter. Matter and antimatter still seem perfectly symmetric. So it's probably not the antimatter strong force that's to blame for the odd imbalance between matter and antimatter back in those earliest moments of the universe.
This is a small contribution to ongoing efforts to resolve the matter-antimatter mystery by conducting not only more precise tests, but also to approach the problem from a new angle. "We did not expect to solve the whole thing," Tang told Gizmodo. But they did learn something valuable about the antimatter strong force. And one day, all that accumulated knowledge will crack the case.
STAR Collaboration. (2015) "Measurement of interactions between antiprotons," Nature. Published online November 4, 2015.
Images courtesy of Brookhaven National Laboratory/RHIC.