The different energy ranges reveal different aspects of the plasma. Raghav Kunnawalkam Elayavalli, a physicist at Vanderbilt University, did their Ph.D. work at the LHC, but recently became a member of the STAR and sPHENIX collaborations to focus on the particles coming out of lower-energy collisions. “They are closer to the scale of the plasma; they talk to it a lot more,” Kunnawalkam Elayavalli says. “Think of it like a party: there's a lot of people, and you're making a beeline to the exit. But if you're kind of slow and you don't want to leave that fast, you get a chance to talk to people on your way out.” Because particles flying through the quark-gluon plasma at RHIC take longer to move through it, they can extract more information from it. “The things we're trying to measure are the transport properties—the average distance you can go without interacting with another particle,” they add. “It tells us about the fundamental scale of the plasma.”

RHIC is designed to collide heavy ions, such as the nuclei of gold atoms. In this case, though, researchers were interested in near misses, not collisions. As the gold nuclei zing at near light speed through the collider, they create an electromagnetic field that generates photons. When two gold nuclei come close to one another but don’t collide, the photons may ping off the neighboring nuclei. These near misses used to be considered background noise, says STAR collaborator Raghav Kunnawalkam Elayavalli, a physicist at Vanderbilt University. But looking at the close-call events “opened up a whole new field of physics that initially was not accessible,” Kunnawalkam Elayavalli says.