Smashing Nuclei to Glimpse What Came After the Big Bang

A few millionths of a second after the big bang, the whole universe was a hot, dense, mix of particles called the quark-gluon plasma (QGP). For almost a decade, scientists have been able to recreate this plasma by smashing heavy nuclei, such as gold, into each other at nearly the speed of light.
 
Now, a large international team of scientists have shown that they can also produce QGP from collisions of smaller, lighter nuclei. This corroborates results from previous experiments, and can help scientists learn about the early universe and the strong interaction in ways they cannot learn from smaller system collisions.
 
Professor Stefan Bathe (GC/Baruch, Physics), CUNY graduate students Daniel Richford, Zachary Rowan, and Zhiyan Wang, and former graduate student Jason Bryslawskyj are members of the PHENIX collaboration and are authors on the paper, which appears in Nature Physics.
 
Quarks and gluons make up the more familiar protons and neutrons, which are the building blocks of an atom’s nucleus. In the QGP, quarks and gluons exist in a fluid-like state with almost zero viscosity. The PHENIX collaboration uses the particle collider known as RHIC at Brookhaven National Laboratory to smash nuclei into each other at enormous speeds. This creates QGP for a fraction of a second and lets researchers study the early-universe mixture.
 
Scientists previously thought that collisions of small nuclei would produce something too small and short-lived to be QGP.
 
“There were some hints that there might be QGP in smaller systems as early as 2010, but nobody could believe it.,” Bathe said, “so we needed to collect a lot of evidence.” This latest paper provides that evidence by studying initial geometries and flow patterns of collisions between gold and a proton, deuteron, and helium-3 nucleus, individually.
 
“The fact that it exists in small collision systems can tell us something new about the properties of the QGP,” Bathe says. A better understanding of the QGP can help researchers unravel how matter developed in the early milliseconds of our universe.
 
Scientists can also use this information to build a better understanding of the strong interaction, one of four fundamental forces of nature along with the weak interaction, electromagnetism, and gravity. The strong interaction governs the QGP’s behavior.
 
“We know the theory behind the strong interaction, but it is complicated to calculate and predict all the emergent phenomena. We are experimentally trying to see if the predictions of the strong interaction are correct, trying to learn something that theory alone can't tell us,” Bathe says.
 
Bathe has been involved in the PHENIX experiment since he was a graduate student and traveled from Germany to the U.S. to spend half a year at Brookhaven.
 
“When I was a student in 1999, the PHENIX experiment was just being constructed,” Bathe says. “This was very exciting because the accelerator was new and there were many people working together from all over the world.” Later he returned to Brookhaven in a postdoctoral position and remained involved as an analysis coordinator and deputy spokesperson after coming to CUNY.
 
Now the PHENIX project is coming to a close. A new project and new detector at RHIC, sPHENIX, will take its place.
 
“sPHENIX will also study the QGP,” Bathe says, “and will start collecting data in 2023. We need to understand other signatures of the QGP that we could not measure with PHENIX.”