RHIC Shutdown at Brookhaven: Quark-Gluon Plasma, Cold Nuclear Matter, and the Road Ahead

Science Friday host Flora Lichtman sits down with Gene Van Buren, a nuclear physicist at Brookhaven National Laboratory, to reflect on the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and its final collisions. They recount how RHIC transformed our understanding of matter at extreme temperatures, revealing a liquid-like quark-gluon plasma with unusually low viscosity, challenging initial expectations of a gaseous state. The discussion covers why large nuclei are used in collisions, what the term heavy ion means, and how RHIC’s results inform future research. As RHIC closes, scientists outline plans for studying cold nuclear matter with a new collider, a decade-long upgrade that will repurpose existing rings to accelerate electrons into nuclei, and the global, collaborative nature of high-energy physics.

Introduction and context

Flora Lichtman introduces the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and explains that its last collisions mark the end of an era for US high-energy nuclear physics. The discussion frames RHIC as a facility that studied the quark-gluon plasma (QGP) and the transition of nuclear matter under extreme conditions.

"it's absolutely a celebration" - Flora Lichtman

Foundations: why RHIC and what it studied

The guests review the original goal for RHIC, which was to explore the quark-gluon plasma, a state of matter hypothesized at very high temperatures and densities. They describe how heavy-ion collisions help scientists probe the interactions of quarks and gluons and how these experiments inform our understanding of the universe's earliest moments.

From gas to liquid: the surprising nature of the QGP

A key finding from RHIC was that the produced plasma behaved not as a gaseous state but as a liquid with exceptionally low viscosity, near the quantum limit. This liquid, often described as a perfect fluid, provided unprecedented insight into the behavior of strongly interacting matter under extreme conditions.

"the first goal was to study what we believed was going to be the quark gluon plasma" - Gene Van Buren

What heavy ions do and how they differ from lighter systems

The conversation breaks down the concept of heavy ions, explaining that a heavy ion is an atom with a very large nucleus, used to maximize the amount of nuclear matter in collisions. This bulk behavior is necessary to observe collective phenomena and bulk properties such as fluid-like motion in the QGP.

"A heavy ion is an ion where the nucleus is very large" - Flora Lichtman

Why shutdown, and what comes next: probing cold nuclear matter

Despite the exciting science, the field has decided to pivot from heating matter to study hot plasma toward probing cold nuclear matter. The plan is to build a new collider that collides very small objects with large nuclei, enabling electron-nucleus scattering to illuminate the structure and movement inside nuclei when they are cold. This shift reflects a strategy to extract more complete understanding from different regimes of nuclear matter.

Royalties of this pivot include using one storage ring from RHIC and replacing the other with an electron accelerator, with detectors designed to observe these new collisions. The timeline is about a decade to complete these upgrades and to bring online the associated research program.

"Nature is actually doing this all the time, and it has been for millennia" - Flora Lichtman

Timeline, data, and the US role in global physics

The guests discuss what the next decade looks like for US physics in a world with an interconnected global community. They acknowledge that while the US will face challenges from a temporary lull in large facilities, scientific analysis and collaboration continue globally, with physicists around the world contributing to the search for answers about matter under extreme and cold conditions. The host notes the emotional dimension of closing a long-running project, while the guest emphasizes the scientific positives and stored data to be analyzed for years to come.

"The next effort is to probe cold nuclear matter" - Flora Lichtman

Closing thoughts

The interview closes with reflections on the community, the data already collected, and the work still ahead—an ongoing, global pursuit of understanding the fundamental nature of matter, even as the US builds toward a new era of collider science.