By Ashley Yeager

Protons and neutrons "melt" to produce a plasma of freely interacting quarks and gluons. Credit: RHIC/BNL.

On July 20, readers of the journal Science are in for quite a treat — a clear, concise explanation of what matter looked like in the early universe and how scientists study it.

Science has “published very few articles in this field, as is generally the case in nuclear and particle physics, so it was a nice surprise when they invited us to write the piece,” said Duke theoretical physicist, Berndt Mueller, a co-author of the review article.

Subatomic scientists rarely try to present their latest discoveries in widely accessible terms, which may be why the journal does not run many articles related to nuclear physics, he said.

In the piece, Mueller and his co-author, Barbara Jacak, an experimental physicist at Stony Brook University, describe the most recent discoveries and remaining questions about matter in the early universe, a primordial soup called the quark-gluon plasma.

One of the most puzzling questions for physicists in this field how the subatomic particles that make matter — quarks and gluons — behave when they are “liberated,” or broken apart, as they were in the early phase of the cosmos. “We now know, thanks to the experiments at the Relativistic Heavy Ion Collider (RHIC), that they behave completely differently than theorists thought, but we only know some aspects, many others remain to be explored,” Mueller said.

The “big” question is what structure the primordial soup of quarks and gluons takes at extremely high temperatures‚ beyond two trillion degrees Celsius. “We don’t know. It’s somehow made up of quarks and gluons. It’s like saying that liquid water is made up of the elements hydrogen and oxygen, but not knowing that they form water, or H2O molecules, and that those again cluster in teaming little molecular clusters,” Mueller said.

Gold particles collide, forming a soup of quarks and gluons. Click the image to watch a more-detailed video about the quark-gluon soup.Credit: RHIC/BNL.

Answering the structure question may also help to better explain how some atoms at the opposite extreme of the temperature scale‚ a minute fraction of a degree above absolute zero, act as a nearly perfect fluid, flowing almost without resistance, just as the quark-gluon plasma does. The answer could also offer ways to comprehend black holes, string theory, and extra dimensions.

To make those connections, the authors argue that physicists will need to explore how quark matter evolves over a range of energies, temperatures and densities. They can use the Large Hadron Collider (LHC) in Europe to probe the universe’s primordial soup at the highest range of energies. But RHIC seems to be able to operate at the energy “sweet spot” for exploring the transition from ordinary matter to the quark-gluon plasma.

The scientists will talk more about future plans and research at RHIC and LHC at the Quark Matter 2012 conference in Washington, D.C. on August 12-18, which the Duke nuclear theory group helped to organize. Jacak, the leader of a 500-member international collaboration performing experiments at RHIC and a member of the National Academy of Sciences, will be at Duke on Sept. 5 to give a talk about hot nuclear matter research.

Citation: The Exploration of Hot Nuclear Matter. Jacak, B., and Mueller, B. 2012. Science. 337: 310-314.
DOI: 10.1126/science.1215901