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Experiments reveal new techniques in studying quark-gluon plasma


Experiments at RHIC and the LHC have complementary strengths in studying the quark-gluon plasma, a state of matter in which quarks come unbound. Image: Brookhaven National Laboratory

Carefully examining H2O molecules at room temperature will tell you a lot about the structure of water. But you need to vary the conditions to gain much insight into how it becomes vapor or ice.

In the past year, scientists at two large particle accelerators have been making these types of adjustments, studying their subject at a wide range of energies. Only they’re not changing the temperature of water; they’re tweaking a state of matter 100,000 times hotter than the interior of the Sun – the quark-gluon plasma.

“We are rapidly entering an era of detailed investigation,” said CERN theoretical physicist Urs Wiedemann. “At the moment, nature has given us just the right tools to study the properties of the QGP.”

The quark-gluon plasma is a state of matter in which quarks, which usually exist only in pairs or threes, float freely in a hot cosmic soup. Theorists think the universe existed in this state microseconds after the big bang, just before cooling into the normal state of matter we see today.

Understanding the properties of the QGP does not fully explain how the universe formed the way it did, Wiedemann said.

“It’s like asking how a child’s nutrition at age 10 affected his height at age 18," he said. "Clearly his nutrition affected it, but it’s just one of many factors.”

Still, he said, it’s worth studying. Of the multiple phase transitions that theorists think the universe underwent after the big bang, only the one between the QGP and normal matter is currently accessible to man-made experiments.

The Relativistic Heavy-Ion Collider, or RHIC, at Brookhaven National Laboratory and the Large Hadron Collider, or LHC, at CERN complement one another well in the study of the quark-gluon plasma. RHIC is able to create the QGP in collisions at a wide variety of energies, though it lasts only moments before cooling back into regular matter. The LHC is able to create the quark-gluon plasma at high energies, which lasts longer before returning to a normal state.

The STAR and PHENIX experiments at RHIC and the ALICE, CMS and ATLAS experiments at the LHC presented preliminary results of their latest studies of the QGP this week at the Quark Matter conference in Washington D.C.

The LHC has spent a month each of the past two years colliding heavy lead ions to study the QGP at high energy. This year, RHIC diversified, colliding uranium ions with one another and colliding gold ions with copper ions, among other combinations. They did this at energies ranging from 7.7 to 200 GeV.

“What’s really great and new is that we have a lot of knobs we know how to control,” said PHENIX Spokesperson Barbara Jacak. “That’s what excites me about the field right now.”

In addition, the RHIC scientists presented new data from control experiments in which the quark-gluon plasma is not created.

The RHIC experiments have begun to feel out a possible boundary between regular matter and the QGP, which could occur below 39 GeV.

They and experiments at the LHC have also significantly advanced their studies of charm quarks, heavy particles affected by the QGP. Just as watching an ice cube melt can tell you about the temperature of the air around it, watching a charm quark and its antiparticle break apart can tell you something about the characteristics of the QGP around it.

“It’s been a long road to understand how all of that works,” said ALICE physicist Peter Jacobs of Lawrence Berkeley National Laboratory.

The ALICE experiment benefitted this year from the completion of a significant piece of its detector, its electromagnetic calorimeter, which has allowed ALICE scientists to make a much more detailed study of jets of particles that are suppressed as they attempt to move through the quark-gluon plasma.

Results at this year's conference may be only preliminary, but scientists are encouraged by the progress they've made on multiple fronts on the path toward understanding the birth of the universe.