Measuring matter hotter than the sun

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A computer rendering of a collision of two beams of gold ions in the STAR detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The beams travel in opposite directions at nearly the speed of light before colliding. The PHENIX detector (below) records many different particles emerging from RHIC collisions, including photons, electrons, muons, and quark-containing particles called hadrons. (Courtesy: Brookhaven National Laboratory)

VANDERBILT (US)—Scientists have created an exotic state of matter with a temperature of four trillion degrees Celsius. It’s the hottest temperature ever achieved in a laboratory and 250,000 times hotter than the heart of the sun.

The results were announced at the annual meeting of the American Physical Society in New York. Details of the findings will be published in Physical Review Letters.

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The team, which included three Vanderbilt University physicists, produced this super-heated state of matter by accelerating gold nuclei to nearly the speed of light in opposite directions and then colliding them in the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory—the largest particle accelerator in the world dedicated to nuclear physics research.

The new temperature measurement has strengthened the physicists’ conviction that they have managed to recreate the quark-gluon plasma (QGP), a state of matter that may have existed fractions of seconds after the Big Bang.

For the last nine years, Vanderbilt physicists Victoria Green, Charles Maguire, and Julia Velkovska have played a key role in the design and operation of PHENIX, the largest of the four detectors positioned around the accelerator’s 2.4-mile-circumference.

“The temperature measurement was made at PHENIX by measuring the photons, the light particles, that were created with the plasma and escaped without interacting with other particles,” says Greene, professor of physics and executive dean in the College of Arts and Science, who contributed to the paper reporting the result.

To do this, the scientists figured out a clever way to separate the primordial photons from those coming from more mundane sources by comparing the photons produced in the collisions of the gold nuclei with those produced in simple collisions between two protons.

RHIC scientists don’t know the exact temperature at which the protons and neutrons in atoms “melt” into their constituent parts, quarks and gluons. But they do think it is considerably below four trillion degrees. So the new measurement increases their confidence that the matter they are creating and studying is indeed the sought-for QGP.

Their observations of the most primordial of all primordial soups is improving scientists’ understanding of the “strong force,” the force that acts within the nucleus of the atom and holds it together.

Originally, they had predicted that the QGP would act like a gas, with all the particles moving independently. They were surprised to find that the quarks appear to be bound together in a way similar to the molecules in a liquid. This is forcing theoreticians back to their drawing boards.

Increased knowledge about the nature of the QGP can also improve cosmologists’ understanding of the origin of the universe. For the first few microseconds after the Big Bang, cosmologists calculate that temperatures were so hot that a quark-gluon plasma should have formed. However, the universe immediately began to expand and cool. As it cooled below the melting point of protons and neutrons, these particles began forming much like rain drops condensing out of steam.

Brookhaven officials have announced that they will be upgrading RHIC over the next five years to get better measurements of the QGP’s temperature, viscosity, and other basic properties. Greene, Maguire, and Velkovska will continue to participate in the RHIC collaboration, but they have also joined the scientific team using one of the detectors (the Compact Muon Solenoid) on the new Large Hadron Collider, the world’s most powerful particle accelerator operated by the European Organization for Nuclear Research (CERN) in Switzerland. Here they hope to study the behavior of the quark-gluon plasma at even more extreme temperatures.

They are in the process of creating a control room at Vanderbilt that will allow them to monitor the quality of the data and analyze the results in near real time without leaving home.

Vanderbilt news: www.vanderbilt.edu/news

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