At the right temperature protons and neutrons ‘melt’ to become a plasma of their constituent particles: quarks and gluons. New experiments at the Relativistic Heavy Ion Collider (RHIC), at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have now determined that the temperature at which quark-gluon plasma (QGP) forms is approximately 4 trillion degrees Celsius. The number is difficult to comprehend; it’s 250,000 times hotter than the center of the Sun. According to current theory, this was the temperature only a few milliseconds after the Big Bang.
The RHIC achieved this astounding temperature (for the first time ever in a lab) by running atoms of gold around its 2.4 mile (3.25 kilometers) track and smashing them together. At collision, the instant of highest temperature lasts no longer than it takes light to cross a single proton.
The incredible numbers of nuclear physics are impressive, but the meat of the experiment is the growing insight into the behavior of the most fundamental particles of matter. In a sense, this insight began with the accepted analytical theory that when quarks and gluons separated from protons and neutrons it would be in the form of a gas. In 2005, the RHIC performed a series of experiments that showed this was not the case; they formed a liquid – plasma. In fact, it was a perfect liquid with quarks and gluons closely interacting yet totally without resistance or viscosity.
The next step was to pin down the temperature at the plasma formation.
In the papers published in 2005, RHIC physicists laid out a plan of crucial measurements to clarify the nature and constituents of the “perfect” liquid. Measuring the temperature early in the collisions was one of those goals. Models of the evolution of the matter produced in RHIC collisions had suggested that the initial temperature might be high enough to melt protons, but a more direct measurement of the temperature required detecting photons — particles of light — emitted near the beginning of the collision, which travel outward undisturbed by their surroundings.
“This was an extraordinarily challenging measurement,” explained Barbara Jacak, a professor of physics at Stony Brook University and spokesperson for the PHENIX collaboration. “There are many ways that photons can be produced in these violent collisions. We were able to ‘eliminate’ the contribution from these other sources by exploiting RHIC’s flexibility to measure them directly and to make the same measurement in collisions of protons, rather than of gold nuclei. Thus we could pin down excess production in the gold-gold collisions, and determine the temperature of the matter that radiated the excess photons. By matching theoretical models of the expanding plasma to the data, we can determine that the initial temperature of the ‘perfect’ liquid has reached about four trillion degrees Celsius.
These are difficult (and expensive) steps, but they bring particle physicists further down the path to explaining the most fundamental particles (that we know of). The field is called quantum chromodynamics, the theory of behavior for the smallest components of the nucleus. Among other things, from such knowledge may eventually flow explanations for the Big Bang and what happened thereafter.