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 Quarks become Un-gluon-ed
 

Despite billions of years of history to choose from, Robert L. Sugar is interested in just a few fractions of a second -- the very first ones.

This physicist from the University of California at Santa Barbara (UCSB) is part of a nine-member, multi-institutional team of high-energy physicists that is recreating the first billionths of a second following the big bang -- the explosion of energy that gave birth to the universe. In those microseconds, when temperatures are thought to have exceeded one trillion degrees Kelvin, physicists believe that matter was a formless plasma of quarks and gluons -- the infinitesimally small particles from which nuclei of atoms are built.

Sugar and his colleagues are attempting to simulate these extreme conditions to address fundamental questions about the origins of the universe. The team -- physicists from the University of Arizona, UCSB, University of Colorado, Florida State University, Indiana University, University of the Pacific, University of Utah, Washington University, and NORDITA -- comprise the MIMD Lattice Calculation (MILC) Collaboration, a Department of Energy Grand Challenge initiative that is using parallel computing to model quantum chromodynamics (QCD). QCD is an intriguing and relatively new theory, having arisen within the past quarter century, that describes the "strong" nuclear force that binds quarks and gluons. The model being created by the MILC Collaboration, and simulated using Alliance computers, may help confirm this early plasma state of matter. It will deepen physicists' understanding of how the elementary particles unite and, in particular, how they decay.

 

Quarks and quantum chromodynamics

Although no one has observed isolated quarks and gluons, the theory governing their behavior -- quantum chromodynamics -- is well-tested and, therefore, accepted as one of four theories that govern all of nature, with gravity, electromagnetism, and the weak nuclear force being the other three theories. QCD tells us that quarks are the subatomic particles that, in addition to forming protons and neutrons, are the building blocks for a host of short-lived particles observed in high-energy physics experiments. These quarks come in six varieties, each with successively greater mass: up, down, strange, charm, bottom, and top. Quarks typically are found in pairs or triplets. Although these particles are continually in motion, physicists can approximate the forms of matter that various combinations of quarks produce. For example, it is thought that a proton contains two up quarks and one down quark, and a neutron, two down quarks and one up quark. Other particles, called mesons, consist of a quark and an antiquark, its antimatter opposite.

Binding, or "gluing," the quarks together are gluons, the carriers of the strong nuclear force. This force is the strongest in nature and behaves unlike any other. Whereas other forces decrease in strength with distance, the strong force increases indefinitely as quarks move apart. As a result, the attempts of physicists to replicate the plasma state by removing a quark from a proton is something of a catch-22. Rather than separating the quark, the energy exerted to pull it free creates a bigger and bigger energy field. Eventually the energy field is so large that it spontaneously produces a quark-antiquark pair. Instead of getting a free quark, they produce two hadrons (particles made of two or more quarks). Although physicists have yet to to free single quarks from the bundles that contain them, they believe it can be done. At extremely high temperatures, such as those that existed at the birth of the universe, the energy may be sufficient for the familiar particles of protons and neutrons to come unglued, breaking into quarks and gluons.

Nuclear physicists are now planning experiments that will replicate these extreme conditions. If the MILC Collaboration can simulate this quark-gluon plasma, their work will assist these other physicists in establishing the experimental conditions necessary for verifying this plasma state.

 

Transition to the quark-gluon plasma

The calculations by the MILC Collaboration are focusing on how quarks and gluons behave when they are heated by taking into account the influence of the strange quark. Previous numerical studies have considered only the up and down quarks because physicists believe that lighter quarks spur the transition to a quark plasma. The strange quark, though significantly heavier than either the up or down quark (which have almost identical mass), is the only other quark light enough to have an appreciable impact on the transition. Preliminary calculations have identified the range of thermodynamic properties where the transition will occur. The current calculations are aimed at determining the precise nature of this transition. Says Sugar, "It might be a rapid crossover or a bona fide phase transition between ordinary matter and the quark-gluon plasma, depending on the effects of the strange quarks."

Recreating this transition, though, is no small undertaking, which is where the Alliance comes in. QCD calculations are among the most challenging numerical calculations in science. For example, one of the MILC's recent studies of the decay constants of heavy-light mesos required hundreds of thousands of processor-hours.

The MILC Collaboration has made significant progress, in large part, by not restricting their computations to single processors or even single computing systems with many processors. They have developed a highly portable MIMD (multiple instruction, multiple data) code with which they can run QCD calculations on many different high-performance parallel machines -- without having to modify the codes extensively for each machine. They recently ported their MIMD codes to two of the Alliance's SGI CRAY Origin2000s -- at NCSA and Boston University -- to simulate the complicated QCD phenomena. Because runs with different parameters do not depend on each other directly, Sugar and his colleagues can take advantage of the Alliance computers at NCSA and Boston to run several jobs simultaneously.

Moving code between the Alliance computers was especially easy, says Sugar, because of the uniform computing environment. "Basically, we didn't have to alter our code at all moving from NCSA to BU," he says. "The Alliance is clearly providing a very important set of resources to the national community."

Already the MILC collaboration is generating some of the most promising new knowledge in particle physics. With more computing power, and scientific insight, they may soon know what happened during those first split seconds of history.


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