Improved Actions for Staggered Quarks

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Improved Actions for Staggered Quarks

Robert Sugar, University of California, Santa Barbara, at a PSC seminar.

Quantum chromodynamics, or QCD, is the theory of the strongest force in nature, the force that holds the nucleus of an atom together. QCD tells us that protons and neutrons are made of bundles of particles called quarks held together by other particles called gluons - the strongest imaginable glue. According to QCD, quarks and gluons are the most fundamental particles in nature, the essential building blocks of all matter.

With support from DOE's Office of Science, a nationwide group of physicists, the MILC (MIMD Lattice Computation) collaboration, have used PSC's CRAY T3E to develop computational methods for QCD. The theory has withstood many tests, says MILC physicist Robert Sugar of the University of California, Santa Barbara, but it's been difficult to extract predictions that can be measured against experiment. Because of the complexities of the theory, QCD is one of the most demanding computational projects in science.

QCD theory is defined in the space-time continuum, but to do numerical simulations requires treating space and time as finite, on a four-dimensional grid called a lattice. The lattice, however, introduces inaccuracies - called finite-lattice spacing effects. To obtain reliable results, therefore, the lattice spacing must be decreased (and the number of grid points correspondingly increased), which quickly eats up computing capability.

To escape this conundrum, the MILC collaboration and other lattice-gauge theory groups have worked to develop more sophisticated schemes - called "improved actions" - for putting the theory on the lattice. Through an extensive series of computations, MILC researchers have tested improved actions for a particular formulation of QCD known as Kogut-Susskind or staggered quarks. "We've used PSC's T3E and other resources," says Sugar, "to develop a family of improved actions for the Kogut-Susskind formulation of lattice quarks." Tests show a significant reduction in finite-lattice spacing effects. As a result of this success, MILC is embarking on a major program of production runs.

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