Doug Toussaint, Tom Burch, and Kostas Orginos, University of Arizona
Claude Bernard, Washington University
Tom DeGrand, University of Colorado
Carleton DeTar and Pierre Lacock, University of Utah
Steve Gottlieb, Indiana University
Jim Hetrick, University of the Pacific
Urs Heller, Supercomputer Computations Research Institute (SCRI),
Florida State University
Craig McNeile, Liverpool University
Kari Rummukainen, Nordic Institute for Theoretical Physics (NORDITA)
Bob Sugar, University of California, Santa Barbara

Research Objectives

The MILC collaboration has developed and tested improved actions for Kogut-Susskind fermions in lattice QCD. These actions greatly reduce the effects of the nonzero lattice spacing on physical results. We are now ready to use these results, combined with a well-understood improved action for the gluon fields, in a large-scale simulation of QCD, including three flavors of dynamical quarks, as in the real world. Finer lattices will be used to study the hadron spectrum, form factors and decay constants of heavy-light mesons, and masses of hybrid mesons.

Computational Approach

	The vector meson mass in units of r1 is plotted versus the squared lattice spacing for several combinations of gauge and quark actions. All of the points have been interpolated in quark mass to the point where the pion mass is 0.78 r1.

We use the standard "refreshed molecular dynamics" method to generate sample configurations, or lattices, with a probability proportional to their weight in the imaginary time quantum chromodynamics partition function. Expectation values of quantum mechanical operators can be estimated by averaging the operator over the set of configurations of the gluon fields. The computation of the force, or acceleration of the gluon fields, coming from the dynamical quarks requires the solution of a sparse matrix problem where the matrix is Hermitian and positive definite. We use the conjugate gradient algorithm to get an approximate solution to the sparse matrix problem. For each lattice link included in the quark action, the force on the corresponding gauge field is computed by parallel transporting the source and result of the sparse matrix computation from both ends of the path in the action to the lattice point where the force is being computed, and taking the outer product of these two vectors.

Accomplishments

We have developed code to simulate with fairly general fat gauge field link actions, and used this code to investigate flavor symmetry breaking in a number of different quark actions. We find that these fat link actions all reduce the flavor symmetry breaking, with the higher amounts of fattening giving greater reductions. These actions are now ready for use in full-scale simulations, generating lattices which can be used to study many aspects of QCD, including the effects of dynamical quarks on the hadron spectrum and on hadronic matrix elements.

Significance

Publications

K. Orginos and D. Toussaint, "Testing improved actions for dynamical Kogut-Susskind quarks," Phys. Rev. D 59, 014501 (1999).

K. Orginos, D. Toussaint, and R. L. Sugar, "Variants of fattening and flavor symmetry restoration," Phys. Rev. D (in press). E-print hep-lat/9903032.

K. Orginos and D. Toussaint, "Tests of improved Kogut-Susskind fermion actions," Nucl. Phys. B (Proc. Suppl.) 73, 909 (1999). E-print hep-lat/9809148.

http://www.physics.arizona.edu/~doug/flavor_breaking