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Quantum Chromodynamics (QCD) is the leading theory in the Standard Model of particle physics of the strong interactions between color charged particles (quarks) and the particles that bind them (gluons). Analogous to the way that electrically charged particles exchange photons to create an electromagnetic field,quarks exchange gluons to form a very strong color force field. Contrary to the electro-magnetic force, the strong force binding quarks does not get weaker as the particles get farther apart. As such, at long distances (low energies), quarks have not been observed independently in experiment and, due to their strong coupling, perturbative techniques, which have been so successful in describing weak interactions in Quantum Electrodynamics (QED), diverge for the low-energy regime of QCD. Instead, hybrid Monte Carlo (HMC) simulations are employed in an attempt to numerically predict physical observables in accelerator experiments.
A significant amount of the computational time in large HMC simulations of lattice QCD is spent inverting the discrete Dirac operator. Unfortunately, traditional covariant finite difference discretizations of the Dirac operator present serious challenges for standard iterative methods. For interesting physical parameters, the discretized operator is large and ill-conditioned, and has random coefficients. More recently, adaptive algebraic multigrid (AMG) methods have been shown to be effective preconditioners for Wilson's discretization of the Dirac equation. This talk presents an alternate discretization of the Dirac operator based on least-squares finite elements. The discretization is systematically developed and physical properties of the resulting matrix system are discussed. Finally, numerical experiments are presented that demonstrate the effectiveness of adaptive smoothed aggregation (αSA ) multigrid as a preconditioner for the discrete field equations resulting from applying the proposed least-squares FE formulation to a simplified test problem, the 2d Schwinger model of QED. |