C. Morningstar

QCD STRING FORMATION AND THE CASIMIR ENERGY

Three distinct scales are identified in the excitation spectrum of the gluon field around a static quark-antiquark pair as the color source separation R is varied. The spectrum, with string-like excitations on the largest length scales of 2-3 fm, provides clues in its rich fine structure for developing an effective bosonic string description. New results are reported from the three-dimensional Z(2) and SU(2) gauge models, providing further insight into the mechanism of bosonic string formation. The precocious onset of string-like behavior in the Casimir energy of the static quark-antiquark ground state is observed below R=1 fm where most of the string eigenmodes do not exist and the few stable excitations above the ground state are displaced. We find no firm theoretical foundation for the widely held view of discovering string formation from high precision ground state properties below the 1 fm scale.

Two-particle Correlation Functions with Distilled Propagators

Carnegie Mellon UniversityE-mail:chikhimw@cmu.eduFor the Hadron Spectrum CollaborationCorrelation functions of the simplest multi-particle state will be presented using distilled quarkpropagators. The I =2 ππstate can be simulated without computing disconnected diagrams andthus is the simplest two-particle state that can be studied with quark sources placed on a singletime-slice alone. We study the quality of the signals of thisππcorrelation function using thequark-smearingguided distillation method. Results will be presented for ππcorrelation functionscomputed on dynamical, anisotropic lattices.The XXVII International Symposium on Lattice Field Theory -LAT2009July 26-31 2009Peking University, Beijing, China

Towards the determination of the spectrum of QCD using a space-time lattice

Progress by the Lattice Hadron Physics Collaboration in determining the baryon and meson resonance spectrum of QCD using Monte Carlo methods with space- time lattices is described. The extraction of excited-state energies necessitates the evaluation of correlation matrices of sets of operators, and the importance of extended three-quark operators to capture both the radial and orbital structures of baryons is emphasized. The use of both quark-field smearing and link-field smearing in the operators is essential for reducing the couplings of the operators to the high-frequency modes and for reducing statistical noise in the correlators. The extraction of nine energy levels in a given symmetry channel is demonstrated, and identifying the continuum spin quantum numbers of the levels is discussed.

Excited-state hadron masses from lattice QCD

Progress in computing the spectrum of excited baryons and mesons in lattice QCD is described. Large sets of spatially-extended hadron operators are used. The need for multi-hadron operators in addition to single-hadron operators is emphasized, necessitating the use of a new stochastic method of treating the low-lying modes of quark propagation which exploits Laplacian Heaviside quark-field smearing. A new glueball operator is tested and computing the mixing of this glueball operator with a quark-antiquark operator and multiple two-pion operators is shown to be feasible.

Hadronic Resonances from Lattice QCD

The determination of the pattern of hadronic resonances as predicted by Quantum Chromodynamics requires the use of non‐perturbative techniques. Lattice QCD has emerged as the dominant tool for such calculations, and has produced many QCD predictions which can be directly compared to experiment. The concepts underlying lattice QCD are outlined, methods for calculating excited states are discussed, and results from an exploratory Nucleon and Delta baryon spectrum study are presented.

Group Theoretical Construction of Baryon Operators using All-to-All Quark Propagators

We describe a method to construct irreducible baryon operators using all-to-all quark propagators. It was demonstrated earlier that a large basis of exten ded baryon operators on anisotropic, quenched lattices can be used to reliably extract the masses of 5 or more excited states in the nucleon channel. All-to-all quark propagators are expected to be needed when studying these excited states on light, dynamical configurations because c ontributions from multi-particle states are expected to be significant. The dilution method is used to approximate the all-to-all quark propagators. Low-lying eigenmodes can also be used if necessary. For efficient computation of matrix elements of the interpolating operators, the algori thms should exploit the fact that many extended baryon operators can be obtained from the different linear combinations of three-quark colour-singlet operators. The sparseness of the diluted no ise vectors also afford several computation simplifications. Some preliminary results are present ed for nucleon effective masses.

Nuclear Physics from Lattice QCD: The Spectrum, Structure and Interactions of Hadrons

The importance of lattice QCD to our understanding of the structure, spectroscopy, and interaction of hadrons is decribed. Recent accomplishments in each of these areas is outlined, and the opportunities emerging with increasing computational power are identified. Milestones at the 10 Tflops-years, 100 Tflops-years and Petaflops-years scales are presented.

The excitation spectrum of the Wilson surface and QCD string theory

The low energy excitation spectrum of the critical Wilson surface is discussed between the roughening transition and the continuum limit of lattice QCD. The fine structure of the spectrum is interpreted within the framework of two-dimensional conformal field theory.