Running Masses in the Nucleon and its Resonances

An overarching scientific challenge for the coming decade is to discover the meaning of confinement, its relationship to dynamical chiral symmetry breaking (DCSB) - the origin of visible mass - and the connection between them. In progressing toward meeting this challenge, significant progress has been made using continuum methods in QCD. For example, a novel understanding of gluon and quark confinement and its consequences has begun to emerge from quantum field theory; a clear picture is being drawn of how hadron masses emerge dynamically in a universe with light quarks; and ground-state hadron wave functions with a direct connection to QCD are becoming available, which reveal that quark-quark correlations are crucial in hadron structure. There is growing experimental support for this body of predictions in both elastic and nucleon-to-resonance-transition form factors.Comment: 10 pages, 6 figures. Contribution to the proceedings of NSTAR2015, the 10th International Workshop on the Physics of Excited Nucleons, 25-28 May 2015, Suita Campus, Osaka University, Osaka, Japa

String breaking

We numerically investigate the transition of the static quark-antiquark string into a static-light meson-antimeson system. Improving noise reduction techniques, we are able to resolve the signature of string breaking dynamics for Nf=2 lattice QCD at zero temperature. We discuss the lattice techniques used and present results on energy levels and mixing angle of the static two-state system. We visualize the action density distribution in the region of string breaking as a function of the static colour source-antisource separation. The results can be related to properties of quarkonium systems.Comment: 8 pages, Talk given at the Workshop on Computational Hadron Physics, Nicosia, Cyprus, 14--17 September 200

Unquenching the Quark Model and Screened Potentials

The low-lying spectrum of the quark model is shown to be robust under the effects of `unquenching'. In contrast, the use of screened potentials is shown to be of limited use in models of hadrons. Applications to unquenching the lattice Wilson loop potential and to glueball mixing in the adiabatic hybrid spectrum are also presented.Comment: 6 pages, 3 ps figures, revtex. Version to appear in J. Phys.

Demonstration of string breaking in quantum chromodynamics by large-scale eigenvalue computations

We present results of our ongoing determination of "string breaking" in quantum chromodynamics (QCD) including two dynamical light quarks. Our investigation of the fission of the string between a heavy (static) quark and a corresponding antiquark into a meson-antimeson system is based on dynamical configurations of size 24(3) x 40. The all-to-all light quark propagators occurring in the transition element are computed from a set of 200 low-lying eigenmodes of the Hermitian Wilson-Dirac matrix which encodes the effect of the dynamical quarks. These eigenmodes are calculated on the 1312-node IBM p690 system at the John von Neumann Institute in Julich. Combining the eigenvalue computations with a variety of ground state enhancing optimization methods we determine the matrix elements of the two-by-two system with so far unprecedented accuracy. We observe-for the first time ever in a simulation of 4-dimensional lattice-QCD-level-splitting as the perfect signature for dynamical string breaking between ground state and excited potential. (c) 2005 Elsevier B.V. All rights reserved