Excited Form Factors USQCD Collaboration Project (Since July 2007) Goal   This class-B proposal aims to explore excited-state form factors in dynamical lattice QCD calculations. Our main physics goal is to extract the axial coupling constant and electromagnetic form factors of N-N′ and N′-N′ from first-principles calculation. These numbers will be of great value to experimentalists and phenomenologists in analyzing data. People   Saul Cohen Robert Edwards Huey-Wen Lin (PI) David Richards Background   The new data provided from experiments at Jefferson Laboratory, MIT-Bates, LEGS, Mainz, Bonn, GRAAL, and Spring-8 offer a new opportunity to understand in detail how nucleon resonance (N*) properties emerge from the nonperturbative aspects of QCD. For example, the extraction of N* → Nγ transition form factors could help us to understand the dynamical origins of the confinement of constituent quarks and the associated meson cloud. There are various QCD-based hadron models such as the well developed constituent quark model and the covariant model based on Dyson-Schwinger Equations. Figure 1 shows preliminary data from CLAS collaboration that covers a large range of photon virtuality Q2. In the region Q2 ≤ 1.5 GeV2, both the transverse amplitude, A1/2(Q2), and the longitudinal amplitude, S1/2(Q2), drop rapidly in magnitude. This is well described in relativistic quark models with light-cone dynamics, and the sign is consistent with the non-relativistic version. However, in the low-Q2 region, A1/2(Q2) becomes negative; this is not understood within constituent quark models and requires inclusion of meson degrees of freedom. At the EBAC, dynamical reaction models have been developed to interpret extracted N* parameters in terms of QCD. A model-independent study of these quantities from lattice QCD will serve as valuable help to phenomenologists in analyzing experimental data and be theoretical ground for better understanding of low-Q2 physics. Progress   Exploratory studies on spectrum using "black-box method" Preliminary exploratory on quenched configurations Form factors Talks   Parameter Tuning of Three-Flavor Dynamical Anisotropic Clover Action Lattice '07, Regensburg, Germany, 2007 July 30–August 5 Lattice QCD Beyond Ground States 4th International Workshop on Numerical Analysis and Lattice QCD, Yale University, New Haven, Connecticut, 2007 May 1–3 Publications   "Parameter Tuning of Three-Flavor Dynamical Anisotropic Clover Action", Huey-Wen Lin, Robert G. Edwards, Bálint Joó, JLAB-THY-07-732, PoS-LAT2007-119, arXiv:0709.4680 [hep-lat]. "Lattice QCD Beyond Ground States", Huey-Wen Lin, Saul D. Cohen, JLAB-THY-07-720, arXiv:0709.1902 [hep-lat]. Links   USQCD Collaboration Hep-lat archives SLAC SPIRES — HEP Database Lattice 2008, Williamsburg, VA, July 14–19

Last updated: 2008 Jan. 24