Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session G12: Invited Session: The Nucleon Resonance Spectrum |
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Sponsoring Units: DNP Chair: Volker Burkert, Thomas Jefferson National Accelerator Facility Room: Grand Hall East A/B |
Sunday, April 1, 2012 8:30AM - 9:06AM |
G12.00001: Light Baryon Spectroscopy at Jefferson Lab: What have we learned about excited baryons? Invited Speaker: Volker Crede Nucleons are complex systems of confined quarks and gluons and exhibit the characteristic spectra of excited states. These states serve as an excellent probe of quantum chromodynamics (QCD), the fundamental theory of strong interaction. Highly-excited states are sensitive to the details of quark confinement, which is only poorly understood within QCD. This is the regime of non-perturbative QCD and it is one of the key issues in hadronic physics to identify the corresponding relevant degrees of freedom and the effective forces between them. In recent years, lattice-QCD has made significant progress toward understanding the spectra of hadrons. On the experimental side, high-energy electrons and photons are a remarkably clean probe of hadronic matter, providing a microscope for examining atomic nuclei and the strong nuclear force. For more than a decade, laboratories worldwide have accumulated data for such investigations, resulting in a number of surprising discoveries and contributing to our understanding of the nucleon, its underlying quark structure, and the dynamics of the strong interaction. Current experimental efforts utilize highly-polarized frozen-spin (butanol) targets and deuterium targets in combination with polarized photon beams. These are important steps toward so-called complete experiments that will allow us to unambiguously determine the scattering amplitudes in the underlying reactions and to identify resonance contributions. In my talk, I will give an overview of the excited baryon program at Jefferson Lab and I will discuss recent results from (double-)polarization experiments. [Preview Abstract] |
Sunday, April 1, 2012 9:06AM - 9:42AM |
G12.00002: Coupled-channels analyses of meson production data and identification of nucleon resonances Invited Speaker: Satoshi Nakamura An important challenge in hadron physics is to understand the structure of the nucleon in terms of Quantum Chromodynamics (QCD). Since the nucleon is a composite particle, its structure is closely related to the spectrum and structure of its excited states. Because the nucleon excited states show up as resonances in $\pi$($\gamma$)-induced meson production reactions on the nucleon, their existence and their properties (mass, width, branching ratios, etc.) can be revealed by a careful partial wave analysis of the reaction data. Because several meson-baryon channels are strongly coupled in the reactions, the unitarity requires a coupled-channels analysis. The mass and width of a nucleon resonance are identified by real and imaginary parts of a pole position in the partial wave amplitude, and a partial width is calculated with the residue of the pole. Those extracted properties are to be understood with Lattice QCD and/or hadron structure models. Analysis methods can be classified into two classes: one of them is based on a dynamical model in which meson-baryon potentials are derived from a set of interaction Lagrangians, and a set of coupled-channels scattering equations is solved to calculate the scattering amplitude. The other approach is based on a parametrization of the amplitudes using the K-matrix. In this presentation, I will give an overview of those coupled-channels analyses of meson production data. Although there are several groups working on their coupled-channels analyses, I will mainly discuss an analysis done by the Excited Baryon Analysis Center (EBAC) in Jefferson Lab as a representative of the dynamical approach. For the parametrization approach, I will mainly review an analysis done by the Bonn-Gachina group. I explain their models, methods and database, and show the quality of description of the data with their models, and present resonance properties extracted from their models. [Preview Abstract] |
Sunday, April 1, 2012 9:42AM - 10:18AM |
G12.00003: Lattice QCD sprectrum of excited states of the nucleon Invited Speaker: Stephen Wallace Lattice QCD results are presented for the spectrum of excited states of the nucleon. Matrices of correlation functions are calculated using lattice operators that incorporate up to two covariant derivatives in combinations that transform according to SU(2) symmetry restricted to the lattice. Although the lattice has cubic symmetry, identification of continuum SU(2) spins is straightforward using such operators. Overlaps of the operators with the lattice QCD states obtained by diagonalizing matrices of correlation functions provide the link of continuum spins to lattice states. Spins up to 7/2 are identified clearly. Evidence for an approximate realization of rotational symmetry in the spectrum is presented, which helps to explain why the continuum spins can be identified. In lattice simulations with pion mass equal to 392 MeV, the low-lying excited states of lattice QCD are found to have the same spin quantum numbers as the states of SU(6)xO(3) symmetry. The lattice QCD spectra are inconsistent with either a quark-diquark model or parity doubling of states. They suggest that the Roper resonance may have a complex structure consisting of contributions from L=0, 1 and 2. [Preview Abstract] |
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