Session FC: Mini Symposium on Meson Electroproduction and Nucleon Structure

Spectrum of Excited Baryons from Lattice QCD

Recent lattice QCD results for the excited states of baryons $N$, $\Delta$, $\Lambda$, $\Sigma$, $\Xi$ and $\Omega$ are reviewed. Three-quark baryon operators are used that incorporate up to two covariant derivatives. The quark spins and derivatives are combined to obtain operators that transform as representations of continuum spins $J = \frac{1}{2}, \frac{3}{2}, \frac{5}{2}$ and $\frac{7}{2}$. Operators corresponding to continuum $J$ values are subduced to the irreducible representations allowed by the lattice and are used to calculate the excited state spectra. Strong overlaps of a lattice state with the operators subduced from one of the continuum values of $J$ provides the identification of the spin of the state. The dominant flavor of each lattice state is identified in a similar fashion from the definite flavor symmetry of each of the operators. The result is that the flavor structure of the lattice spectrum is found to be in good accord with $SU(6)\times O(3)$ symmetry, e.g., the same number of low-lying excited states is found for each spin and baryon flavor. Information about hybrid baryons is obtained by the use of operators that include a chromo-magnetic gluon field, which allows some lattice states with substantial hybrid content to be identified.   

Light-Cone Distribution Amplitudes of N$^*$(1535) from Lattice QCD

We present the results of a lattice calculation of the first two moments of the light-cone distribution amplitudes of the lowest positive parity resonance, N$^*$(1535). The calculation is done on lattices with Nf = 2 flavors of dynamical Clover fermions. We have used lattices with several volumes, lattice spacings and pion masses, the lightest one being close to the physical pion mass. To select the N* state, we have used improved parity projection and the variational approach.   

Nucleon and N* masses and form factors from a contact interaction

In the Schwinger-Dyson and bound state equation framework, we use a contact interaction that reproduces the low energy interactions of QCD. We use the Faddeev equation to determine masses for the ground and low-lying excited states of the Baryon octet. We then use these amplitudes in a form factor calculation to obtain elastic and transition form factors.   

Form Factors in Contac Interaction

We explore how elastic and transition form factors for excited baryons can be evaluated using a momentum independent interaction, regularized in a symmetry-preserving manner. The fact that the di-quark decomposition of the baryons is rather precise and simplifies the problem considerably helps attack the problem.   

Excited nucleon spectrum and structure

The longitudinal and transverse helicity amplitudes for all the known Nstar resonances have been calculated in a systematic way within a quark model. They will be presented and compared with a large number of experimental data recently obtained. A discussion on their structure will be also given. Results regarding the Unquenching of the quark model will be also presented and discussed.   

Exclusive $\pi^{0}$ electroproduction in the resonance region

We report the analysis of single $\pi^{0}$ electroproduction in the resonance region to study the electromagnetic excitation of the nucleon resonances. The study is aimed at understanding the internal structure and dynamics of the nucleon. The experiment was performed using an unpolarized cryogenic hydrogen target and polarized electron beam with the CLAS detector at Jefferson Lab. The new measurements produce a data base with high statistics and large kinematic coverage for the hadronic invariant mass W up to 1.8 GeV and in the momentum transfer $Q^{2}$ range of 0.4 - 1.0 (GeV/c)$^{2}$ with full angular coverage in the Center of Mass system. This data set is currently being analyzed using a Unitary Isobar Model to determine the $Q^{2}$ dependence of resonance transition amplitudes for several excited states of the nucleon. Preliminary results will be presented.   

G14 Polarization Observables for Double Charged Pion Photo-production.

The work to be presented is part of the JLab N* program to search for new resonances by partial wave analysis of the unpolarized cross sections and many polarization observables. This talk will describe the recently completed g14/E06-101 experiment on polarized HD in Hall B of Jefferson Lab, including preliminary results for two charged pion photo-production asymmetries on both polarized protons and polarized quasi-free neutrons. The same polarization observables for a proton target are compared with previous published experiment with good agreement, and the new results for quasi-free neutrons will also be given. Significant differences between proton and neutron targets are evident in both the Dalitz distributions and the polarization observables.