[Jlab-seminars] Theory Center Seminar

[Mary Fox]

Theory Center Seminar
Monday, Feb. 22, 2010
1:00 p.m. (coffee at 12:45 p.m.)
CEBAF Center, Room L104

Kanzo Nakayama
University of Georgia, Athens*/

/**The Phase and Pole Structure of /N/*(1535) and the Issue of 
sigma_n/sigma_p in eta-Photoproduction within /U/-chi /PT

/*The nature of some baryonic resonances is still an unresolved issue. 
The case of the /N/*(1535) is particularly interesting in this respect 
due to the nearby eta-/N/ threshold and interference with the 
/N/*(1650). The /N/*(1535) has been described as a threshold effect, as 
a genuine 3-quark resonance, or as dynamically generated from the 
interaction of the octet of baryons with the octet of mesons. In the 
scheme of dynamical generation, predictions for the interaction of the 
/N/*(1535) with the photon can be made. In this study, we simultaneously 
analyze the role of the /N/*(1535) in the pi-/N/ --> pi-/N/ and the 
gamma-/N/ --> pi-/N/ reactions within a /U_x PT/ approach and compare to 
the respective amplitudes from partial wave analysis.

Within the same approach, we also address the issue of the recently 
discovered enhancement of eta photoproduction on the quasi-free neutron 
at energies around sqrt s ~ 1.67 GeV. The quasi-free cross sections on 
proton and neutron, sigma_n and sigma_p , can be quantitatively 
explained. In this study, the main source for the peak in sigma_n 
/sigma_p is shown to be a coupled channel effect in /S/ wave that 
explains the dip-bump structure in the gamma-n --> eta-n. This is a 
direct consequence of the underlying hadron dynamics which, in the 
present model, is driven by the Weinberg-Tomozawa contact interaction 
with a relatively strong coupling to the /K/-Lambda and /K/-Sigma 
channels through the SU(3) structure in conjuntion with the Kaon decay 
constant of /f_K / ? 113 MeV.
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