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Theory Center Seminar<br>
Monday, Feb. 22, 2010<br>
1:00 p.m. (coffee at 12:45 p.m.)<br>
CEBAF Center, Room L104<br>
<br>
Kanzo Nakayama<br>
University of Georgia, Athens<b><i><br>
<br>
</i></b><b>The Phase and Pole Structure of <i>N</i>*(1535) and the
Issue of sigma_n/sigma_p in eta-Photoproduction within <i>U</i>-chi <i>PT<br>
<br>
</i></b>The nature of some baryonic resonances is still an unresolved
issue. The case of the <i>N</i>*(1535) is particularly interesting in
this respect due to the nearby eta-<i>N</i> threshold and interference
with the <i>N</i>*(1650). The <i>N</i>*(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 <i>N</i>*(1535)
with the photon can be made. In this study, we simultaneously analyze
the role of the <i>N</i>*(1535) in the pi-<i>N</i> --> pi-<i>N</i>
and the gamma-<i>N</i> --> pi-<i>N</i> reactions within a <i>U<sub>x</sub>PT</i>
approach and compare to the respective amplitudes from partial wave
analysis.
<br>
<br>
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<sub>n</sub> and sigma<sub>p</sub>, can be
quantitatively explained. In this study, the main source for the peak
in sigma<sub>n</sub>/sigma<sub>p</sub> is shown to be a coupled channel
effect in <i>S</i>
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 <i>K</i>-Lambda
and <i>K</i>-Sigma channels through the SU(3) structure in conjuntion
with the Kaon decay constant of <i>f<sub>K</sub></i> ≈ 113 MeV. <br>
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