Bonn 2010 – wissenschaftliches Programm

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HK: Fachverband Physik der Hadronen und Kerne

HK 26: Hadronenstruktur und -spektroskopie VI

HK 26.5: Vortrag

Dienstag, 16. März 2010, 17:45–18:00, HG IV

η Photoproduction off 3He — •Lilian Witthauer — Department of Physics, University of Basel, Switzerland

Quark models predict many more states in the excitation spectrum of the nucleon than observed in experiments. Most previous experiments investigating the excitation spectrum of nucleons are based on elastic scattering of charged pions. This makes it probable that the data is biased against states that couple only weakly to Nπ. Hence, one can study different excitation mechanisms and channels as the photoproduction of mesons via excitation of nucleon resonances.

Measurements at ELSA in Bonn investigated the quasi-free η photoproduction off the neutron (I. Jaeglé et al., Phys. Rev. Lett 100 (2008) 252002). The resulting cross section shows a narrow structure at W=1.68 GeV with a width smaller than 60 MeV. This structure is only visible in the cross section on the neutron and not in that on the proton. Experiments of the GRAAL collaboration (V.Kuznetsov et al., arXiv:hep-ex/0606065v2) and LNS-Sendai (F. Miyahara et al., Prog. Theor. Phys. Suppl. 168 (2007) 90) showed the same effect in quasi-free photoproduction off the deuteron. In order to exclude any possibility that the structure could arise from nuclear effects (re-scattering of mesons, final state interaction) we have studied it for a nucleon system with different momentum distribution and different neutron/proton ratio, namely 3He. Preliminary results for the quasi-free η cross section obtained in coincidence with recoil nucleons will be discussed.

Supported by Swiss National Fund and DFG.

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