/ 04 09 03 2 v 1 1 0 Se p 20 04 1 η photoproduction off the neutron at GRAAL :

New (preliminary) data on η photoproduction on the neutron are presented. These data reveal a resonant structure at W=1.67 GeV. Meson photoproduction on the neutron may provide essentially new information regarding the spectrum of baryons. An example is given by a model 1 which exploits the SU(6) symmetry and assumes single-quark transitions from ground nucleons to the [70, 1 − ] supermultiplet. The model predicts only weak photoexcitation of the D 15 (1675) resonance from the proton target. Conversely, photon-neutron couplings of D 15 (1675) calculated in the framework of this approach are not small. Measurements of the relative strength of photoneutron/photoproton interaction are therefore an important testing ground for this (and others) theoretical approaches. Another example is possible photoexcitation of the non-strange pen-taquark state, which is associated with the second member of an antide-cuplet of exotic baryons 2,3. Evidence for the lightest member of the an-tidecuplet, the Θ + (1540) baryon, is now being widely discussed 4. It can be produced, in particular, by photoexcitation of the nucleon. However, exact SU (3) F would forbid the proton photoexcitation into the proton-like antidecuplet member. The chiral soliton model predicts that photoexci-tation of the non-strange pentaquark has to be suppressed on the proton and should occur mainly on the neutron, even after accounting for SU (3) F violation 5. Estimates of the mass and width of the non-strange pentaquark are ambiguous. As initial input, the mass was used to be 1.71 GeV 2 , with the width estimated ∼ 40 MeV. More recent evaluation of the chi-ral soliton approach led to the range of 1.65 – 1.69 GeV 6. In the di-quarks approach 3 , the mass of the pentaquark with hidden strangeness is quoted about 1.7 GeV. Modified partial wave analysis of the πN scattering 7 suggests two possible candidates, at 1.68 GeV and/or at 1.73 GeV, with the total width about 10 MeV and the partial width for πN decay mode less than 0.5 MeV. Thus, photo-neutron excitation data, and their comparison with