Ja n 20 18 Observation of Narrow N + ( 1685 ) and N 0 ( 1685 ) Resonances in γN → πηN Reactions

Understanding the internal structure of the nucleon is a key task in the domain of hadronic physics. Suggested in the 60th the approximate flavor SU(3) symmetry of QCD led to a remarkably successful classification of lowlying mesons and baryons. Many properties of baryons known at that time were transparently explained by the Constituent Quark Model (CQM) [1] that treats baryons as bound systems of three effective (constituent) quarks. CQM-based calculations predicted a rich spectrum of baryon resonances with widths varying from ∼ 80 to ∼ 400 MeV. Nevertheless, in spite of significant efforts, many of the predicted resonances still escape from reliable experimental identification (the so-called “missing resonances”). The Chiral Soliton Model (χSM) is an alternative picture of baryons. It treats them as space/flavor rotational excitations of a classical object a soliton of the chiral field. The model predicts the lowest-mass baryon multiplets to be the (8, 1/2) octet and the (10, 3/2) decuplet exactly as CQM does. χSM also predicts the existence of long-lived exotic particles [2]. Therefore the search for light-quark exotic states may provide critical benchmarks to examine two different approaches and to establish the connection between them. In this context the observation of a narrow enhancement at W ∼ 1.68 GeV in the γn → ηn excitation function (the so-called “neutron anomaly”) at GRAAL, CBELSA/TAPS, LNS and A2@MAMI C [3–7] might be quite important. Narrow structures at the same energy were also observed in Compton scattering on the neutron γn → γn [8] and in the beam asymmetry for the η photoproduction of the proton γp → ηp [9] (see alsociteann). The recent data on the beam asymmetry for Compton scattering on the proton γp → γp [11], the precise data for the γn → ηn [12] and π−p → π−p [13] reactions revealed two narrow structures at W ∼ 1.68 and W ∼ 1.72 GeV. The whole complex of experimental observations may signal the existence of one (N(1685)) or two (N(1685) and N(1726)) narrow nucleon resonances. The proper-