Properties of nucleon and $Delta$ resonances are derived from a multichannel partial wave analysis. The statistical significance of pion and photo-induced inelastic reactions off protons are studied in a multichannel partial-wave analysis.
The $gamma + p rightarrow K^0 + Sigma^+$ photoproduction reaction is investigated in the energy region from threshold to $E_gamma = 2250$,MeV. The differential cross section exhibits increasing forward-peaking with energy, but only up to the $K^*$ th
reshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the $K^*Lambda$ and $K^*Sigma$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^*$ production threshold to an s-channel mechanism associated with the formation of a dynamically generated $K^*$-hyperon intermediate state.
Evidence is reported for two nucleon resonances with spin-parity $J^P=1/2^-$ and $J^P=3/2^-$ at a mass just below 1.9,GeV. The evidence is derived from a coupled-channel analysis of a large number of pion and photo-produced reactions. The two resonan
ces are nearly degenerate in mass with two resonances of the same spin but positive parity. Such parity doublets are predicted in models claiming restoration of chiral symmetry in high-mass excitations of the nucleon. Further examples of spin parity doublets are found in addition. Alternatively, the spin doublet can be interpreted as member of the 56-plet expected in the third excitation band of the nucleon. Implications for the problem of the {it missing resonances} are discussed.
Results from a multi-channel partial wave analysis of elastic and inelastic $pi N$ and $gamma N$ induced reactions are presented. The analysis evidences the existence of a spin-quartet of nucleon resonances with total angular momenta $J^P=1/2^+,...,
7/2^+$. All states fall into a $pm130$,MeV mass gap centered at 1.97,GeV. The spin quartet is at variance with S-wave diquark configurations required in classical di-quark models.
Results of a partial wave analysis of new high-statistics data on $gamma pto peta$ from MAMI are presented. A fit using known broad resonances and only standard background amplitudes can not describe the relatively narrow peaking structure in the cro
ss section in the mass region of 1660-1750 MeV which follows a minimum. An improved description of the data can be reached by either assuming the existence of a narrow resonance at a mass of about 1700 MeV with small photo-coupling or by a threshold effect. In the latter case the observed structure is explained by a strong (resonant or non-resonant) $gamma ptoomega p$ coupling in the $S_{11}$ partial wave. When the beam asymmetry data, published by part of the GRAAL collaboration, are included in the fit, the solution with a narrow $P_{11}$ state is slightly preferred. In that fit, mass and width of the hypothetical resonance are determined to $Msim$1694 MeV and $Gammasim 40$ MeV, respectively, and the photo-coupling to $sqrt{{rm Br}_{eta N}} A_{1/2}^p sim 2.6cdot 10^{-3}$ GeV$^{-1/2}$. High precision measurements of the target asymmetry and $F$-observable are mandatory to establish the possible existence of such a narrow state and to provide the necessary information to define which partial wave is responsible for the structure observed in the data.
Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68~GeV at the electron accelerator ELSA. The beam asymmetry~$Sigma$ has been extracted for $115^circ < theta_{rm c.m.} < 155^circ$
of the $pi^0$~meson and for $theta_{rm c.m.} < 60^circ$. The new beam asymmetry data improve the world database for photon energies above 1.5~GeV and, by covering the very forward region, extend previously published data for the same reaction by our collaboration. The angular dependence of $Sigma$ shows overall good agreement with the SAID parameterization.
