V. A. Nikonov

Properties of baryon resonances from a multichannel partial wave analysis

Properties of nucleon and Δ resonances are derived from a multichannel partial wave analysis of pion and photo-induced reactions off protons. This paper summarizes the latest results on masses, widths, and decay properties of nucleon and Δ resonances.

New results on the Roper resonance and the P-11 partial wave

Abstract Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the P 11 partial wave are studied in a combined analysis of pion- and photo-induced reactions. We find the Roper pole at { ( 1371 ± 7 ) − i ( 92 ± 10 ) } MeV and an elasticity of 0.61 ± 0.03 . The largest decay coupling is found for the Nσ ( σ = ( π π ) - S -wave). The analysis is based on new data on γ p → p π 0 π 0 for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths.

Quasifree Photoproduction ofηMesons off the Neutron

Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.

Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Σ, was measured at ELSA in the reaction γ p → η p using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range Eγ = 800 to 1400 MeV with the maximum polarisation of Pγ = 49% obtained at 1305 MeV. Both dominant decay modes of the η into two photons and 3π were used to extract the beam asymmetry from the azimuthal modulation of the cross section. The measurements cover the angular range Θcm ≃ 50 – 150 degrees. Large asymmetries up to 80 % are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn–Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly. PACS. 13.60.-r Photon and charged-lepton interactions with hadrons – 13.60.Le Meson production – 13.88.+e Polarization in interactions and scattering – 14.20.Gk Baryon resonances with S=0

Decays of baryon resonances into ΛK+, ΣK+ and Σ+K0

Abstract.Cross-sections, beam asymmetries, and recoil polarisations for the reactions γp → K+Λ;γp → K+Σ0, and γp → K0Σ+ have been measured by the SAPHIR, CLAS, and LEPS Collaborations with high statistics and good angular coverage for centre-of-mass energies between 1.6 and 2.3 GeV. The combined analysis of these data with data from π and η photoproduction reveals evidence for new baryon resonances in this energy region. A new P11 state with mass 1840 MeV and width 140 MeV was observed contributing to most of the fitted reactions. The data demand the presence of two D13 states at 1875 and, optimistically, at 2170 MeV.

High statistics study of the reaction gamma p -> p pi(0)eta

Photoproduction off protons of the pπ0η three-body final state was studied with the Crystal Barrel/TAPS detector, at the electron stretcher accelerator ELSA in Bonn, for incident energies from the π0η production threshold up to 2.5 GeV. Differential cross sections and the total cross sections are presented. The use of linearly polarized photons gives access to the polarization observables Σ, Is, and Ic, the latter two characterize beam asymmetries in case of three-body final states. Δ(1232)η, N(1535)1/2−π, and pa0(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and Δ resonances, for which so far only fair evidence was reported. A large number of decay modes of known nucleon and Δ resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and Δ resonances.

Quasi-free photoproduction of η-mesons off the deuteron

Abstract.Precise data for quasi-free photoproduction of η-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5GeV. The η-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to η-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the η-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free η-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the η-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W ≈ 1665 MeV and FWHM Γ ≈ 25 MeV. The data are compared to the results of different models.

Radiative decays of basic scalar, vector and tensor mesons and the determination of the P-wave q\(\) multiplet

Abstract.We perform simultaneous calculations of the radiative decays of scalar mesons f0(980)↦γγ, a0(980)↦γγ, vector meson φ(1020)↦γf0(980), γa0(980), γπ0, γη, γη′ and tensor mesons a2(1320)↦γγ, f2(1270)↦γγ, f2(1525)↦γγ, assuming all these states to be dominantly the q ones. A good description of the considered radiative decays is reached by using almost the same radial wave functions for scalar and tensor mesons that supports the idea for the f0(980), a0(980) and a2(1320), f2(1270), f2(1525) to belong to the same P-wave q multiplet.

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Abstract.Photoproduction of π0-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For Eγ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.

Baryon resonances and polarization transfer in hyperon photoproduction

Abstract.A partial-wave analysis of data on photoproduction of hyperons including single and double polarization observables is presented. The large spin transfer probability reported by the CLAS Collaboration can be successfully described within an isobar partial-wave analysis.