Search Results (16 total)

Evidence is reported for the existence of a parity doublet of Δ resonances with total angular momentum J=3/2 from photoproduction of the pπ⁰η final state. The two parity partners Δ(1920)P₃₃ and Δ(1940)D₃₃ make significant contributions to the reaction. Cascades of resonances into Δ(1232)η, N(1535)π, and Na₀(980) are clearly observed.

Information on hadron properties in the nuclear medium has been derived from the photoproduction of ω mesons on the nuclei C, Ca, Nb, and Pb using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the ω-meson cross section on the nuclear mass number has been compared with three different types of models: a Glauber analysis, a Boltzmann-Uehling-Uhlenbeck analysis of the Giessen theory group, and a calculation by the Valencia theory group. In all three cases, the inelastic ω width is found to be 130–150  MeV/c² at normal nuclear matter density for an average 3-momentum of 1.1  GeV/c. In the rest frame of the ω meson, this inelastic ω width corresponds to a reduction of the ω lifetime by a factor ≈30. For the first time, the momentum dependent ωN cross section has been extracted from the experiment and is in the range of 70 mb.

The photoproduction of ω mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the ω meson via the reaction γ+A→ω+X→π⁰γ+X^′. Results obtained for Nb are compared to a reference measurement on a LH₂ target. While for recoiling, long-lived mesons (π⁰, η, and η^′), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for ω mesons produced on the Nb target. For momenta less than 500  MeV/c an in-medium ω meson mass of M_medium=[722_-4⁺⁴(stat)_-5⁺³⁵(syst)]  MeV/c² has been deduced at an estimated average nuclear density of 0.6ρ₀.

Helicity-dependent total photoabsorption cross sections on the deuteron have been measured for the first time at ELSA (Bonn) in the photon energy range from 815 to 1825 MeV. Circularly polarized tagged photons impinging on a longitudinally polarized LiD target have been used together with a highly efficient 4π detector system. The data around 1 GeV are not compatible with predictions from existing multipole analyses. From the measured energy range an experimental contribution to the GDH integral on the neutron of [33.9±5.5(stat)±4.5(syst)] μb is extracted.

Totaland differential cross sections for the reaction p(γ,η)p havebeen measured for photon energies in the range from 750 MeV to 3 GeV. Thelow-energy data are dominated by the S₁₁ wave which has two poles in the energy region below 2 GeV. Eleven nucleonresonances are observed in their decay into pη.At medium energies we find evidence for a new resonance N(2070)D₁₅ with (M,Γ)=(2068±22, 295±40)  MeV.At γ energiesabove 1.5 GeV, a strong peak in the forward direction develops, signalingthe exchange of vector mesons in the t channel.

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2–2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2–2.9 GeV yields [254±5(stat)±12(syst)]  μb with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68–1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4π detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9±2.4(stat)±2.2(syst)]   μb.

The helicity dependence of the γ→p→→pπ⁰ reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D₁₃(1520) resonance and are used to determine its helicity amplitudes.

A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200<E_γ<800 MeV. The experiment used a 4π detection system, a circularly polarized tagged photon beam, and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability γ₀ determined from the data are 226±5(stat)±12(syst) μb and -187±8(stat)±10(syst)×10^-6 fm⁴, respectively, for 200<E_γ<800 MeV.

The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability γ₀.

At the tagged photon facility PHOENICS at the Bonn accelerator ELSA a measurement of the target asymmetry of the reaction γp→pη from threshold to 1150 MeV has been performed. Simultaneously the reaction γp→pπ⁰ has been measured in the first resonance region. Results are presented for both reactions. The target asymmetry data are suited to put considerable constraints on the model parameters used for the theoretical description of meson photoproduction.

We present new break-up and coherent data for η meson photoproduction on the deuteron, using a deuterium target and tagged bremsstrahlung photons up to 1 GeV. The differential cross sections for the coherent process were measured from threshold to 800 MeV. They are much smaller than those previously reported. The break-up channel provides a direct measurement of the neutron to proton differential cross section ratios. At the S₁₁(1535) resonance peak, σ_n/σ_p  =  0.68±0.06 leading to an isoscalar to isovector amplitude ratio of A_s/A_υ  =  0.096±0.02.

A Reply to the Comment by N. C. Mukhopadhyay, J.-F. Zhang, and M. Benmerrouche.

We have measured precise total and differential cross sections for the reaction γp→pη from threshold to 790 MeV at the MAMI accelerator in Mainz with the neutral meson spectrometer TAPS. Resonance parameters of the S₁₁(1535) resonance and the electromagnetic coupling γp→S₁₁ have been extracted from the data. Contributions from the D₁₃(1520) resonance to η photoproduction in the threshold region have been identified for the first time via interference terms in the angular distributions.

The total cross section for γp→ηp near threshold has been measured using the PHOENICS tagging system at the ELSA electron facility of the Physikalisches Institut der Universität Bonn. The photons are created by bremsstrahlung, and are tagged by measuring the momentum of each electron after the photon has been emitted. The recoil proton from γp→ηp is detected by the AMADEUS counter setup in coincidence with the tagging system. Data were taken with AMADEUS at 3.3° in the laboratory, where the large Jacobian increases our event rate so that we obtain the cross section from threshold (E_γ=707.2 MeV) to E_γ≃720 MeV with adequate statistics. The γp→ηp events are identified by kinematics, dE/dx, and timing information. We find that in our energy region the production cross section is consistent with S-wave production.