Browsing by Author "Leukel, R."

Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV < E- < 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X cross-sections, on, the other hand, are significantly smaller than predicted by the quasi-free process for E-gamma greater than or similar to 250 MeV. Associating this failure with the pi (o) photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value.

The differential cross sections for Compton scattering from the proton have been measured at MAMI with three different detector setups. There is a good agreement among all the results. The theoretical calculation based on dispersion relations shows that there is no drastic change necessary in the parameters of this approach.

The reaction gammap --> pi (+)pi (o)n has been measured at MAMI for photon energies up to 820 MeV. Invariant mass spectra of the particles in the final state (pi (+)n), (pi (o)n), (pi (+)pi (o)) have been determined for several bins of incident photon energy. Differences in pi (+)pi (o) and simultaneously measured pi (o)pi (o) invariant mass distributions are assigned to a rho branch of the D-13(1520) nucleon resonance.

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (pi (0)pi N-0) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Delta (1232)-resonance as an intermediate state.

Coherent photoproduction of eta -mesons from the deuteron has been measured from threshold up to E-gamma approximate to 750 MeV using the photon spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI. For the first time, differential coherent cross-sections have been deduced from the coincident detection of the eta -meson and the recoil deuteron. A missing-energy analysis was used for the suppression of background events so that a very clean identification of coherent eta -photoproduction was achieved. The resulting cross-sections agree with previous experimental results except for angles around 90 degrees in the gammad c.m. system where they are smaller. They are compared to various model calculations.

Quasi-free photoproduction of eta-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement eta-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final-state interaction effects are negligible, an almost constant ratio of sigma(n)/sigma(p) = 0.66 +/- 0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison of the inclusive d(gamma,eta)X cross section to the free-proton cross-section via a participant-spectator model for the deuteron cross-section (sigma(n)/sigma(p) = 0.67 +/- 0.07). The angular dependence of the ratio agrees with the expected small deviation of neutron and proton cross-section from isotropy due to the influence of the D-13(1520)-resonance. The energy dependence of the ratio in the excitation energy range of the S-11(1535)-resonance disfavors model predictions which try to explain this resonance as KSigma bound state.

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)] mub with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

A direct measurement of the helicity dependence of the total photoabsorption cross section an the proton was carried out at MAMI (Mainz) in the energy range 200 < E- < 800 MeV. The experiment used a 4 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 gamma (0) determined from the data are 226 +/- 5(stat) +/- 12(syst) mub and -187 +/- 8(stat) +/- 10(syst) x 10(-6) fm(4), respectively, for 200 < E- < 800 MeV.

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 4pi 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)] mub.

The helicity dependence of the (gamma) over right arrow(p) over right arrow -+ peta reaction has been measured for the first time at a center-of-mass angle theta(eta) = 70degrees in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam. and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis.

The helicity dependence of the (γ) over right arrow(p) over right arrow -->ppi(0) 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 4pi -detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D-13(1520) resonance and are used to determine its helicity amplitudes.

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 4 pi-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 gamma(0).

The helicity dependence of the total cross section for the (γ) over right arrow(p) over right arrownpi(+) pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV The measurement was performed with the large acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, due to the intermediate production of the D-13 resonance. (C) 2002 Elsevier Science B.V. All rights reserved.

A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both gammap --> Npi channels in the Delta region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4pi detection system, a circularly polarized photon beam, and a longitudinally polarized frozen-spin target. These data are included in the database to perform a multipole analysis to determine the properties of the Delta(1232)-resonance. For the resonant Delta(1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations.

The helicity dependence of the total cross section for the (gamma) over right arrow(p) over right arrow -> p pi(0)pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV. The measurement, performed at the tagged photon beam facility of the MAMI accelerator in Mainz, used the large acceptance detector DAPHNE and a longitudinally polarized frozen-spin target. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, most likely due to the intermediate production of the D-13(1520) resonance. However, the contribution of the antiparallel spin configuration, arising from other reaction mechanisms, is also not negligible. This result gives important new information to resolve the existing model discrepancies in the identification of the nucleon resonances contributing to this channel. (c) 2005 Elsevier B.V. All rights reserved.

Differential cross-sections for Compton scattering from the proton hal e been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of theta (lab)(gamma) = 59 degrees -155 degrees and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be <()over bar> = [11.9 +/- 0.5(stat.) -/+ 1.3(syst.) +/- 0.3(mod.)] (.) 10(-4) fm(3) and <()over bar> = [1.2 +/- 0.7(stat.) +/- 0.3(syst). +/- 0.4(mod.))] (.) 10(-4) fm(3). These results confirm the Baldin sum rule which was re-evaluated to be <()over bar> + <()over bar> = [13.8 +/- 0.4] 10(-4) fm(3). We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability gamma (pi) beyond the t-channel pi (0)-exchange.

Coherent and incoherent pi(0) photoproduction off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from pi(0) threshold up to 300 MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total pi(0) production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have bees obtained.

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 4 pi 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)] mu b is extracted.

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 mub at E-gamma = 740 MeV. Dalitz plots Of m(2)(pi (0), pi (0)) versus m(2)(p, pi (0)) for seven bins of incident photon energy have been analysed. For E-gamma > 610 MeV, a strong contribution of a sequential decay is observed with the Delta (1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the D-13(1520)- and also the P-11(1440)-resonance.

The reaction gammad --> pi (0)X has been measured with TAPS at MAMI in the energy range E(gamma) = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2 sigma. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m less than or equal to 2100 MeV/c(2) were deduced. They are in the range 2-5 mub averaged over the 0.8 MeV energy resolution.