M. Dieterle

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.

Narrow structure in the excitation function of η photoproduction off the neutron

The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)  MeV and Γ=(30±15)  MeV. For the first time precise results for the angular dependence of this structure have been extracted.

Measurement of the Transverse Target and Beam-Target Asymmetries in η Meson Photoproduction at MAMI

We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γ p →ηp reaction up to a center-of-mass energy of W=1.9  GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.

K+Λ and K+Σ0 photoproduction with fine center-of-mass energy resolution

Measurements of γp→K+Λγp→K+Λ and γp→K+Σ0γp→K+Σ0 cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel K+K+ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy (W) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark–diquark dynamics in the nucleon.

Quasifree photoproduction of η mesons off protons and neutrons

Differential and total cross sections for the quasifree reactions γp→ηp and γn→ηn have been determined at the MAMI-C electron accelerator by using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes η→2γ and η→3π0→6γ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The η-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree-proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the S11(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy W=(1670±5) MeV with an intrinsic width of Γ≈30 MeV.

Measurements of double-polarized compton scattering asymmetries and extraction of the proton spin polarizabilities.

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γ(E1E1)=-3.5±1.2, γ(M1M1)=3.16±0.85, γ(E1M2)=-0.7±1.2, and γ(M1E2)=1.99±0.29, in units of 10(-4)  fm(4).

The polarization observables T, P, and H and their impact on gamma p -< p pi(0) multipoles

Data on the polarization observables T, P, and H for the reaction gamma p -> p pi(0) are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction

N(1520) 3/2(-) Helicity Amplitudes from an Energy-Independent Multipole Analysis Based on New Polarization Data on Photoproduction of Neutral Pions

New data on the polarization observables T, P, and H for the reaction γp→pπ(0) are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

Measurement of the beam-helicity asymmetry I-circle dot in the photoproduction of pi(0)-pairs off the proton and off the neutron

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions (gamma) over right arrowp -< p pi(0)pi(0) and (gamma) over right arrowd -< (n)p pi(0)pi(0), (gamma) over right arrowd -< (p)n pi(0)pi(0) off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow magnetic spectrometer. Decay photons from the pi(0) mesons, recoil protons, and recoil neutrons were detected in the 4 pi covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons. This demonstrates that the free-nucleon behavior of such observables can be extracted from measurements with quasi-free nucleons, which is the only possibility for the neutron. Contrary to expectations, the measured asymmetries are very similar for reactions off protons and neutrons. The results are compared to the predictions from the Two-Pion MAID reaction model and (for the proton) also to the Bonn-Gatchina coupled channel analysis

Measurement of

Differential cross sections for the gamma p -> pi(0)p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W = 1.9 GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of pi(0) photoproduction data by similar to 47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.