Publications (81)208.38 Total impact
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ABSTRACT: Total cross sections, angular distributions, and invariantmass distributions have been measured for the photoproduction of $\pi^0\pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400~MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariantmass distributions) for free and quasifree protons are almost identical in shape, but differ in absolute magnitude up to 15\%. Thus, moderate finalstate interaction effects are present. The data for quasifree neutrons are similar to the proton data in the second resonance region (final state invariant masses up to $\approx$1550~MeV), where both reactions are dominated by the $N(1520)3/2^\rightarrow \Delta(1232)3/2^+\pi$ decay. At higher energies, angular and invariantmass distributions are different. A simple analysis of the shapes of the invariantmass distributions in the third resonance region is consistent with strong contributions of an $N^{\star}\rightarrow N\sigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $\Delta\pi$ intermediate state for the neutron. The data are compared to predictions from the TwoPionMAID model and the BonnGatchina coupled channel analysis. 
Dataset: The isospin structure of photoproduction of πη pairs from the nucleon in the threshold region
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ABSTRACT: Photoproduction of πηpairs from nucleons has been investigated from threshold up to incident photon energies of ≈ 1.4 GeV. The quasifree reactions γp→pπ0η, γn→nπ0η, γp→nπ+η, and γn→pπη were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a freeproton target were also studied to investigate finalstate interaction effects (for neutral pions the freeproton results could be compared to previous measurements; the γp→nπ+η reaction was measured for the first time). For the π0η final state coherent production via the γd→dπ0η reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost 4π coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation σ(pπ0η)≈σ(nπ0η)≈2σ(pπη)≈2σ(nπ+η) as expected for a dominant contribution from a δ*→ηδ(1232)→πηN reaction chain, which is also supported by the shapes of the invariantmass distributions of nucleonmeson and πη pairs. The experimental results are compared to the predictions from an isobar reaction model. 
Article: The isospin structure of photoproduction of pieta pairs from the nucleon in the threshold region
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ABSTRACT: Photoproduction of $\pi\eta$pairs from nucleons has been investigated from threshold up to incident photon energies of $\approx$~1.4~GeV. The quasifree reactions $\gamma p\rightarrow p\pi^0\eta$, $\gamma n\rightarrow n\pi^0\eta$, $\gamma p\rightarrow n\pi^+\eta$, and $\gamma n\rightarrow p\pi^\eta$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a freeproton target were also studied to investigate finalstate interaction effects (for neutral pions the freeproton results could be compared to previous measurements; the $\gamma p\rightarrow n\pi^+\eta$ reaction was measured for the first time). For the $\pi^0\eta$ final state coherent production via the $\gamma d\rightarrow d\pi^0\eta$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4\pi$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $\sigma(p\pi^0\eta)$ $\approx$ $\sigma(n\pi^0\eta)$ $\approx$ $2\sigma(p\pi^\eta)$ $\approx$ $2\sigma(n\pi^+\eta)$ as expected for a dominant contribution from a $\Delta^{\star}\rightarrow\eta\Delta(1232)\rightarrow\pi\eta N$ reaction chain, which is also supported by the shapes of the invariantmass distributions of nucleonmeson and $\pi$$\eta$ pairs. The experimental results are compared to the predictions from an isobar reaction model.  [Show abstract] [Hide abstract]
ABSTRACT: Differential cross sections for the gamma p > pi^0 p reaction have been measured with the A2 taggedphoton facilities at the Mainz Microtron, MAMI C, up to the centerofmass 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 ~47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partialwave 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 partialwave analysis obtained after adding the new data into the fit is presented. 
Article: First measurement of target and beamtarget asymmetries in the $\gamma p\to \pi^0\eta p$ reaction
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ABSTRACT: The first data on target and beamtarget asymmetries for the $\gamma p\to\pi^0\eta p$ reaction at photon energies from 1050 up to 1450 MeV are presented. The measurements were performed using the Crystal Ball and TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. The general assumption that the reaction is dominated by the $\Delta 3/2^$ amplitude is confirmed. The data are in particular sensitive to small contributions from other partial waves.  [Show abstract] [Hide abstract]
ABSTRACT: The first data on target and beamtarget asymmetries for the γp→π0ηp reaction at photon energies from 1050 up to 1450 MeV are presented. The measurements were performed using the Crystal Ball and TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. The general assumption that the reaction is dominated by the Δ3/2 amplitude is confirmed. The data are in particular sensitive to small contributions from other partial waves. 
Dataset: 1501.00289

Dataset: 1501.00289
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ABSTRACT: Incoherent η photoproduction on the deuteron is studied, with the main emphasis on the role of finalstate interactions. In addition to the previously studied mechanisms of NN and ηN rescatterings, the role of an intermediate pion exchange is considered in detail, where first a pion is photoproduced on one nucleon and then rescatters into an η meson on the other, the spectator nucleon. It is found that the role of this pionmediated contribution is comparable in size to that of ηN rescattering. Results for total and semiinclusive differential cross sections and associated polarization observables are presented. In particular, polarization observables show significant sensitivity to finalstate interactions. 
Dataset: 1501.00289
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ABSTRACT: Incoherent $\eta$ photoproduction on the deuteron is studied with the main emphasis on the role of final state interactions. In addition to the previously studied mechanisms of $NN$ and $\eta N$ rescatterings, the role of an intermediate pion exchange is considered in detail, where first a pion is photoproduced on one nucleon and then rescatters into an $\eta$ meson on the other, the spectator nucleon. It is found, that the role of this pion mediated contribution is comparable in size to that of $\eta N$ rescattering. Results for total and semiinclusive differential cross sections and associated polarization observables are presented. In particular polarization observables show a significant sensititvity to final state interactions.  [Show abstract] [Hide abstract]
ABSTRACT: On the basis of an analysis of the available experimental data for double neutral pion photoproduction on nucleons, estimates have been obtained of the relative contribution of isoscalar and isovector transitions to the amplitude of this process. The obtained results are used to calculate the cross section of coherent photoproduction of pairs of neutral pions on a deuteron and on the threenucleon isotope of helium. A strong sensitivity of the cross section to peculiarities of the isotopic structure of the onenucleon amplitude is demonstrated.  [Show abstract] [Hide abstract]
ABSTRACT: Coherent photoproduction of π0π0π0π0 on the deuteron and 3He is calculated. The isoscalar and isovector parts of the elementary photoproduction amplitude were determined by fitting the measured total cross section on protons and neutrons. The dependence of the cross section on the isospin of the target nucleus is discussed.  [Show abstract] [Hide abstract]
ABSTRACT: The helicity dependences of the differential cross sections for the semiinclusive γ 3He → π0X and γ 3He → π±X reactions have been measured for the first time in the energy region 200 < E γ 450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised highpressure 3He gas target. Hadronic products were measured with the largeacceptance Crystal Ball detector complemented with additional devices for chargedparticle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the FixArenhövel model. The effect of the intermediate excitation of the Δ(1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicitydependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on 3He and suggest that a polarised 3He target can provide valuable information on the corresponding polarised quasifree neutron reactions. 
Article: Measurement of the Transverse Target and BeamTarget Asymmetries in η Meson Photoproduction at MAMI
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ABSTRACT: We present new data for the transverse target asymmetry T and the very first data for the beamtarget asymmetry F in the $\vec \gamma \vec p\to\eta p$ reaction up to a centerofmass energy of W=1.9 GeV. The data were obtained with the CrystalBall/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 $\eta$ meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.  [Show abstract] [Hide abstract]
ABSTRACT: Differential and total cross sections for the quasifree reactions $\gamma p\rightarrow\eta p$ and $\gamma n\rightarrow\eta n$ have been determined at the MAMIC electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energytagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0 \rightarrow 6\gamma$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $\eta$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 finalstate 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 neutrontoproton crosssection ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a centerofmass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $\Gamma\approx 30$ MeV.  [Show abstract] [Hide abstract]
ABSTRACT: A new measurement of the rare, doubly radiative decay eta>pi^0 gamma gamma was conducted with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. New data on the dependence of the partial decay width, Gamma(eta>pi^0 gamma gamma), on the twophoton invariant mass squared, m^2(gamma gamma), as well as a new, more precise value for the decay width, Gamma(eta>pi^0 gamma gamma) = (0.33+/0.03_tot) eV, are based on analysis of 1.2 x 10^3 eta>pi^0 gamma gamma decays from a total of 6 x 10^7 eta mesons produced in the gamma p > eta p reaction. The present results for dGamma(eta>pi^0 gamma gamma)/dm^2(gamma gamma) are in good agreement with previous measurements and recent theoretical calculations for this dependence. 
Dataset: PhysRevLett.112.142001
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ABSTRACT: Precise angular distributions have been measured for the first time for the photoproduction of $\pi^{0}$mesons off neutrons bound in the deuteron. The effects from nuclear Fermi motion have been eliminated by a complete kinematic reconstruction of the final state. The influence of finalstateinteraction effects has been estimated by a comparison of the reaction cross section for quasifree protons bound in the deuteron to the results for free protons and then applied as a correction to the quasifree neutron data. The experiment was performed at the tagged photon facility of the Mainz Microtron MAMI with the Crystal Ball and TAPS detector setup for incident photon energies between $0.45$~GeV and $1.4$~GeV. The results are compared to the predictions from reaction models and partialwave analyses based on data from other isospin channels. The model predictions show large discrepancies among each other and the present data will provide much tighter constraints. This is demonstrated by the results of a new analysis in the framework of the BonnGatchina coupledchannel analysis which included the present data. 
Publication Stats
773  Citations  
208.38  Total Impact Points  
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 Physical Review C (22)
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Institutions

20002015

Tomsk Polytechnical University
 Mathematical Physics International Laboratory
Tomsk, Tomsk, Russia


20022011

Johannes GutenbergUniversität Mainz
 Institute of Nuclear Physics
Mayence, RheinlandPfalz, Germany
