Publications (71)129.51 Total impact
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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.08/2014;  [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.Phys. Rev. C. 07/2014; 90(1).  [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.05/2014; 
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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.Physical Review Letters 04/2014; 112(14). · 7.73 Impact Factor 
Dataset: PhysRevLett.112.142001
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ABSTRACT: Beamhelicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixedcharge pion pairs in the reactions $\boldsymbol{\gamma}p\rightarrow n\pi^0\pi^+$ off free protons and $\boldsymbol{\gamma}d\rightarrow (p)p\pi^0\pi^$ and $\boldsymbol{\gamma}d\rightarrow (n)n\pi^0\pi^+$ off quasifree 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 GlasgowMainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from $\pi^0$ mesons were detected in the 4$\pi$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasifree protons, suggesting that the quasifree neutron results are also a close approximation of the freeneutron asymmetries. A comparison of the results to the predictions of the TwoPionMAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleonresonance region.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The problem of determining completely the spin amplitudes of photoproduction of two pseudoscalar mesons on a nucleon from observables is studied. The procedure of reconstruction of the scattering matrix elements from a complete set of observables is based on the expressions of all observables as quadratic hermitean forms in the reaction matrix elements which are derived explicitly. Their inversion allows one to find explicit solutions for the reaction matrix elements in terms of observables. Two methods for finding a complete set of observables are presented. In particular, one set was found, which does not contain a triple polarization observable.02/2014; 89(3). 
Dataset: paper7
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ABSTRACT: Quasifree photoproduction of $\eta$mesons has been measured off nucleons bound in $^3$He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost $4\pi$ covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The $\eta$mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasifree protons and quasifree neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the $S_{11}$(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasifree neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasifree protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final state interaction effects.12/2013;  [Show abstract] [Hide abstract]
ABSTRACT: The photoproduction of $\eta$mesons off nucleons bound in $^2$H and $^3$He 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 $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0$ decays and the recoil nucleons were detected with an almost $4\pi$ 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 $\gamma n\rightarrow n\eta$. 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$\pm$5) MeV and $\Gamma$ =(30$\pm$15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.Physical Review Letters 11/2013; 111(23). · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Coherent photoproduction of π0η on the deuteron and 3He and 4He nuclei is considered in the energy region from threshold to a laboratory photon energy Eγ=1.2 GeV. The transition amplitude is derived in the impulse approximation. The effects of pion absorption are included by means of the FernbachSerberTaylor model. For the reactions on d and 3He the interaction of the produced η meson with the recoiling nucleus is taken into account. The corresponding ηd and η3He scattering amplitudes are obtained as solutions of the fewbody equations for ηNN and η3N systems. The impact of this interaction on the differential cross section in the region of small relative ηnuclear momenta is discussed.Physical Review C 11/2013; · 3.72 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The Dalitz decay eta > e^+ e^ gamma has been measured in the gamma p > eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used in this work is one order of magnitude greater than in any previous measurement of eta > e^+ e^ gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/ 0.15_stat +/ 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta > e^+ e^ gamma and eta > mu^+ mu^ gamma decays as well as with recent formfactor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta > e^+ e^ gamma decay.09/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Analysis of the partial wave structure of gamma p > pi0 eta p reaction is presented in the energy region from threshold up to the total center of mass energy W = 1.9 GeV. Angular distributions measured with the Crystal Ball/TAPS hermetic detector system at the Mainz Microtron MAMI are expanded in terms of spherical harmonics. The relation of the extracted moments to the partial wave structure of the reaction amplitude is discussed and compared with predictions from model calculations.Nuclear Physics A 07/2013; · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The g p > K^0 Sigma^+ reaction has been measured from threshold to Eg=1.45 GeV (W_cm=1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3pi^0 p final state, assuming K^0_S > pi^0 pi^0 and Sigma^+ > pi^0 p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve the empirical knowledge about the g p > K^0 Sigma^+ reaction in the measured energy range. The results are compared to previous measurements and model predictions. It was demonstrated that adding the present g p > K^0 Sigma^+ results to existing data allowed a better description of this reaction with various models.06/2013; 
Article: PWA tools in Hadronic Spectroscopy
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ABSTRACT: The miniproceedings of the Workshop on PWA tools in Hadronic Spectroscopy held in Mainz from February 18th to 20th, 2013.04/2013;  [Show abstract] [Hide abstract]
ABSTRACT: A truncated partial wave analysis for the photoproduction of two pseudoscalar mesons on a nucleon is discussed with respect to the determination of a complete set of observables. For the selection of such a set we have applied a criterion previously developed for photo and electrodisintegration of a deuteron, which allows one to find a ‘minimal’ set of observables for determining the partial wave amplitudes up to possible discrete ambiguities. The question of resolving the remaining ambiguities by invoking additional observables is discussed for the simplest case, when the partial wave expansion is truncated at the lowest total angular momentum of the final state Jmax=1/2. The resulting ‘fully’ complete set, allowing an unambiguous determination of the partial wave amplitudes, is presented.Physical Review C 04/2013; 87(4). · 3.72 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Beamhelicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions $\vec{\gamma}p\rightarrow p\pi^0\pi^0$ and $\vec{\gamma}d\rightarrow (n)p\pi^0\pi^0$, $\vec{\gamma}d\rightarrow (p)n\pi^0\pi^0$ off free protons and off quasifree 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 quasifree protons. This demonstrates that the freenucleon behavior of such observables can be extracted from measurements with quasifree 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 TwoPionMAID reaction model and (for the proton) also to the BonnGatchina coupled channel analysis.Physics Letters B. 04/2013; 721(s 4–5).  [Show abstract] [Hide abstract]
ABSTRACT: Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca, and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pionpion invariant mass spectra for heavy nuclei to small invariant masses has been observed for $\pi^0$ pairs but also for the mixedcharge pairs. The precise results allow for the first time a modelindependent analysis of the influence of pion finalstate interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible inmedium modifications of the $\sigma$meson. Results from a transport model calculation reproduce the shape of the invariantmass distributions for the mixedcharge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for $\sigma$inmedium modification.Physics Letters B. 04/2013; 722(s 1–3).  [Show abstract] [Hide abstract]
ABSTRACT: Coherent photoproduction of $\pi^0$mesons from threshold ($E_{th} \approx$ 136 MeV) throughout the $\Delta$resonance region and of $\eta$mesons close to the production threshold ($E_{th} \approx$ 570 MeV for $\eta$) has been measured for $^7$Li nuclei. The experiment was performed using the taggedphoton beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$\pi$ solidangle electromagnetic calorimeter. The reactions were identified by a combined invariantmass and missingenergy analysis. A comparison of the pion data to planewave impulse modelling tests the nuclear mass form factor. So far coherent $\eta$production had been only identified for the lightest nuclear systems ($^2$H and $^3$He). For $^3$He a large enhancement of the cross section above planewave approximations had been reported, indicating the formation of a quasibound state. The present Lidata for $\eta$production agree with a planewave approximation. Contrary to $^3$He, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected formfactor dependence were observed.03/2013;
Publication Stats
310  Citations  
129.51  Total Impact Points  
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Institutions

2000–2013

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


1997–2011

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