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M. Ostrick

Publications and source records attributed to M. Ostrick.

At least 73 records · Page 4Linked to original sources
arXiv2014

Quasifree photoproduction of $η$ mesons off protons and neutrons

Differential and total cross sections for the quasifree reactions $γp\rightarrowηp$ and $γn\rightarrowηn$ have been determined at the MAMI-C electron accelerator 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 $η\rightarrow 2γ$ and $η\rightarrow 3π^0 \rightarrow 6γ$ 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 $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $Γ\approx 30$ MeV.

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arXiv2014

$K^+Λ$ and $K^+Σ^0$ photoproduction with fine center-of-mass energy resolution

Measurements of $γp \rightarrow K^{+} Λ$ and $γp \rightarrow K^{+} Σ^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ 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.

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arXiv2014

Measurement of polarisation observables in $K^0_sΣ^+$ photoproduction off the proton

The reaction $γ\, p \rightarrow K^0_S\,Σ^+$ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to $E_γ= 2250$ MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay $Σ^+ \rightarrow p π^0$ up to $E_γ= 1650$ MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed cusp-like structure near the $K^*$ thresholds is discussed.

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arXiv2014

A new measurement of the rare decay eta -> pi^0 gamma gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

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 two-photon 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.

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arXiv2014

Measurement of the g p -> K^0 Sigma^+ reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron

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, by 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 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 is demonstrated that adding the present g p -> K^0 Sigma^+ results to existing data allowed a better description of this reaction with various models.

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arXiv2014

A new determination of the eta transition form factor in the Dalitz decay eta -> e^+ e^- gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

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 form-factor 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.

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arXiv2014

Light Meson Dynamics Workshop. Mini proceedings

The mini-proceedings of the Light Meson Dynamics Workshop held in Mainz from February 10th to 12th, 2014, are presented. The web page of the conference, which contains all talks, can be found at https://indico.cern.ch/event/287442/overview .

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arXiv2014

Photoproduction of pi0-mesons off neutrons in the nucleon resonance region

Precise angular distributions have been measured for the first time for the photoproduction of $π^{0}$-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 final-state-interaction effects has been estimated by a comparison of the reaction cross section for quasi-free protons bound in the deuteron to the results for free protons and then applied as a correction to the quasi-free 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 partial-wave 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 Bonn-Gatchina coupled-channel analysis which included the present data.

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arXiv2014

Measurement of the beam-helicity asymmetry $I^{\odot}$ in the photoproduction of $π^0π^{\pm}$-pairs off protons and off neutrons

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions $\boldsymbolγp\rightarrow nπ^0π^+$ off free protons and $\boldsymbolγd\rightarrow (p)pπ^0π^-$ and $\boldsymbolγd\rightarrow (n)nπ^0π^+$ 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-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from $π^0$ mesons were detected in the 4$π$ 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 quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.

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arXiv2013

Quasi-free photoproduction of eta-mesons off 3He nuclei

Quasi-free photoproduction of $η$-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 $4π$ covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The $η$-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free 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 quasi-free 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 quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final state interaction effects.

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arXiv2013

A narrow structure in the excitation function of eta-photoproduction off the neutron

The photoproduction of $η$-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 $η\rightarrow 2γ$ and $η\rightarrow 3π^0$ 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 $γn\rightarrow nη$. 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$\pm$5) MeV and $Γ$ =(30$\pm$15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.

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arXiv2013

Search for narrow six-quark states in the reactions $γd\to πγnn$

We study the reactions $γd\toπγNN$ with the aim to search for six-quark states, the decay of which into two nucleons is forbidden by the Pauli exclusion principle. These states with the masses $M<2m_N+m_π$ should mainly decay by emitting a photon. This is a new class of metastable six-quark states with the decay widths $<< 1$keV. The recent experiment at Proton Linear Accelerator of INR (Moscow) suggests a possibility of the existence of such states with masses 1904, 1926, and 1942 MeV. The proposed experiment at MAMI-C could provide an unique opportunity to observe such dibaryon states in mass region up to 2000 MeV and determine their masses and quantum numbers.

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arXiv2013

Measurement of the $^{1}H$($\vecγ$, $\vec{p}$)$π^{0}$ reaction using a novel nucleon spin polarimeter

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon, pioneering a novel polarimetry technique with wide application to future nuclear and hadronic physics experiments. The commissioning measurement of polarization transfer in the $^{1}H$($\vecγ$,$\vec{p}$)$π^{0}$ reaction in the range $0.4<E_γ<1.4$ GeV is highly selective regarding the basic parameterizations used in partial wave analyses to extract the nucleon excitation spectrum. The new data strongly favor the recently proposed Chew-Mandelstam formalism.

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arXiv2013

Accurate Test of Chiral Dynamics in the \boldmath$\vecγ p \rightarrow π^0p$ Reaction

A precision measurement of the differential cross sections $dσ/dΩ$ and the linearly polarized photon asymmetry $Σ\equiv (dσ_\perp - dσ_\parallel) \slash (dσ_\perp + dσ_\parallel)$ for the $\vecγ p \rightarrow π^0p$ reaction in the near-threshold region has been performed with a tagged photon beam and almost $4π$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multi-photon detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the $S$- and all three $P$-wave amplitudes for the first time and provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.

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arXiv2013

Study of the partial wave structure of $π^0η$ photoproduction on protons

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.

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arXiv2013

Measurement of the beam-helicity asymmetry $I^{\odot}$ in the photoproduction of $π^0$-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 $\vecγp\rightarrow pπ^0π^0$ and $\vecγd\rightarrow (n)pπ^0π^0$, $\vecγd\rightarrow (p)nπ^0π^0$ 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 $π^0$ mesons, recoil protons, and recoil neutrons were detected in the 4$π$ 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.

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arXiv2013

Double pion photoproduction off nuclei - are there effects beyond final-state interaction?

Photoproduction of $π^{0}π^{0}$ and $π^{0}π^{\pm}$ 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$π$ electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass spectra for heavy nuclei to small invariant masses has been observed for $π^0$ pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the $σ$-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge 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 $σ$-in-medium modification.

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