819 research outputs found
Photoproduction of eta-mesons from light nuclei
In a series of experiments coherent and quasifree eta-photoproduction from
light nuclei (4He, 3He, 2H) was investigated with the TAPS-detector at the
Mainz MAMI-accelerator. The experiments were motivated by two different
subjects: the determination of the isospin structure of the electromagnetic
excitation of the S11(1535) resonance and the study of the eta-nucleon and
eta-nucleus interaction at small momenta. The results for the deuteron and 4He
are summarized and first preliminary results for 3He are presented.Comment: 5 pages, 3 figures, Proceedings of Meson2002 workshop, Kraka
Study of Non-Strange Baryon Resonances with Meson Photoproduction
Photoproduction of mesons is an excellent tool for the study of nucleon
resonances. Complementary to pion induced reactions, photoproduction on the
free proton contributes to the determination of the basic properties of nucleon
resonances like excitation energy, decay widths, spin, and the coupling to the
photon. Photoproduction from light nuclei, in particular from the deuteron,
reveals the isospin structure of the electromagnetic excitation of the nucleon.
During the last few years, progress in this field has been substantial. New
accelerator facilities combined with state-of-the-art detector technologies
have pushed the experiments to unprecedented sensitivity and precision. The
experimental progress has been accompanied by new developments for the reaction
models, necessary to extract the properties of the nucleon states, and for
modern hadron models which try to connect these properties to QCD. The emphasis
of this review lies on the experimental side and focuses on experiments aiming
at precise studies of the low-lying nucleon resonances.Comment: 87 pages, 67 figures, Prog. Part. Nucl. Phys., accepte
Photoproduction of Mesons off Light Nuclei -- the Search for \eta -Mesic Nuclei
Photoproduction of \eta mesons off light nuclei (d, 3He, 7Li) has been
measured at the tagged photon beam of the Mainz MAMI accelerator with the
combined Crystal Ball/TAPS detection system. Special attention was given to the
threshold behavior of the reactions in view of possible indications for the
formation of (quasi-) bound \eta -nucleus states, so-called \eta -mesic nuclei.
A very strong threshold enhancement of coherent \eta photoproduction off 3He
was found and coherent \eta photoproduction off 7Li was observed for the first
time. Preliminary results will be discussed.Comment: Invited talk at the International Symposium on Mesic Nuclei, Krakow,
16 June 201
Photoproduction of mesons off nuclei
Recent results for the photoproduction of mesons off nuclei are reviewed. These experiments have been performed for two major lines of research related to the properties of the strong interaction. The investigation of nucleon resonances requires light nuclei as targets for the extraction of the isospin composition of the electromagnetic excitations. This is done with quasi-free meson photoproduction off the bound neutron and supplemented with the measurement of coherent photoproduction reactions, serving as spin and/or isospin filters. Furthermore, photoproduction from light and heavy nuclei is a very efficient tool for the study of the interactions of mesons with nuclear matter and the in-medium properties of hadrons. Experiments are currently rapidly developing due to the combination of high quality tagged (and polarized) photon beams with state-of-the-art 4Ď€ detectors and polarized target
Nuclear mass form factors from coherent photoproduction of πmesons
Abstract.: Data for coherent photoproduction of π0 mesons from nuclei ( 12C, 40Ca, 93Nb, natPb), recently measured with the TAPS detector at the Mainz MAMI accelerator, have been analyzed in view of the mass form factors of the nuclei. The form factors have been extracted in plane-wave approximation of the A(γ,π0)A reaction and corrected for final-state interaction effects with the help of distorted-wave impulse approximations. Nuclear mass rms radii have been calculated from the slope of the form factors for q2 → 0. Furthermore, the Helm model (hard-sphere form factor folded with Gaussian) was used to extract diffraction radii from the zeroes of the form factor and skin thicknesses from the position and height of its first maximum. The diffraction radii from the Helm model agree with the corresponding charge radii obtained from electron scattering experiments within their uncertainties of a few per cent. The rms radii from the slope of the form factors are systematically lower by up to 5% for PWIA and up to 10% for DWIA. Also the skin thicknesses extracted from the Helm model are systematically smaller than their charge counterpart
Surface and volume effects in the photoabsorption of nuclei
Recent experimental results for meson photoproduction from nuclei obtained
with TAPS at MAMI are analyzed in view of the suppression of the second nucleon
resonance region in total photoabsorption. The cross sections can be split into
a component from the low density surface region of nuclei and a component which
scales more like the nuclear volume. The energy dependence of the surface
component is similar to the deuteron cross section, it shows a clear signal for
the second resonance peak assigned to the excitation of the P11(1440),
D13(1520), and S11(1535). The volume component behaves differently, it is
lacking the second resonance peak and shows an enhancement at intermediate
photon energies.Comment: accepted for publication in Eur. J. Phys.
Photoproduction of mesons from nuclei - In-medium properties of hadrons
Recent experimental results for the in-medium properties of hadrons obtained
with photoproduction of mesons from nuclei are discussed. The experiments were
done with the TAPS detector at the tagged photon beam of the MAMI accelerator
in Mainz. Measured were the final states , , , and
for C, Ca, Nb, and Pb up to the
second resonance region. The results were used for an investigation of the
in-medium properties of the P(1232), the P(1440), the
D(1520), and the S(1535) resonances. It was found that the cross
sections can be spilt into a component which originates from the low density
surface region of the nuclei and a component which scales like the nuclear
volume. The energy dependence of the surface component is strikingly similar to
the deuteron, it shows a clear signal for the second resonance peak. The volume
component is lacking this peak and shows an enhancement at intermediate
energies. Furthermore the measurement of coherent -photoproduction and
the final state from He is discussed in the context of the search
for -mesic nuclei.Comment: Proceedings of International School of Nuclear Physics, 26th Course,
Lepton Scattering and the Structure of Hadrons and Nuclei, Erice-Sicily,
Italy 16. - 24. September 2004, to be published in Prog. Part. Nucl. Phy
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