24 research outputs found
Properties of (1535) at Finite Density in the Extended Parity-Doublet Models
We improve so called ``naive'' and ``mirror'' models for the positive and
negative parity nucleons, and , by introducing nonlinear terms
allowed by chiral symmetry. Both models in this improvement reproduce the
observed nucleon axial charge in free space and reveal interesting density
dependence of the axial charges for and , and the doublet masses. A
remarkable difference between the two models is found in the off-diagonal axial
charge, , which could appear either as suppression or as enhancement
of decay in the medium.Comment: 14 pages (including two postscript figures), revte
Photoproduction of pions and etas in nuclei
We calculate the cross sections for inclusive one-pion, two-pion and eta
photoproduction in nuclei in the photon energy range from 300 MeV to 900 MeV
within the framework of a semi-classical BUU transport model. Our results are
compared with existing experimental data and discussed with respect to a
calculation of the total photoabsorption cross section.Comment: 30 pages LaTeX including 13 postscript figure
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.
Hadronic properties of the S_{11}(1535) studied by electroproduction off the deuteron
Properties of excited baryonic states are investigated in the context of
electroproduction of baryon resonances off the deuteron. In particular, the
hadronic radii and the compositeness of baryon resonances are studied for
kinematic situations in which their hadronic reinteraction is the dominant
contribution. Specifically, we study the reaction at for kinematics in which the produced hadronic state reinteracts
predominantly with the spectator nucleon. A comparison of constituent quark
model and effective chiral Lagrangian calculations of the shows
substantial sensitivity to the structure of the produced resonance.Comment: 24 pages, 5 figure
Photoproduction of pi0-mesons from nuclei
Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead)
has been studied for incident photon energies from 200 MeV to 800 MeV with the
TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI
accelerator. Data were obtained for the inclusive photoproduction of neutral
pions and the partial channels of quasifree single pi0, double pi0, and
pi0pi+/- photoproduction. They have been analyzed in terms of the in-medium
behavior of nucleon resonances and the pion - nucleus interaction. They are
compared to earlier measurements from the deuteron and to the predictions of a
Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon induced pion
production from nuclei.Comment: 15 pages, 22 figures, accepted for publication in EPJ
Quasi-free photoproduction of η-mesons off 3He nuclei
Quasi-free photoproduction of η-mesons has been measured off nucleons bound in 3He 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 4Ï 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 S11(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
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 4pi detectors and polarized
targets
Mass dependence of -production in heavy ion collisions at 1 A GeV
The production of neutral pions has been studied in the reactions Ar-40+Ca-nat, Kr-86+Zr-nat and (197)AU+(197)AU at 1 A GeV. For high energy pions emitted from the heavier systems a steeper than linear rise of the pion multiplicity with the centrality of the reaction is observed, indicating a pion production process other than binary nucleon-nucleon collisions. At low transverse momenta an enhancement of the pi(0)-multiplicity increasing with the mass of the collision system is found. Systematic discrepancies between the experimental results and recent BW, QMD and Cascade calculations are discussed