33 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
Relativistic Calculations for Incoherent Photoproduction of Eta Mesons
We develop a relativistic model for incoherent eta-photoproduction on nuclei.
The elementary process is described using an effective Lagrangian containing
photons, nucleons, the S11(1535) and D13(1520) nucleon resonances, and rho,
omega, and eta mesons. The nucleon and eta wavefunctions are obtained from
relativistic wave equations. Final-state interactions of the outgoing particles
are included via optical potentials. The effects of these interactions are
found to be large and lead to reduced cross sections.The incoherent cross
sections for isovector transitions are much larger than those for isoscalar
ones. The dominant contributions are those from the S11 and D13 resonances. We
find important interference effects between the contributions of these two
resonances. We give some detailed calculations for the cross sections for
incoherent eta-photoproduction on C-12. We find that the incoherent cross
section for a subset of states in the excitation energy region below 17 MeV are
significantly larger than those of the coherent process. These cross sections
may thus be accessible experimentally.Comment: 24 pages, 7 figures, to be published in Nuclear Physics
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
Quasifree Eta Photoproduction from Nuclei
Quasifree photoproduction from nuclei is studied in the Distorted Wave
Impulse Approximation (DWIA). The elementary eta production operator contains
Born terms, vector meson and nucleon resonance contributions and provides an
excellent description of the recent low energy Mainz measurements on the
nucleon. The resonance sector includes the , and
states whose couplings are fixed by independent electromagnetic
and hadronic data. Different models for the t-matrix are used to
construct a simple optical potential based on a -approximation. We find that the exclusive process can
be used to study medium modifications of the resonances, particularly if
the photon asymmetry can be measured. The inclusive reaction
is compared to new data obtained on , , and is found to
provide a clear distinction between different models for the t-matrix.Comment: 30 pages in RevTeX including 14 embedded PS figures; Replaced with
revised version. Added more discussion about the imaginary part of the eta
optical potentia
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.
Eta bound states in nuclei: a probe of flavour-singlet dynamics
We argue that eta bound states in nuclei are sensitive to the singlet
component in the eta. The bigger the singlet component, the more attraction and
the greater the binding. Thus, measurements of eta bound states will yield new
information about axial U(1) dynamics and glue in mesons. Eta - etaprime mixing
plays an important role in understanding the value of the eta-nucleon
scattering length.Comment: 8 pages, version to appear in PL
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
Dilepton production and -scaling at BEVALAC/SIS energies
We present a dynamical study of production in C + C and Ca + Ca
collisions at BEVALAC/SIS energies on the basis of the covariant transport
approach HSD employing momentum-dependent -meson spectral functions that
include the pion modifications in the nuclear medium as well as the
polarization of the -meson due to resonant scattering. We find
that the experimental data from the DLS collaboration cannot be described
within the -meson spectral function approach. A dropping -mass
scenario leads to a good reproduction of the DLS dilepton data, however,
violates the -scaling of and spectra as observed by the
TAPS collaboration as well as photoproduction on nuclei.Comment: 35 pages, ReVTeX, including 11 postscript figures, UGI-97-06, Nucl.
Phys. A, in pres
eta in the nuclear medium within a chiral unitary approach
The self-energy of an eta meson in the nuclear medium is calculated in a
chiral unitary approach.
A coupled channel Bethe-Salpeter equation is solved to obtain the effective
eta-N interaction in the medium.
The base model reproduces well the free space pi-N elastic and inelastic
scattering at the eta-N threshold or N^*(1535) region.
The Pauli blocking on the nucleons, binding potentials for the baryons and
self-energies of the mesons are incorporated, including the eta self-energy in
a self-consistent way.
Our calculation predicts about -54 -i29 MeV for the optical potential at
normal nuclear matter for an eta at threshold but also shows a strong energy
dependence of the potential.Comment: LaTeX, 13 pages, 9 figure
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