42 research outputs found
Low-energy pions in nuclear matter and 2pi photoproduction within a BUU transport model
A description of low-energy scattering of pions and nuclei within a BUU
transport model is presented. Implementing different scenarios of medium
modifications, the mean free path of pions in nuclear matter at low momenta and
pion absorption reactions on nuclei have been studied and compared to data and
to results obtained via quantum mechanical scattering theory. We show that even
in a regime of a long pionic wave length the semi-classical transport model is
still a reliable framework for pion kinetic energies greater than ~20-30 MeV.
Results are presented on pion-absorption cross sections in the regime of 10 MeV
< E(kin) < 130 MeV and on photon-induced double-pion production at incident
beam energies of 400-500 MeV.Comment: 22 pages, 20 figures Replaced with a revised version. Accepted for
publication in EPJ A. Added a short section on pion reaction and charge
exchange cross-section
Attenuation of phi mesons in gamma A reactions
We present a theoretical analysis of inclusive photoproduction of phi mesons
in nuclei. In particular the dependence of the total phi meson yield on the
target mass number is investigated. The calculations are done using the
semi-classical BUU transport approach that combines the initial state
interaction of the incoming photon with the coupled-channel dynamics of the
final state particles. The conditions of the calculations are chosen such as to
match the set up of a recent experiment performed at SPring8/Osaka. Whereas the
observables prove to be rather sensitive to the phi self energy in the medium,
the attribution of deviations from the standard scenario to a particular
in-medium effect seems to be impossible.Comment: 6 pages, 4 figure
Hadrons in Nuclei -- from High (200 GeV) to Low (1 GeV) energies
The study of the interaction of hadrons, produced by elementary probes in a
nucleus, with the surrounding nuclear medium can give insight into two
important questions. First, at high energies, the production process, the
time-scales connected with it and the prehadronic interactions can be studied
by using the nuclear radius as a length-scale. We do this here by analyzing
data from the EMC and HERMES experiements on nuclear attenuation. Second, at
low energies the spectral function, and thus the selfenergy of the produced
hadron, can be studied. Specifically, we analyze the CBELSA/TAPS data on
production in nuclei and discuss the importance of understanding
in-medium effects both on the primary production cross section and the final
state branching ratio. In both of these studies an excellent control of the
final state interactions is essential.Comment: Lecture given by U. Mosel at International School of Nuclear Physics:
29th Course: Quarks in Hadrons and Nuclei, Erice, Sicily, Italy, 16-24 Sep
200
Inclusive omega photoproduction off nuclei
We investigate inclusive omega photoproduction off complex nuclei,
concentrating on the feasibility to examine a possible in-medium change of the
omega meson properties by observing the pi^0 gamma invariant mass spectrum. The
simulations are performed by means of a BUU transport model including a full
coupled-channel treatment of the final state interactions. In-medium changes of
the omega spectral density are found to yield a moderate modification of the
observables as compared to the situation in free space. Also the effects of a
momentum dependence of the strong omega potential are discussed.Comment: 19 pages, 12 figures, minor corrections, accepted for publication in
EPJ
Hadron attenuation in deep inelastic lepton-nucleus scattering
We present a detailed theoretical investigation of hadron attenuation in deep
inelastic scattering (DIS) off complex nuclei in the kinematic regime of the
HERMES experiment. The analysis is carried out in the framework of a
probabilistic coupled-channel transport model based on the
Boltzmann-Uehling-Uhlenbeck (BUU) equation, which allows for a treatment of the
final-state interactions (FSI) beyond simple absorption mechanisms.
Furthermore, our event-by-event simulations account for the kinematic cuts of
the experiments as well as the geometrical acceptance of the detectors. We
calculate the multiplicity ratios of charged hadrons for various nuclear
targets relative to deuterium as a function of the photon energy nu, the hadron
energy fraction z_h=E_h/nu and the transverse momentum p_T. We also confront
our model results on double-hadron attenuation with recent experimental data.
Separately, we compare the attenuation of identified hadrons (pi^\pm, \pi^0,
K^\pm, p and pbar) on Ne and Kr targets with the data from the HERMES
Collaboration and make predictions for a Xe target. At the end we turn towards
hadron attenuation on Cu nuclei at EMC energies. Our studies demonstrate that
(pre-)hadronic final-state interactions play a dominant role in the kinematic
regime of the HERMES experiment while our present approach overestimates the
attenuation at EMC energies.Comment: 61 pages, 19 figures, version accepted for publication in Phys. Rev.
In-Medium Properties of Hadrons - Observables II
In this review we discuss the observable consequences of in-medium changes of
hadronic properties in reactions with elementary probes, and in particular
photons, on nuclei. After an outline of the theoretical method used we focus on
a discussion of actual observables in photonuclear reactions; we discuss in
detail - and vector-meson production. We show that the
photoproduction data can be well described by final state interactions of the
pions produced whereas the semi-charged channel exhibits a major
discrepancy with theory. For production on nuclei in the TAPS/CB@ELSA
experiment we analyse the decay channel, and illustrate the
strength of the method by simulating experimental acceptance problems.
Completely free of final state interactions is dilepton production in the few
GeV range. We show that the sensitivity of this decay channel to changes of
hadronic properties in medium in photonuclear reactions on nuclei is as large
as in ultrarelativistic heavy ion collisions and make predictions for the
on-going G7 experiment at JLAB. Finally we discuss that hadron production in
nuclei at 10 -- 20 GeV photon energies can give important information on the
hadronization process, and in particular on the time-scales involved. We show
here detailed calculations for the low-energy (12 GeV) run at HERMES and
predictions for planned experiments at JLAB.Comment: Invited Talk by U. Mosel, Proceedings of the Int. School on Nuclear
Physics, 26th Course, "Lepton scattering and the structure of hadrons and
nuclei", Erice (Sicily), September 16th-24th, 2004, short piece of text adde
Modification of the -Meson Lifetime in Nuclear Matter
The photo production of mesons on the nuclei C, Ca, Nb and Pb has
been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon
facility in Bonn. The dependence of the meson cross section on the
nuclear mass number has been compared with three different types of models, a
Glauber analysis, a BUU analysis of the Giessen theory group and a calculation
by the Valencia theory group. In all three cases, the inelastic width
is found to be at normal nuclear matter density for an
average 3-momentum of 1.1 GeV/c. In the restframe of the meson, this
inelastic width corresponds to a reduction of the lifetime by
a factor . For the first time, the momentum dependent N
cross section has been extracted from the experiment and is in the range of 70
mb.Comment: 5 pages, 4 figure
Strangeness production in proton-proton and proton-nucleus collisions
In these lectures we discuss the investigation of the strange meson
production in proton-proton () and in proton-nucleus () reactions
within an effective Lagrangian model. The kaon production proceeds mainly via
the excitations of (1650), (1710), and (1720) resonant
intermediate nucleonic states, in the collision of two initial state nucleons.
Therefore, the strangeness production is expected to provide information about
the resonances lying at higher excitation energies. For beam energies very
close to the kaon production threshold the hyperon-proton final state
interaction effects are quite important. Thus, these studies provide a check on
the models of hyperon-nucleon interactions. The in-medium production of kaons
show strong sensitivity to the self energies of the intermediate mesons.Comment: 16 pages, 9 figures, Talk presented in the workshop on Hadron
Physics, Puri, India, March 7-17,200
A relativistic two-nucleon model for reaction
We investigate the reaction within a covariant two
nucleon model. We focus on amplitudes which are described by creation,
propagation and decay into relevant channel of (1650), (1710), and
(1720) intermediate baryonic resonance states in the initial collision of
the projectile nucleon with one of its target counterparts. This collision is
modeled by the exchange of as well as and mesons. The
bound state nucleon and hyperon wave functions are obtained by solving the
Dirac equation with appropriate scalar and vector potentials. Expressions for
the reaction amplitudes are derived taking continuum particle wave functions in
both distorted wave and plane wave approximations. Detailed numerical results
are presented in the plane wave approximation; estimates of the effects of the
initial and final state interactions are given in the eikonal approximation.
Cross sections of 1 - 2 nb/sr at the peak positions, are obtained for favored
transitions in case of heavier targets.Comment: 36 pages including 14 figures, revised version, more discussions
added, to appear in Nucl. Phys.
Photoproduction of mesons on nuclei near the production threshold
The photoproduction of mesons on LH_2, C and Nb has been studied for
incident photon energies from 900 to 1300 MeV using the CB/TAPS detector at
ELSA. The line shape does not show any significant difference between
the LH_2 and the Nb targets. The data are compared with results of transport
calculations that predict a sensitivity of the line shape to in-medium
modifications near the production threshold on a free nucleon of
= 1109 MeV.}Comment: 5 pages, 3 figure