Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Thursday
Plenary Talk HK 36.2 Thu 11:45 Plenarsaal
Search for Missing Baryon Resonances — •Ulrike Thoma —
Thomas Jefferson National Laboratory, 12000 Jefferson Avenue, Newport
News, VA 23606, USA
It is widely accepted that QCD is most probably the correct theory of
strong interactions. But a major goal of QCD is still unfulfilled: to provide
the theory of quark confinement. Instead, constituent quark models
have been developed which describe the baryon spectrum with good success.
However there is an interesting controversy in baryon spectroscopy.
Constituent quark model calculations predict much more resonances than
have been observed so far. Two very different explanations have been
proposed:
1) The ”missing” states are not missing. They have not been observed
so far because of lack of high quality data in channels different from πN.
If these states decouple from πN they would not have been observed so
far.
2) The ”missing” states are not missing, they do not exist. The
nucleon-resonances could have a quark-diquark structure. This reduces
the number of internal degrees of freedom and therefore the number of
existing states.
Photoproduction experiments investigating channels different from πN
are expected to have a big discovery potential if these states really exist.
This is one of the goals of the CB-ELSA experiment in Bonn and of the
CLASexperiment at Jefferson Laboratory. Recent progress in the search
for these ”missing” states will be discussed.
HK37 Theory V
Plenary Talk HK 36.3 Thu 12:15 Plenarsaal
THE ROLE OF NUCLEAR PHYSICS IN PROVIDING DATA
FOR ASTROPHYSICS — •Stephane Goriely —IAA-Universite
Libre de Bruxelles, CP 226, Campus de la Plaine, B-1050 Brussels
Impressive progress has been made for the last decades in the various
fields related to nuclear astrophysics. However, major problems and
puzzles remain, which challenges continuously the nuclear astrophysics
concepts and findings. To put them on a safer footing requires a deeper
and more precise understanding of the many nuclear physics processes
operating in the astrophysical environment.
More specifically, major difficulties related to the specific conditions of
the astrophysical plasma remain (capture of charged particles at low energies,
large number of nuclei and properties to consider, exotic species,
high-temperature and/or high-density environments, ...). In many astrophysical
scenarios, only theoretical predictions can fill the gaps. The
nuclear ingredients to the reaction or weak interaction models should
preferentially be estimated from microscopic global predictions based on
sound and reliable nuclear models which, in turn, can compete with more
phenomenological highly-parametrized models in the reproduction of experimental
data. The latest developments made in deriving the nuclear
inputs of relevance in astrophysics applications are reviewed. Emphasis
is made on the possibility to make use of reliable microscopic models for
practical applications.
Time: Thursday 14:00–15:30 Room: A
Group Report HK 37.1 Thu 14:00 A
Dynamical Correlations in In-Medium Hyperon and Nucleon
Interactions — •Ch. Keil, H. Lenske, andC. Greiner — Institut
für Theoretische Physik, Universität Gießen, Germany
The extension of isospin nuclear structure physics into the full SU(3)f
flavor sector is investigated in a field-theoretical approach using Dirac-
Brueckner theory and the DDRH field theory for finite nuclei. Baryonbaryon
interactions in free space and hadronic matter are calculated in a
SU(3)f scheme by solving the K-Matrix equations for the lowest meson
nonets and baryon octets. Results for BB interactions in infinite hadronic
matter are presented. The structure of correlated two-baryon wave functions
in nuclear matter is discussed and applications in HBT-analyses of
hyperon production in heavy ion collisions are indicated. DDRH theory is
used to extract density dependent meson-baryon vertices, allowing applications
to finite nuclei in a covariant and thermodynamically consistent
approach. RMF calculations for single Λ hypernuclei are in good agreement
with observed hyperon separation energies and spin-orbit splittings.
The ΛΛ correlation energy, however, derived experimentally from recent
measurements of 6 ΛΛHe is underestimated, indicating that correlation dynamics
are likely to play an important role for in-medium hyperon interactions.
Taking into account loop diagrams extensions of the theory
beyond the ladder-approximation are envisaged. As a first application
we determine from a re-analysis of the Urbana nuclear equation of state
the content of RPA loop diagrams. They are found to introduce a density
dependence, shifting the equilibrium point to the empirical position.
Work supported by BMBF.
HK 37.2 Thu 14:30 A
Medium Effects in Ae, e ′ p Reactions at High Q 2 — •Dimitri Debruyne
and Jan Ryckebusch — INW, proeftuinstraat 86, B-9000
Gent, Belgium
Medium dependencies of bound nucleons are studied in a fully relativistic
and unfactorized framework for the description of exclusive A(e,e’p)
processes. The theoretical framework, which is based on the eikonal approximation,
can accommodate both optical-potential and Glauber approaches
for the treatment of final-state interactions. We have performed
calculations for the target nuclei He4, C12 and O16 in kinematic situations
corresponding with Q 2 values in the range 0.5 ≤Q 2 ≤ 10 (GeV/c) 2 .
One of the major findings of our investigations is that in kinematic
regions where both the optical-potential and the Glauber approach seem
justified, both methods for treating final-state interactions produce comparable
results. We have not found any evidence for the onset of the
color transparency phenomenon below Q 2 ≤ 8(GeV/c) 2 .
Another issue which has received our attention is the predicted medium
modification of the electromagnetic form factors for bound nucleons. By
incorporating model predictions for the medium dependence of the electromagnetic
form factors in our theoretical framework, we can estimate
the effect on the (�e, e ′ �p) observables as a function of the nuclear density
and Q 2 .
HK 37.3 Thu 14:45 A
Strangeness photoproduction on the nucleon in the resonance
region — •Stijn Janssen and Jan Ryckebusch — Proeftuinstraat
86, 9000 Gent, Belgium
A study of three strangeness photoproduction processes on the proton
(γp → K + Λ, γp → K + Σ 0 and γp → K 0 Σ + )withinaneffectiveLagrangian
formalism is presented [1,2]. By comparing model calculations
to the SAPHIR data, we explore the contributions from different N ∗ and
∆ ∗ resonances in the reaction mechanism. Special attention is paid to
the issue of the “missing resonances”. Some of those missing nucleon
states are expected to be revealed in these strange channels. In addition,
we survey the sensitivity of the extracted resonance information to the
uncertainties inherent to the treatment of the background contributions.
We show that those background terms inevitably produce a dominant
part of the reaction amplitude and compare predictions obtained with
three plausible techniques of dealing with those terms. We conclude that
model dependent effects can not be neglected in the analyses at this
stage.
[1] S. Janssen, J. Ryckebusch, W. Van Nespen, D. Debruyne and
T. Van Cauteren, Eur. Phys.J. A 11, 105 (2001)
[2] S. Janssen, J. Ryckebusch, D. Debruyne and T. Van Cauteren,
Phys. Rev. C 65, 015201 (2001)
HK 37.4 Thu 15:00 A
Contribution of Single Pion Photoproduction to Spin Asymmetry
and GDH Sum Rule for the Deuteron — •Eed Darwish 1,2 ,
Hartmuth Arenhövel 1 ,andMichael Schwamb 1 — 1 Institut für
Kernphysik, J. Gutenberg-Universität, J.-J. Becher-Weg 45, D-55099
Mainz, Germany — 2 Physics Department, Faculty of Science, South Valley
University, Sohag, Egypt
The contribution of incoherent single pion photoproduction to the spin
asymmetry for the deuteron is evaluated up to 550 MeV photon energy
with inclusion of NN and πN rescattering in the final state. For the
elementary production operator γN → πN, we have taken into account
the standard pseudovector Born terms as well as the contribution of the
∆(1232) resonance [1]. For the NN and πN interactions we use the separable
representations from Haidenbauer et al. [2] and Nozawa et al. [3],
respectively.