Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Tuesday
HK15 Plenary Session
Time: Tuesday 14:00–16:15 Room: Plenarsaal
Plenary Talk HK 15.1 Tue 14:00 Plenarsaal
Recent Progress in Lattice QCD — •Uwe-Jens Wiese — Institut
fuer Theoretische Physik, Universitaet Bern, Sidlerstrasse 5, CH-3012
Bern
Lattice QCD provides a framework in which one can hope to understand
the strong interactions from first principles. In this approach quark
and gluon fields are regularized nonperturbatively on a Euclidean spacetime
lattice with spacing a. In his original formulation, Wilson avoided
the fermion doubling problem at the cost of breaking chiral symmetry
explicitly at non-zero a. Recently, the fermion doubling problem has
been solved completely. In the resulting lattice fermion formulation chiral
symmetry remains exact already at non-zero a. The lattice QCD path
integral takes the form of a 4-d statistical mechanics system that can be
simulated with Monte Carlo methods. In practice, lattice QCD simulations
must reach the infinite volume limit, the continuum limit of zero
a, as well as the chiral limit of light quarks. Recent progress has been
made in all these directions. Still, the simulation of dynamical quarks remains
the bottleneck of lattice QCD calculations. D-Theory provides an
alternative formulation of lattice QCD in which Wilson’s classical link
matrices are replaced by discrete quantum links to which the efficient
meron-cluster algorithm may be applicable.
Plenary Talk HK 15.2 Tue 14:45 Plenarsaal
Chiral Extrapolation of Lattice QCD data for Baryon Properties
— •Thomas R. Hemmert — Physik Department T39, TU
München
QCD should predict basic baryon properties like masses, magnetic moments,
form factors, etc. However, these quantities are not accessible in
terms of quark-gluon perturbation theory. In this talk we focus on two
techniques—Lattice simulations of QCD and Chiral Perturbation Theory
(ChPT)—which currently begin to develop significant overlap in addressing
such questions. Baryon ChPT has been successfully applied to study
the influence of the spontaneously broken chiral symmetry on low energy
processes involving pions and nucleons. In the present context we are interested
in the explicit breaking of chiral symmetry by the finite (current-)
quark masses in low energy baryon observables. Such observables can be
calculated in computer simulations on a finite space-time grid—Lattice
QCD. However, these simulations are usually not performed with realistic
small quark masses as the simulation of full QCD with three light
flavors u, d, s is technically quite challenging. For reasons of numerical
stability it is standard practice in Lattice QCD to work with sets of quite
heavy quark masses and then extrapolate the results to physical quark
masses. We will show that ChPT via its built-in explicit breaking of chiral
symmetry can provide guidance for this extrapolation procedure that
goes beyond the traditionally used linear ansatz. Furthermore, we discuss
extensions of standard baryon ChPT to so called (partially-) quenched
baryon ChPT that allow to estimate the size of artificial modifications of
HK16 Theory II
baryon properties due the use of the “quenching” simplification.
Plenary Talk HK 15.3 Tue 15:15 Plenarsaal
Electric dipole strength in atomic nuclei – a key to the breaking
of isospin symmetry — •Andreas Zilges — Institut für Kernphysik,
TU Darmstadt, Schlossgartenstrasse 9, D-64289 Darmstadt, Germany
A global or local breaking of the symmetry of proton and neutron distributions
in nuclei leads to electric dipole excitations. The most prominent
feature is the Giant Dipole Resonance (GDR) at energies of about
15 MeV. At the other end of the energetic scale a bound two phonon
octupole–quadrupole excitation has been established as a fundamental
E1 mode around 4 MeV in all medium and heavy mass nuclei.
In recent photon scattering experiments at the Superconducting Darmstadt
Linear Accelerator S–DALINAC we investigated several nuclei near
shell closures in the energy range between the two phonon state and the
neutron threshold. Collective electric dipole strength exhausting up to
one percent of the energy weighted sum rule has been observed. These
excitations are clearly separated from the GDR. Possible interpretations
in terms of a local breaking of isospin symmetry and the influence of a
neutron skin will be discussed.
∗ supported by the DFG (contracts Zi 510/2-1 and FOR 272/2-2).
Plenary Talk HK 15.4 Tue 15:45 Plenarsaal
Nuclear Physics with a Free Electron LASER $ — •N. Pietralla
— Institut für Kernphysik, Universität zu Köln — WNSL, Yale University,
U.S.A.
The electron storage ring at the Duke Free Electron LASER Laboratory
operates at electron energies between 0.2 and 1.1 GeV. Its beam
drives the OK-4 Free Electron LASER with tunable wavelengths in the
optical range. The high photon density inside of the optical cavity enables
one to obtain a large luminosity for Compton scattering processes
between the polarized optical LASER photons and the relativistic electrons
in the ring. Compton scattered photons experience a forwardpeaked
Lorentz-boost by a factor of 106 –108 by transformation to the lab
system and they form after collimation a nearly monochromatic, tunable,
completely polarized γ-ray beam with an intensity of up to 109 γ’s/sec.
This “High Intensity γ-ray Source” (HIγS), with a degree of linear polarization
of Pγ > 99% and a narrow band width of ∆Eγ/Eγ < 4%,
offers new conditions for experiments with photo-nuclear reactions, e.g.,
for the photo-disintegration of the deuteron or for Nuclear Resonance
Fluorescence (NRF) close to the particle emission threshold. First NRF
experiments have recently been performed [1] at HIγS. Parity quantum
numbers of J = 1 states of 138Ba and 88Sr have been measured. The results
demonstrate the experimental progress made by the new technique.
[1] N.Pietralla et al., Phys. Rev. Lett. 88 (2002), in press.
$
Supported by the U.S.-DOE and by the Emmy Noether-Programm of the DFG
under contract Pi 393/1.
Time: Tuesday 16:45–18:45 Room: A
Group Report HK 16.1 Tue 16:45 A
Glueballs and Instantons — •Hilmar Forkel — Institut für Theoretische
Physik, Uni Heidelberg, Philosophenweg 19, 69120 Heidelberg
The impact of QCD instantons on scalar glueball properties is studied
in the framework of an instanton-improved operator product expansion
(IOPE) for the 0 ++ glueball correlation function. Direct instanton contributions
are found to strongly dominate over those from perturbative fluctuations
and soft vacuum fields. All IOPE sum rules, including the one
involving a subtraction constant, show a high degree of stability and are,
in contrast to previous glueball sum rules, consistent with the low-energy
theorem for the zero-momentum correlator. The predicted glueball mass
mG =1.53 ± 0.2 GeV is less sensitive to the instanton contributions then
the glueball coupling (residue) fG =1.01 ± 0.25 GeV, which increases by
about half an order of magnitude. Both glueball properties are shown
to obey scaling relations as a function of the average instanton size and
density.
HK 16.2 Tue 17:15 A
Scale setting with Hypercubic Blocking — •Roland Hoffmann
— Institut für Theoretische Physik, Universität Regensburg, Universitätsstrasse
31, D-93053 Regensburg
We measure the static potential from Wilson loops constructed using
hypercubic blocked (HYP) links. The HYP smearing mixes gauge
links within hypercubes attached to the original link only. The HYP
potential agrees with the potential measured using thin links for distances
r/a ≥ 2. We calculated the lowest order perturbative expansion
of the lattice Coulomb potential of HYP links. These results are used
in analyzing the static potential with Wilson’s action as well as with the
Lüscher-Weisz action. The statistical accuracy of the potential with HYP
links improves by about an order of magnitude, which makes it possible
to determine a reliable scale even with limited statistics.