Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Wednesday
cess π − N → π − γN is presented which demonstrates the feasibility of the
measurement of the pion polarizabilities α and β with the COMPASS
spectrometer at the CERN SPS. Data samples with a total of 2.9 million
events, corresponding to three days of COMPASS data taking, were
generated using the Polaris event generator and have been analyzed to
study event selection algorithms and reconstruction efficiency. For the
hadronic background 4.5 million events have been simulated using the
Fritiof event generator. The results of this study and the accuracy which
can be achieved will be presented. *This project is supported by the
BMBF and the Maier-Leibnitz-Labor, Garching
HK 26.4 Wed 12:00 D
Near-threshold π 0 production from the neutron in d(γ,π 0 n)p —
•D.L. Hornidge for the A2 collaboration — University of Saskatshewan,
Saskatoon, Canada — Johannes Gutenberg–Universität, Mainz
A significant body of experimental results now exists for the proton
photo-pion amplitudes near threshold and is in good agreement with
ChPT predictions. Due to the experimental intricacies involved, there is
a clear lack of data in the threshold region for the neutron. Preliminary
attempts to determine the neutron amplitudes using the coherent production
from the deuteron were hampered by the difficulties in separating
out nuclear effects. For these reasons, a measurement of π 0 photoproduction
in the threshold region from the neutron via the d(γ,π 0 n)p reaction
channel has been performed using the Mainz Microtron (MAMI). Photons
in the energy range Eγ = 95.7 − 208.5 MeV were tagged using
the Glasgow tagger, the TAPSspectrometer was employed to detect the
neutral-pion decay photons, and a segmented liquid-scintillator detector
covering the angular range θn =21.5 ◦ ± 8.5 ◦ , φn =0.0 ◦ ± 8.5 ◦ was
used to detect recoil neutrons under quasi-free kinematical conditions.
Preliminary results will be presented.
HK 26.5 Wed 12:15 D
Pion production in p + d reactions in the resonance region
— H. Machner and •H. Machner for the GEM collaboration and
the GEM collaboration — Institut für Kernphysik, Forschungszentrum
Jülich, Jülich, Germany
Pion production is the first inelastic channel in pp interactions. The
pd → 3 Heπ 0 and pd → 3 Hπ + reactions are the key reactions for the
understanding of meson production on nuclei. We have measured com-
HK27 Heavy Ions III
plete angular distributions and total cross sections for both reactions in
the region of the ∆(1232) resonance for seven different beam momenta.
No data existed for the first reaction in this range, while previous data
for the second reaction were sometimes in disagreement. The recoiling
trinucleons were detected with the GEM detector (Ref. 1). The data
show two two components: one for small momentum transfer and the
second one for large momentum transfer. While the first one can be reproduced
by model calculations the models fail to account for the second
component. The possible reaction mechanisms will be discussed.
[1] M. Betigeri et al., Nuclear Instr. Methods in Physics Research A 421
(1999) 447
HK 26.6 Wed 12:30 D
First results from the new pionic hydrogen experiment —
•M. Hennebach1 , D. F. Anagnostopoulos2 , W. Breunlich3 , H.
Fuhrmann3 , D. Gotta1 , A. Gruber3 , P. Indelicato4 , Y.-W. Liu5 ,
B. Manil4 , V. Markushin5 , N. Nelms6 , A. J. Rusi El Hassani7 ,
L. M. Simons5 ,andH. Zmeskal3 — 1IKP, FZ Jülich — 2Dept. of
Mat. Science, Univ. of Ioannina — 3IMEP, Österr. Akademie der Wiss.,
Wien — 4 Lab. Kastler-Brossel, UPMC Paris — 5 PSI, Villigen — 6 Univ.
of Leicester — 7 Dept. de Phys., Tanger, Morocco
A new high–precision measurement of the strong–interaction shift (ɛ1s)
and broadening (Γ1s) of the ground state in pionic hydrogen (πH) has
been started at PSI. This experiment constitutes a direct measurement
of the πN scattering lengths and is an important test of the methods of
chiral perturbation theory (χPT).
Pions from the πE5 beam at PSI are slowed down into a cryogenic gas
target installed inside the cyclotron trap to form pionic atoms. Resultant
x–rays are reflected onto a large area CCD array by a Bragg spectrometer.
ɛ1s is derived by comparing the measured energy with a pure QED
calculation; Γ1s is obtained after deconvolution of the crystal response
function. Collision processes during the cascade of the pion through the
atomic levels could skew the measured values - molecular formations of
the form [(π − pp)p]ee could shift the measured energy, and Coulomb deexcitation
will increase the width of the line. The possible influence of these
collision processes is investigated by varying the target density. During
the first production run, conducted in spring 2001, measurements ranged
from 3 bar to liquid hydrogen (eqv. pressure ∼700 bar).
Time: Wednesday 10:45–12:45 Room: E
Group Report HK 27.1 Wed 10:45 E
Particle flow at RHIC — •Christian Fuchs, Amand Faessler,
Eugene Zabrodin, andLarissa Bravina — Institut für Theoretische
Physik, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen,
Germany
We investigate directed (v1) and elliptic flow (v2) of hadrons in heavy
ion collisions at RHIC energies, i.e. at √ s = 130 and 200 AGeV. The
microscopic quark-gluon string model (QGSM) is used. Available data
on the elliptic flow from the STAR Collaboration [1] are well described
by this approach, in particular the pt dependence of v2 is reproduced also
at high pt where hydrodynamical models usually fail [2]. The origin of
the elliptic flow within the QGSM model is discussed and predictions for
v1 are given. The success of the QGSM model for the description of the
elliptic flow is thereby due to the variety of string excitations included
in that model. We further present predictions for the flow of negatively
and positively charged kaons at SPS and RHIC. Here the change in the
flow pattern reflects in a clear way the transition from baryon to meson
dominated matter when going from SPS to RHIC energies.
[1] K.H. Ackermann et al., STAR Collab., Phys. Rev. Lett. 86 (2001)
402
[2] E. Zabrodin, C. Fuchs, L. Bravina, A. Faessler, Phys. Lett. B508
(2001) 184
Group Report HK 27.2 Wed 11:15 E
Charmonium Evolution in a Hot and Dense Environment —
•Alberto Polleri 1 , Thorsten Renk 1 , Roland A. Schneider 1 ,
and Wolfram Weise 1,2 — 1 Physik Department, TU München, D-
85747 Garching,Germany — 2 ECT*, I-38050 Villazzano (TN), Italy
It has been suggested long ago that the measurement of the J/ψ production
cross section in ultra-relativistic nuclear collisions can provide
an important signal of quark-gluon deconfinement. Intense theoretical
work has been performed in order to understand the production process
in proton-nucleus collisions. This piece of information is used to provide
a baseline to the study of the subsequent evolution of J/ψ and,
more generally, of charmonia, in the hot and dense medium produced in
more complex high-energy nucleus-nucleus collisions. With this input,
we study the subsequent interactions of charmonia as they collide with
the constituents of the produced fireball. The latter evolves in a manner
controlled by the equation of state as given by lattice QCD, and is constructed
in such a way that the observed hadronic spectra are correctly
reproduced. A kinetic description of charmonium interactions with both
quark-gluon and hadronic degrees of freedom allows to study in microscopic
detail the evolution in different regimes, controlled by collision
energy, kinematics (rapidity and pT ) and geometry (centrality). While
the amount of data collected at the CERN-SPS accelerator is well described,
new predictions for the presently running BNL-RHIC machine
are presented.
[*] Work supported in part by BMBF and GSI.
Group Report HK 27.3 Wed 11:45 E
Chemical Freeze-out of Antihyperons in Relativistic Heavy Ion
Collisions — •Carsten Greiner — Institut fuer Theoretische Physik,
Universitaet Giessen
We elaborate on our recent suggestion on antihyperon production in
relativistic heavy ion collisions solely by means of multi-mesonic (fusiontype)
reactions. It will be shown that the antihyperons are driven towards
chemical equilibrium with pions, nucleons and kaons on a timescale of
1–3 fm/c in a still moderately baryon-dense hadronic environment. Explicit
rate calculations for a dynamical setup will be presented and detail
the proposed picture. For an estimated entropy to baryon ratio of