Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Thursday
HK46 Electromagnetic and Hadronic Probes VI
Time: Thursday 16:00–18:00 Room: D
Group Report HK 46.1 Thu 16:00 D
Exclusive Measurements of the Two-Pion Production
in Proton-Proton Collis ions ∗ — •J. Kress, J. Pätzold, H.
Clement, E. Dorochkevitch, A. Erhardt, G.J. Wagner, andU.
Weidlich for the COSY-TOF collaboration and the PROMICE/WASA
collaboration — Physikalisches Institut der Universität Tübingen
After installation and commissioning of the central calorimeter at
COSY-TOF runs at Tp = 750 MeV and 800 MeV have been carried out
with polarized beam to study the reaction �pp → ppπ + π − over a large
solid angle. Deuterons, protons and pions emerging from the pp collision
in the LH2 target are identified by the ∆E − E method, π + particles in
addition by the recently installed delayed pulse technique. The status
of the data analysis of these runs is presented. Data taken previously
with the PROMICE/WASA detector at CELSIUS in the energy range
Tp = 650-775 MeV have now been fully analyzed. The final results are
discussed. Besides of the exclusive data for the ppπ + π − channel also
integral cross sections for the channels ppπ 0 π 0 and pnπ + π 0 have been
obtained. The exclusive data show that the pp → ppπ + π − reaction at
these energies proceeds via σ exchange and subsequent excitation of
the Roper resonance. Its decay into Nππ is observed to be practically
exclusively into (ππ)I=l=0 pairs, either directly by N ∗ → Nσ or via
N ∗ → ∆π.
∗supported by BMBF (06 TÜ 987) and DFG (European Graduate
School)
Group Report HK 46.2 Thu 16:30 D
Measurement of hadronic cross sections with the KLOE experiment
in Frascati — •Stefan E. Müller for the KLOE collaboration
— Institut für Exp. Kernphysik Universität Karlsruhe, Postfach 3640,
76021 Karlsruhe
The KLOE experiment at the Frascati e + e − collider DAΦNE working
at the φ(1.02 GeV) resonance started data taking in 1999. Although optimized
to study neutral kaons, the KLOE detector is suited to cover a
much wider field of physics. Especially appealing is the measurement of
hadronic cross sections from 1.02 GeV down to the two pion threshold.
The precise determination of these cross sections can be used to lower the
uncertainty of the theoretical calculations for the hadronic contribution
to the muon’s anomalous magnetic moment. Since DAΦNE is working
at a fixed energy, the measurement is done by selecting events where the
e + or the e − emits a hard photon. In this way the collision energy of the
electron and the positron is lowered. The method of the measurement
is presented in detail and first results for σ(e + e − → π + π − )areshown.
In addition further recent results from the KLOE physics program are
presented.
HK 46.3 Thu 17:00 D
ω-Production on a Neutron Target at ANKE — •I. Lehmann 1
and S . Barsov 2 for the ANKE collaboration — 1 Forschungszentrum
Jülich — 2 PNPI, Gatchina
In August 2001 a test beam time to investigate the reaction: pd →
pspdω took place at ANKE/COSY. It could be shown, that the slow
spectator proton (Tkin =2.5 − 30 MeV) can be identified simultaneously
with the fast deuteron (p ≈ 2 GeV/c) in the detection systems of ANKE.
The former is detected in a near-target-silicon telescope and the latter is
identified using the momentum resolution of the magnetic spectrometer
together with energy losses in two layers of scintillators and the response
from inclined Čerenkov counters.
In the talk the methods to determine the ω-cross section from a missing
mass spectrum with large background from multi-pion and ρ-production
will be discussed and preliminary results will be presented.
HK 46.4 Thu 17:15 D
Pion-Proton-Scattering at Low Energies — •Holger Denz,
Johannes Breitschopf, Heinz Clement, Margit Cröni,
Arthur Erhardt, Rudolf Meier, Jens Pätzold, Florian von
Wrochem, andGerhard J. Wagner for the CHAOScollaboration
and the LEPScollaboration — Universität Tübingen, Physikalisches
Institut, Auf der Morgenstelle 14, 72076 Tübingen, Germany
From πp observables, important quantities of the strong interaction
can be extracted: the πNN coupling constant, the πN sigma-termand
the size of isospin symmetry violation. Presently, there is no agreement
on the value of any of these quantities. This is partly due to the unsatisfactory
status of the πp data base, in particular at low energies. New
measurements of πp observables at low energies are aimed at providing
additional information and resolving discrepancies. Elastic scattering
cross sections have been measured with the CHAOSdetector at TRI-
UMF for several pion energies down to 15 MeV. A large angular range
has been covered with special emphasis on forward angle measurements
in the Coulomb-nuclear interference region. Elastic scattering polarization
observables have been measured, both with the CHAOSdetector
and, at PSI, with the LEPS spectrometer and an active polarized target.
An experiment measuring the π − p→ π ◦ n total cross section from
30 to 250 MeV using a transmission technique has started taking data
at PSI. Status and/or results of these experiments will be discussed.
This work is supported by BMBF (06Tü987I) and DFG (Europäisches
Graduiertenkolleg Basel-Tübingen).
HK 46.5 Thu 17:30 D
Total cross section of the π − p → π 0 n charge exchange reaction
— •J. Breitschopf 1 , H. Clement 1 , M. Cröni 1 , H. Denz 1 ,
E. Friedman 2 , P. Jesinger 1 , R. Meier 1 ,andG. J. Wagner 1 —
1 Physikalisches Institut, Universität Tübingen — 2 Racah Institute of
Physics, The Hebrew University, Jerusalem, Israel
The mass difference of up- and down-quarks is the origin of isospin
violation in the strong interaction. Isospin breaking can in principle be
tested in a number of ways in the pion-nucleon system. Currently experimentally
accessible is the comparison of elastic scattering of positively
and negatively charged pions from protons with the charge exchange
reaction π − p → π 0 n (SCX). Analyses of this system, based on the existing
scattering data base, found unexpectedly large isospin breaking in
s-waves for pion energies below 100 MeV. These analyses probably are
not suffficiently restricted by the low amount of SCX data. Therefore,
further measurements of SCX cross sections and analyzing powers are
needed to test this result.
A measurement of the SCX total cross section in the energy region from
30 to 250 MeV has been started at PSI. The experiment employs a transmission
technique using a 4π box detector consisting of thin plastic scintillators.
A first beam time covered the high energy part from 60 to
250 MeV, a second beam time in 2002 will take data from 30 to 90 MeV.
The measuring procedure and the status of the experiment are discussed.
This work is supported by BMBF (06Tü987I) and DFG (Europäisches
Graduiertenkolleg Basel-Tübingen).
HK 46.6 Thu 17:45 D
Investigation of the a + 0 (980) Resonance at ANKE — •V. Kleber
for the ANKE collaboration — Institut für Kernphysik, Forschungszentrum
Jülich, 52425 Jülich
In a recent ANKE beam time at COSY Jülich a first experiment on
the production of scalar mesons in pp collisions was performed. The goal
of this experiment is to investigate the a + 0 (980) resonance, a candidate
for the scalar meson nonet, in the reaction pp → da + 0 .
The a0(980) is known to decay in KK, πη and 2γ. At ANKE the
deuteron and the decay K + or π + are detected. The mesons are identified
by TOF and energy loss, the coincident deuterons by their TOF
relative to the mesons. Subsequently the a + 0 (980) is investigated with a
missing mass analysis.
In case of a K + and a deuteron a clean sample of dK + K 0 events is obtained
due to energy and strangeness conservation. In the missing mass
distribution m(pp,d) a narrow structure is observed mainly given by the
KK production threshold (mmin=991 MeV) and the COSY beam energy
of T=2.65 GeV (mmax=1038 MeV).
In the missing mass distribution m(pp,dπ + ) a clear peak from η mesons
on a broad background is seen as well as a shoulder in the distribution
m(pp,d) at the a0(980) mass which reveals a peak structure after background
subtraction.