70 research outputs found
Photoproduction of pi0 omega off protons for E(gamma) < 3 GeV
Differential and total cross-sections for photoproduction of gamma proton to
proton pi0 omega and gamma proton to Delta+ omega were determined from
measurements of the CB-ELSA experiment, performed at the electron accelerator
ELSA in Bonn. The measurements covered the photon energy range from the
production threshold up to 3GeV.Comment: 8 pages, 13 figure
In-medium mass from the reaction
Data on the photoproduction of mesons on nuclei have been
re-analyzed in a search for in-medium modifications. The data were taken with
the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in
Bonn. First results from the analysis of the data set were published by D.
Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a
lowering of the mass in the nuclear medium by 14 at normal nuclear
matter density. The extracted line shape was found to be sensitive to
the background subtraction. For this reason a re-analysis of the same data set
has been initiated and a new method has been developed to reduce the background
and to determine the shape and absolute magnitude of the background directly
from the data. Details of the re-analysis and of the background determination
are described. The signal on the target, extracted in the
re-analysis, does not show a deviation from the corresponding line shape on a
target, measured as reference. The earlier claim of an in-medium mass
shift is thus not confirmed. The sensitivity of the line shape to
different in-medium modification scenarios is discussed.Comment: 13 pages and 11 figures, submitted for publicatio
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
K^0 pi^0 Sigma^+ and K^*0 Sigma^+ photoproduction off the proton
The exclusive reactions and , leading to the p 4 final state, have
been measured with a tagged photon beam for incident energies from threshold up
to 2.5 GeV. The experiment has been performed at the tagged photon facility of
the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were
combined to a photon detector system of almost 4 geometrical acceptance.
Differential and total cross sections are reported. At energies close to the
threshold, a flat angular distribution has been observed for the reaction
suggesting dominant s-channel production.
and higher lying hyperon states have been observed. An
enhancement in the forward direction in the angular distributions of the
reaction indicates a -channel exchange
contribution to the reaction mechanism. The experimental data are in reasonable
agreement with recent theoretical predictions.Comment: 11 pages, 13 figures, submitted to EPJ
Photoproduction of mesons off nuclei
Recent results for the photoproduction of mesons off nuclei are reviewed.
These experiments have been performed for two major lines of research related
to the properties of the strong interaction. The investigation of nucleon
resonances requires light nuclei as targets for the extraction of the isospin
composition of the electromagnetic excitations. This is done with quasi-free
meson photoproduction off the bound neutron and supplemented with the
measurement of coherent photoproduction reactions, serving as spin and/or
isospin filters. Furthermore, photoproduction from light and heavy nuclei is a
very efficient tool for the study of the interactions of mesons with nuclear
matter and the in-medium properties of hadrons. Experiments are currently
rapidly developing due to the combination of high quality tagged (and
polarized) photon beams with state-of-the-art 4pi detectors and polarized
targets
Study of the reaction
The reaction has been studied with the CBELSA
detector at the tagged photon beam of the Bonn electron stretcher facility. The
reaction shows contributions from , and
as intermediate states. A partial wave analysis suggests that the
reaction proceeds via formation of six resonances,
, , ,
, , , and two
nucleon resonances and , for which pole
positions and decay branching ratios are given.Comment: 13 pages, 10 figures, 7 table
Neutral pion photoproduction off protons in the energy range 0.3 GeV < E(gamma) < 3 GeV
Single pi0 photoproduction has been studied with the CB-ELSA experiment at
Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental
setup covers a very large solid angle of about 98% of 4 pi. Differential cross
sections (d sigma)/(d Omega) have been measured. Complicated structures in the
angular distributions indicate a variety of different resonances being produced
in the s channel intermediate state gamma p --> N* (Delta*) --> p pi0. A
combined analysis including the data presented in this letter along with other
data sets reveals contributions from known resonances and evidence for a new
resonance N(2070)D15.Comment: LaTeX file, 4 pages, 4 encapsulated postscript figures, submitted to
Phys. Rev. Lett. The publication of hep-ex/0407022 is accompanied by
hep-ex/0311045 on photoproduction of eta mesons. Reference [3]: changed,
reference [17]: citation added. Figure 3, 4: SAID added up to 2 GeV for
comparison, update
Photoproduction of eta mesons off protons for photon energies from 0.75 GeV to 3 GeV
Total and differential cross sections for the reaction p(gamma, eta)p have
been measured for photon energies in the range from 750 MeV to 3 GeV. The
low-energy data are dominated by the S11 wave which has two poles in the energy
region below 2 GeV. Eleven nucleon resonances are observed in their decay into
p eta. At medium energies we find evidence for a new resonance N(2070)D15 with
(mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a
strong peak in forward direction develops, signalling the exchange of vector
mesons in the t channel.Comment: LaTeX, 4 pages including 4 eps-figures, submitted to Phys. Rev. Lett.
The publication of hep-ex/0311045 is accompanied by hep-ex/0407022 on
photoproduction of neutral pions, submitted to Phys. Rev. Lett. Fits
published in the latest version are based on additional data, new beam
asymmetry data from GRAAL are included, for instance. The data demanded more
resonant contributions which were studied in detail. PWA reference adde
First observation of in-medium modifications of the omega meson
The photoproduction of omega mesons on nuclei has been investigated using the
Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The
aim is to study possible in-medium modifications of the omega meson via the
reaction A(gamma, omega)X. Results obtained for Nb are compared to a reference
measurement on a liquid hydrogen target. While for recoiling, long-lived mesons
(pi, eta and etaprime), which decay outside of the nucleus, a difference in the
lineshape for the two data samples is not observed, we find a significant
enhancement towards lower masses for omega mesons with low momenta produced on
the Nb target.Comment: Latex, 4 pages, 4 figures, accepted by Phys. Rev. Lett; references
re-arranged, now references in chronological order in first paragrap
N* and Delta* decays into N pi0 pi0
Decays of baryon resonances in the second and the third resonance region into
N pi0 pi0 are studied by photoproduction of two neutral pions off protons.
Partial decay widths of N* and Delta* resonances decaying into Delta(1232) pi,
N(\pi\pi)_{S}, N(1440)P_{11} pi, and N(1520)D_{13} pi are determined in a
partial wave analysis of this data, and data from other reactions. Several
partial decay widths were not known before. Interesting decay patterns are
observed which are not even qualitatively reproduced by quark model
calculations. In the second resonance region, decays into Delta(1232) pi
dominate clearly. The N(\pi\pi)_{S}-wave provides a significant contribution to
the cross section, especially in the third resonance region. The P_{13}(1720)
properties found here are at clear variance to PDG values.Comment: 13 pages, 4 figures, long author's lis
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