312 research outputs found
In-Medium Properties of Hadrons
A diverse experimental program for the study of the photoproduction of mesons
off nuclei has been carried out - and is still running - at the Mainz MAMI and
Bonn ELSA electron accelerators with the TAPS, Crystal Barrel, and Crystal Ball
calorimeters. It is motivated as a detailed study of the in-medium properties
of hadrons and the meson - nucleus interactions. Typical examples for the
in-medium behavior of vector mesons (), scalar mesons (), and
nucleon resonances (P(1232), S(1535), D(1520)) are
discussed. Special attention is paid to meson - nucleus final state
interactions.Comment: Proceedings of the Meson2006 IX Int. Workshop on Production,
Properties and Interaction of Mesons, Cracow, Poland 09 - 14 June, 2006, to
be published in Int. J. Mod. Phys.
Investigation of the anomaly in eta-photoproduction off the neutron
Quasi-free photoproduction of eta-mesons off the neutron and off the proton
has been studied using a deuterium target and bremsstrahlung photons produced
by MAMI-C with incident energies up to 1.5 GeV. The eta-mesons were detected in
coincidence with the recoil nucleons thus a fully exclusive measurement was
performed. Preliminary results show a bump-like structure in the excitation
function for the neutron close to W 1675 MeV which is not seen for
the proton. Considering the experimental resolution and using a Breit-Wigner
fit the width of this structure was approximated below 50 MeV.Comment: oral contribution to the workshop on physics of the excited nucleon -
NSTAR 2009, Beijing, April 200
Studying the properties in pA collisions via the decay
Within transport calculations we study the production and decay of
-mesons in reactions at COSY energies including elastic and
inelastic rescattering, the Dalitz decay
as well as rescattering. The resulting invariant mass
distributions indicate that in-medium modifications of the -meson may
be observed experimentally.Comment: 5 pages, espcrc2-style, including 5 ps-figure
Medium effects in the production and pi0 gamma decay of omega-mesons in pA collisions in the GeV region
The omega-resonance production and its pi0 gamma decay in p-A reactions close
to threshold is considered within the Intranuclear Cascade (INC) Model. The pi0
gamma invariant mass distribution shows two components which correspond to the
omega decay 'inside' and 'outside' the nucleus, respectively. The 'inside'
component is distorted by medium effects, which introduce a mass shift as well
as collisional broadening for the omega-meson and its decaying pion. The
relative contribution of the 'inside' component is analyzed in detail for
different kinematical conditions and nuclear targets. It is demonstrated that a
measurement of the correlation in azimuthal angle between the pi0 and gamma
momenta allows to separate events related to the 'inside' omega decay from
different sources of background when uncorrelated pi0's and gamma's are
produced.Comment: 24 pages, 11 figure
Transition form factors of the N*(1535) as a dynamically generated resonance
We discuss how electromagnetic properties provide useful tests of the nature
of resonances, and we study these properties for the N*(1535) which appears
dynamically generated from the strong interaction of mesons and baryons. Within
this coupled channel chiral unitary approach, we evaluate the A_1/2 and S_1/2
helicity amplitudes as a function of Q^2 for the electromagnetic N*(1535) to
gamma* N transition. Within the same formalism we evaluate the cross section
for the reactions gamma N to eta N. We find a fair agreement for the absolute
values of the transition amplitudes, as well as for the Q^2 dependence of the
amplitudes, within theoretical and experimental uncertainties discussed in the
paper. The ratios obtained between the S_1/2 and A_1/2 for the neutron or
proton states of the N*(1535) are in qualitative agreement with experiment and
there is agreement on the signs. The same occurs for the ratio of cross
sections for the eta photoproduction on neutron and proton targets in the
vicinity of the N*(1535) energy. The global results support the idea of this
resonance as being dynamically generated, hence, largely built up from meson
baryon components. However, the details of the model indicate that an admixture
with a genuine quark state is also demanded that could help obtain a better
agreement with experimental data.Comment: 19 pages, 16 figures, REVTe
Spectator detection for the measurement of proton neutron interactions at ANKE
A telescope of three silicon detectors has been installed close to the
internal target position of the ANKE spectrometer, which is situated inside the
ultra-high vacuum of the COSY-Juelich light-ion storage ring. The detection and
identification of slow protons and deuterons emerging from a deuterium
cluster-jet target thus becomes feasible. A good measurement of the energy and
angle of such a spectator proton (p_sp) allows one to identify a reaction as
having taken place on the neutron in the target and then to determine the
kinematical variables of the ion-neutron system on an event-by-event basis over
a range of c.m. energies.
The system has been successfully tested under laboratory conditions. By
measuring the spectator proton in the p d to p_sp d pi^0 reaction in
coincidence with a fast deuteron in the ANKE Forward Detector, values of the p
n to d pi^0 total cross-section have been deduced. Further applications of the
telescope include the determination of the luminosity and beam polarisation
which are required for several experiments.Comment: 16 pages, 9 figure
Exclusive measurement of coherent eta photoproduction from the deuteron
Coherent photoproduction of eta mesons from the deuteron has been measured
from threshold up to incident photon energies of 750 MeV using the photon
spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI.
For the first time, differential coherent cross sections have been deduced from
the coincident detection of the eta meson and the recoil deuteron. A missing
energy analysis was used for the suppression of background events so that a
very clean identification of coherent eta-photoproduction was achieved. The
resulting cross sections agree with previous experimental results except for
angles around 90 deg in the photon-deuteron cm-system where they are smaller.
They are compared to various model calculations.Comment: 4 pages, 4 figure
Near threshold eta meson production in the d+d->alpha+eta reaction
The d+d->alpha+eta reaction has been investigated near threshold using the
ANKE facility at COSY-Juelich. Both total and differential cross sections have
been measured at two excess energies, Q=2.6 MeV and 7.7 MeV, with a
subthreshold measurement being undertaken at Q=-2.6 MeV to study the physical
background. While consistent with isotropy at the lower energy, the angular
distribution reveals a pronounced anisotropy at the higher one, indicating the
presence of higher partial waves. Options for the decomposition into partial
amplitudes and their consequences for determination of the s-wave eta-alpha
scattering length are discussed.Comment: 8pp, fig.3 added, normalisation in eq.4.1 correcte
Pluto: A Monte Carlo Simulation Tool for Hadronic Physics
Pluto is a Monte-Carlo event generator designed for hadronic interactions
from Pion production threshold to intermediate energies of a few GeV per
nucleon, as well as for studies of heavy ion reactions. This report gives an
overview of the design of the package, the included models and the user
interface.Comment: XI International Workshop on Advanced Computing and Analysis
Techniques in Physics Research, April 23-27 2007, Amsterdam, the Netherland
Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei
The ratio of forward K+ production on copper, silver and gold targets to that
on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as
a function of the kaon momentum p_K using the ANKE spectrometer at
COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250
MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the
K+A system. This opens a new way to investigate the interaction of K+-mesons in
the nuclear medium. Our data are consistent with a K+A nuclear potential of
V_K~20 MeV at low kaon momenta and normal nuclear density. Given the
sensitivity of the data to the kaon potential, the current experimental
precision might allow one to determine V_K to better than 3 MeV.Comment: 9 pages, 3 figures; changed conten
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