202 research outputs found
Eta and eta-prime meson production in the reaction pn -> dM near threshold
The two-step model with pi-, rho- and omega- exchanges taken into account is
applied to investigate the reactions pn -> d eta and pn -> d eta-prime. The
existing experimental data on the reaction pn -> d eta are analysed and
predictions for the cross section of the reaction pn -> d eta-prime are
presented. It is found that pi- as well as rho exchange yield significant
contributions in both reactions. The effect of the final state interaction is
also studied.Comment: 15 pages, 4 figure
Theoretical Description of the pd -> pd eta Reaction Near Threshold
The contributions of three different types of driving terms are included in
the estimation of the pd -> pd eta reaction at low energies. Near threshold, it
is predicted that a two-step model involving an intermediate pion should be the
most important but, as the energy approaches the threshold for eta production
in the free nucleon--nucleon reaction, a pick-up mechanism with a spectator
proton should become dominant. The total cross sections are underestimated by
about a factor of two compared to experimental data but the discrepancies in
the angular distributions are more serious, with no sign in the data for the
peaks corresponding to the pick--up diagram
On the close to threshold meson production in neutron-neutron collisions
A method of measuring the close to threshold meson production in
neutron-neutron collisions is described where the momenta of the colliding
neutrons can be determined with the accuracy obtainable for the proton-proton
reaction. The technique is based on the double quasi-free nn --> nn X^0
reaction, where deuterons are used as a source of neutronsComment: 6 pages, 2 figures, to be published in Phys. Lett.
A Covariant OBE Model for Production in NN Collisions
A relativistic covariant one boson exchange model, previously applied to
describe elastic nucleon-nucleon scattering, is extended to study
production in NN collisions. The transition amplitude for the elementary
BN->N process with B being the meson exchanged (B=, ,,
, and ) are taken to be the sum of four terms
corresponding to s and u-channels with a nucleon or a nucleon isobar
N*(1535MeV) in the intermediate states. Taking the relative phases of the
various exchange amplitudes to be +1, the model reproduces the cross sections
for the reactions in a consistent manner. In the limit where all
's are produced via N^* excitations, interference terms between the
overall contributions from the exchange of pseudoscalart and scalar mesons with
that of vector mesons cancel out. Consequently, much of the ambiguities in the
model predictions due to unknown relative phases of different vector
pseudoscalar exchanges are strongly reduced.Comment: 40 pages, 15 figure
A pellet tracking system for hadron physics experiments
Frozen microspheres of hydrogen (pellets) are used as targets in the hadron physics experiment WASA (Forschungszentrum Jülich, Germany) [1] and will also be used in the future PANDA experiment at FAIR (GSI, Darmstadt, Germany) [2]. The interaction region is defined by the overlap of the pellet stream and the accelerator beam and has a size of a few millimeters. One would like to know the interaction point more precisely, to have better possibility to reconstruct particle tracks and events. One would also like to suppress background events that do not originate in a pellet, but e.g. may occur in residual gas in the beam pipe. A solution is provided by the presented pellet tracking system, for which a prototype [3] has been developed in Uppsala. The goal is to track individual pellets in order to know their position at the time of an interaction. The design of such a system, simulation techniques and results are presented
e^+e^- production from pp reactions at BEVALAC energies
We have performed a detailed study of dilepton production from pp collisions
including the subthreshold production via baryonic resonances (N(1520),
N(1700)) in addition to the conventional dilepton sources as and Dalitz decays and direct decays of vector mesons (). The role of baryonic resonances in production from
nucleon-nucleon collisions is studied in comparison to the DLS data which are
well described.Comment: 19 pages, revtex, including 7 postscript figures, to be published in
Nucl. Phys.
Dilepton production and -scaling at BEVALAC/SIS energies
We present a dynamical study of production in C + C and Ca + Ca
collisions at BEVALAC/SIS energies on the basis of the covariant transport
approach HSD employing momentum-dependent -meson spectral functions that
include the pion modifications in the nuclear medium as well as the
polarization of the -meson due to resonant scattering. We find
that the experimental data from the DLS collaboration cannot be described
within the -meson spectral function approach. A dropping -mass
scenario leads to a good reproduction of the DLS dilepton data, however,
violates the -scaling of and spectra as observed by the
TAPS collaboration as well as photoproduction on nuclei.Comment: 35 pages, ReVTeX, including 11 postscript figures, UGI-97-06, Nucl.
Phys. A, in pres
Near-Threshold eta Meson Production in Proton-Proton Collisions
The production of eta mesons has been measured in the proton-proton
interaction close to the reaction threshold using the COSY-11 internal facility
at the cooler synchrotron COSY. Total cross sections were determined for eight
different excess energies in the range from 0.5 MeV to 5.4 MeV. The energy
dependence of the total cross section is well described by the available
phase-space volume weighted by FSI factors for the proton-proton and proton-eta
pairs.Comment: 9 pages, 1 table, 5 figure
Eta physics at threshold
The production of eta and eta-prime mesons in elementary nucleon-nucleon
collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE.
The magnitude and energy dependence of the total cross section as well as the
occupation distribution of the phase space serve as observables for
investigating the mechanisms underlying the production processes and the
interaction of mesons with nucleons. The precise data on the eta and eta-prime
creation via the pp --> pp eta(eta-prime) reactions allowed to settle the
general features of the eta and eta-prime meson production and revealed the
sensitivity of the mentioned observables to the nucleon-nucleon-meson final
state interaction. The particular production properties, like for example the
determination of the dominating exchange processes which lead to the excitation
of the S_11 nucleon isobar in the case of eta creation, must be established by
confrontation with other observables. The present status of this investigation
with an emphasis on the results of the COSY-11 collaboration is briefly
presented. The available data are interpreted in view of the production
mechanism and the meson-nucleon interaction.Comment: 3 pages, 3 figures, Presented at Conference on Quarks and Nuclear
Physics (QNP 2002), Julich, Germany, 9-14 Jun 200
Phenomenology of pp->pp eta reaction close to threshold
The recent high statistics measurement of the pp -> pp eta reaction at an
excess energy Q=15.5 MeV has been analysed by means of partial wave
decomposition of the cross section. Guided by the dominance of the final state
1S0 pp interaction (FSI), we keep only terms involving the FSI enhancement
factor. The measured p-p and p-eta effective mass spectra can be well
reproduced by lifting the standard on-shell approximation in the enhancement
factor and by allowing for a linear energy dependence in the leading 3P0->1S0,s
partial wave amplitude. Higher partial waves seem to play only a marginal role
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