13 research outputs found
Form factors and photoproduction amplitudes
We examine the use of phenomenological form factors in tree level amplitudes
for meson photoproduction. Two common recipes are shown to be fundamentally
incorrect. An alternate form consistent with gauge invariance and crossing
symmetry is proposed.Comment: To be published in PR
Threshold eta and eta' electroproduction off nucleons
The electroproduction of eta and eta' mesons on the proton and the neutron is
investigated at tree level within the framework of U(3) chiral perturbation
theory. In addition to the Born terms low-lying resonances such as the vector
mesons and J^P= 1/2^+, 1/2^- baryon resonances are included explicitly and
their contributions are calculated. Results for the separated differential
cross sections are presented.Comment: 24 pages, 7 figure
photoproduction near threshold
In this work, the meson photoproduction near threshold is
studied in the quark model framework. A pseudovector effective Lagrangian is
introduced for the coupling and the newly published data from
the SAPHIR Collaboration provide good constraints to this parameter.
Corrections of order for the electromagnetic interaction vertex
are taken into account, which produce corrections of order to the
transition amplitude for . Some low-lying
resonances, , , and are found to
have significant contributions. A bump structure around 2 GeV
is found arising from the terms in the harmonic oscillator basis. The
beam polarization asymmetries are predicted and can be tested against the
forthcoming data from GRAAL.Comment: Revtex, 3 eps figures, 1 table; Resubmitted to Phys. Rev.
K0-Sigma+ Photoproduction with SAPHIR
Preliminary results of the analysis of the reaction p(gamma,K0)Sigma+ are
presented. We show the first measurement of the differential cross section and
much improved data for the total cross section than previous data. The data are
compared with model predictions from different isobar and quark models that
give a good description of p(gamma,K+)Lambda and p(gamma,K+)Sigma0 data in the
same energy range. Results of ChPT describe the data adequately at threshold
while isobar models that include hadronic form factors reproduce the data at
intermediate energies.Comment: 4 pages, Latex2e, 4 postscript figures. Talk given at the
International Conference on Hypernuclear and Strange Particle Physics
(HYP97), Brookhaven National Laboratory, USA, October 13-18, 1997. To be
published in Nucl. Phys. A. Revised version due to changes in experimental
dat
Evidence for the positive-strangeness pentaquark in photoproduction with the SAPHIR detector at ELSA
The positive--strangeness baryon resonance is observed in
photoproduction of the final state with the SAPHIR detector at
the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the invariant mass distribution with a confidence level. We find
a mass MeV and an upper limit of the width
MeV at 90% c.l. From the absence of a signal in
the invariant mass distribution in at the
expected strength we conclude that the must be isoscalar.Comment: 9 pages, 4 figure
Energy Dependence of the Near-Threshold Total Cross-Section for the pp --> pp eta' Reaction
Total cross sections for the pp --> pp eta' reaction have been measured in
the excess energy range from Q = 1.53 MeV to Q = 23.64 MeV. The experiment has
been performed at the internal installation COSY-11 using a stochastically
cooled proton beam of the COoler SYnchrotron COSY and a hydrogen cluster
target. The determined energy dependence of the total cross section weakens the
hypothesis of the S-wave repulsive interaction between the eta' meson and the
proton. New data agree well with predictions based on the phase-space
distribution modified by the proton-proton final-state-interaction (FSI) only.Comment: 12 pages, 1 table, 4 figure
Eta, eta-prime photoproduction and electroproduction off nucleons
The photo- and electroproduction of eta, eta-prime mesons on nucleons are
investigated within a relativistic chiral unitary approach based on coupled
channels. The s-wave potentials for electroproduction and meson-baryon
scattering are derived from a chiral effective Lagrangian which includes the
eta-prime as an explicit degree of freedom and incorporates important features
of the underlying QCD Lagrangian such as the axial U(1) anomaly. The effective
potentials are iterated in a Bethe-Salpeter equation and cross sections for
eta, eta-prime photo- and electroproduction from nucleons are obtained. The
results for the eta-prime photoproduction cross section on protons reproduce
the appearance of an S_{11} resonance around 1.9 GeV observed at ELSA. The
inclusion of electromagnetic form factors increases the predicted eta
electroproduction cross sections on the proton, providing a qualitative
explanation for the hard form factor of the photocoupling amplitude observed at
CLAS.Comment: 26 page
eta-prime Meson Production in Nucleon-Nucleon Collisions Near The Threshold
The production of mesons in the reactions and at threshold-near energies is analyzed within a covariant effective
meson-nucleon theory. The description of cross section and angular
distributions of the available data in this kinematical region in the
channel is accomplished by including meson currents and nucleon currents with
the resonances , and . Predictions
for the channel are given. The di-electron production from subsequent
Dalitz decay is also
calculated and numerical results are presented for intermediate energy and
kinematics of possible experiments with HADES, CLAS and KEK-PS
-meson production in proton-proton collisions
The production of -mesons in proton-proton collisions is investigated
within a relativistic meson-exchange model of hadronic interactions. The
experimental prerequisites for extracting the coupling strength from
this reaction are discussed. In the absence of a sufficient set of data, which
would enable an accurate determination of the coupling strength, we
perform a combined analysis, based on some reasonable assumptions, of the
existing data for both - and -meson production. We find that the
recent data from the DISTO collaboration on the angular distribution of the
meson indicate that the coupling constant is small. The
analysis yields values for that are compatible with the OZI rule.Comment: 21 pages, 3 figures, submitted to Phys.Rev.