141 research outputs found
Evidence for the fourth P11 resonance predicted by the constituent quark model
It is pointed out that the third of five low-lying P11 states predicted by a
constituent quark model can be identified with the third of four states in a
solution from a three-channel analysis by the Zagreb group. This is one of the
so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb
group to the pi N -> eta N data is the crucial element in finding this fourth
resonance in the P11 partial wave.Comment: 8 pages, revtex; expanded acknowledgement
The detailed mechanism of the eta production in pp scattering up to the Tlab = 4.5 GeV
Contrary to very early beliefs, the experimental cross section data for the
eta production in proton-proton scattering are well described if pi and only
eta meson exchange diagrams are used to calculate the Born term. The inclusion
of initial and final state interactions is done in the factorization
approximation by using the inverse square of the Jost function. The two body
Jost functions are obtained from the S matrices in the low energy effective
range approximation. The danger of double counting in the p-eta final state
interaction is discussed. It is shown that higher partial waves in
meson-nucleon amplitudes do not contribute significantly bellow excess energy
of Q=100 MeV. Known difficulties of reducing the multi resonance model to a
single resonance one are illustrated.Comment: 10 pages, 5 figures, corrected typos in relation (3), changed content
(added section with differential cross sections
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
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
The (1535) and (1650) Resonances in Meson-Baryon Unitarized Coupled Channel Chiral Perturbation Theory
The wave meson-baryon scattering is analyzed for the strangeness S=0
sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral
Symmetry. Four channels have been considered: , , ,
. The needed two particle irreducible matrix amplitude is taken from
lowest order Chiral Perturbation Theory in a relativistic formalism and low
energy constants are fitted to the elastic phase-shifts and the and cross section data. The position of
the complex poles in the second Riemann sheet of the scattering amplitude
determine masses and widths of the (1535) and (1650)
resonances, in reasonable agreement with experiment. A good overall description
of data, from threshold up to 2 GeV, is achieved keeping in mind that
the two pion production channel has not been included.Comment: 35 pages, LaTeX + 7 ps-figure files. Some minor mistakes have been
corrected for and a new appendix discussing the matching to HBChPT has been
also adde
eta d scattering in the region of the S11 resonance
We have studied the reaction eta d -> eta d close to threshold within a
nonrelativistic three-body formalism. We considered several eta N and NN
models, in particular potentials with separable form, fitted to the low-energy
eta N and NN data to represent the two-body interactions. We found that with
realistic two-body interactions a quasibound state does not exist in this
system, although there is an enhancement of the cross section by one order of
magnitude, in the region near threshold, which is a genuine three-body effect
not predicted within the impulse approximation.Comment: 18 pages Revtex, 2 figure
Relativistic Calculations for the Exclusive Photoproduction of Eta Mesons from Complex Nuclei
A relativistic model for the quasifree photoproduction of eta meson from
complex nuclei is developed. The interactions between fields are introduced
through effective Lagrangians. Contributions from several nucleon resonances as
well as nucleon Born terms and vector meson exchange diagrams are included.
Nucleon and eta wavefunctions are solutions of Dirac and Klein-Gordon
equations, respectively. Final state interactions of the outgoing particles are
included via optical potentials. The effects of these interactions on the cross
sections and photon asymmetries are studied and are found to be large.
Calculations indicate that at energies near threshold the exclusive reaction
takes place mainly through formation of the S_{11}(1535) resonance. Comparisons
with the non-relativistic calculations show differences between the two
approaches both for the cross sections and photon asymmetries. We give some
detailed predictions for the reaction observables for exclusive photoproduction
on ^{12}C, ^{16}O and ^{40}Ca.Comment: Latex, 27 pages, 13 uuencoded postscript figures. Accepted for
publication in Phys. Rev.
The energies and residues of the nucleon resonances N(1535) and N(1650)
We extract pole positions for the N(1535) and N(1650) resonances using two
different models. The positions are determined from fits to different subsets
of the existing , and data
and found to be 1515(10)--i85(15)MeV and 1660(10)--i65(10)MeV, when the data is
described in terms of two poles. Sensitivity to the choice of fitted data is
explored. The corresponding and residues of these poles
are also extracted.Comment: 9 page
eta N S-wave scattering length in a three coupled channel, multiresonance, unitary model
The S-wave scattering length for eta-N elastic scattering is extracted from
the S-wave T-matrix in a three coupled channel, multiresonance unitary model.
Results are compared with values already reported in literature which are
obtained applying multichannel, but single resonance -- no background models. A
dispersion among the previously published values of the real part of the S-wave
scattering length is observed. We demonstrate that the reported spread
originates from the strong sensitivity of the scattering length upon the small
variation of the used input resonance parameters. In addition, we show that
eta-N scattering length value obtained in single resonance -- no background
models significantly increases if background term is added in a unitary way. We
question the reliability of previously reported values based only on the single
resonance -- no background models, and demonstrate that the value of the eta-N
S-wave scattering length obtained in this publication is much more realistic
because of the multiresonance and unitary approach.Comment: revtex, 20 pages + 3 figures (PostScript: gzip + uuencode) included,
submitted to Phys. Rev. C, brief Reports
Pion-nucleon scattering in a meson-exchange model
The pi-N interaction is studied within a meson-exchange model and in a
coupled-channels approach which includes the channels pi-N, eta-N, as well as
three effective pi-pi-N channels namely rho-N, pi-Delta, and sigma-N. Starting
out from an earlier model of the Julich group systematic improvements in the
dynamics and in some technical aspects are introduced. With the new model an
excellent quantitative reproduction of the pi-N phase shifts and inelasticity
parameters in the energy region up to 1.9 GeV and for total angular momenta J
leq 3/2 is achieved. Simultaneously, good agreement with data for the total and
differential pi-N -> eta-N transition cross sections is obtained. The
connection of the pi_N dynamics in the S_{11} partial wave with the reaction
pi-N -> eta-N is discussed.Comment: 32 pages, 9 figure
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