790 research outputs found
eta-photoproduction in the resonance energy region
The production in the nucleon resonance energy region is studied
within the unitary coupled-channels effective Lagrangian approach of the
Giessen model. We demonstrate that the second peak recently observed in the
cross section of photoproduction on the neutron at =1.66 GeV
can be explained in terms of coupled-channel effects due to and
resonance excitations
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
Photoproduction of Baryons Decaying into N pi and N eta
A combined analysis of photoproduction data on \gamma p to \pi N, eta N was
performed including the data on K Lambda and K Sigma. The data are interpreted
in an isobar model with s--channel baryon resonances and pi, rho,(omega), K,
and K^* exchange in the t--channel. Three baryon resonances have a substantial
coupling to eta N, the well known N(1535)S_{11}, N(1720)P_{13}, and
N(2070)D_{15}. The inclusion of data with open strangeness reveals the presence
of further new resonances, N(1840)P_{11}, N(1875)D_{13} and N(2170)D_{13}.Comment: 13 pages, 14 figure
Evidence for a negative-parity spin-doublet of nucleon resonances at 1.88\,GeV
Evidence is reported for two nucleon resonances with spin-parity
and at a mass just below 1.9\,GeV. The evidence is derived from a
coupled-channel analysis of a large number of pion and photo-produced
reactions. The two resonances are nearly degenerate in mass with two resonances
of the same spin but positive parity. Such parity doublets are predicted in
models claiming restoration of chiral symmetry in high-mass excitations of the
nucleon. Further examples of spin parity doublets are found in addition.
Alternatively, the spin doublet can be interpreted as member of the 56-plet
expected in the third excitation band of the nucleon. Implications for the
problem of the {\it missing resonances} are discussed.Comment: 10 pages, 3 figures, 4 tables, minor revisio
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In-sample forecasting: A brief review and new algorithms
Statistical methods often distinguish between in-sample and out-of-sample approaches. In particular this is the case when time is involved. Then often time series methods are proposed that extrapolate past patterns into the future via complicated recursion formulas. Standard statistical inference is on the other hand concerned with estimating parameters within the given sample. This review paper is about a statistical methodology, where all parameters are estimated in-sample while producing a forecast out-of-sample without recursion or extrapolation. A new super-simulation algorithm ensures a faster implementation of the simplest and perhaps most important version of in-sample forecasting
Electromagnetic Meson Production in the Nucleon Resonance Region
Recent experimental and theoretical advances in investigating electromagnetic
meson production reactions in the nucleon resonance region are reviewed.Comment: 75 pages, 42 figure
Exclusive electroproduction of K+ Lambda and K+ Sigma^0 final states at Q^2 = 0.030-0.055 (GeV/c)^2
Cross section measurements of the exclusive p(e,e'K+)Lambda,Sigma^0
electroproduction reactions have been performed at the Mainz Microtron MAMI in
the A1 spectrometer facility using for the first time the Kaos spectrometer for
kaon detection. These processes were studied in a kinematical region not
covered by any previous experiment. The nucleon was probed in its third
resonance region with virtual photons of low four-momenta, Q^2= 0.030-0.055
(GeV/c)^2. The MAMI data indicate a smooth transition in Q^2 from
photoproduction to electroproduction cross sections. Comparison with
predictions of effective Lagrangian models based on the isobar approach reveal
that strong longitudinal couplings of the virtual photon to the N* resonances
can be excluded from these models.Comment: 16 pages, 7 figure
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