33 research outputs found
Photoproduction of Pentaquark in Feynman and Regge Theories
Photoproduction of the Theta+ pentaquark on the proton is analyzed by using
an isobar and a Regge models. The difference in the calculated total cross
section is found to be more than two orders of magnitude for a hadronic form
factor cut-off Lambda > 1 GeV. Comparable results would be obtained for 0.6 <
Lambda < 0.8 GeV. We also calculate contribution of the Theta+ photoproduction
to the GDH integral. By comparing with the current phenomenological
calculation, it is found that the GDH sum rule favors the result obtained from
Regge approach and isobar model with small Lambda.Comment: 5 pages, 5 figures, submitted to Phys.Rev.C as a Rapid Communicatio
Analysis of resonance multipoles from polarization observables in eta photoproduction
A combined analysis of new eta photoproduction data for total and
differential cross sections, target asymmetry and photon asymmetry is
presented. Using a few reasonable assumptions we perform the first
model-independent analysis of the E0+, E2- and M2- eta photoproduction
multipoles. Making use of the well-known A3/2 helicity amplitude of the
D13(1520) state we extract its branching ratio to the eta-N channel,
Gamma(eta,N)/Gamma = (0.08 +- 0.01)%. At higher energies, we show that the
photon asymmetry is extremely sensitive to small multipoles that are excited by
photons in the helicity 3/2 state. The new GRAAL photon asymmetry data at
higher energy show a clear signal of the F15(1680) excitation which permits
extracting an F15(1680)->eta,N branching ratio of (0.15 +0.35 -0.10)%.Comment: 14 pages of LATEX including 7 postscript figure
Kaon photoproduction on the nucleon: Contributions of kaon-hyperon final states to the magnetic moment of the nucleon
By using the Gerasimov-Drell-Hearn (GDH) sum rule and an isobaric model of
kaon photoproduction, we calculate contributions of kaon-hyperon final states
to the magnetic moment of the proton and the neutron. We find that the
contributions are small. The approximation of sigma_{TT'} by sigma_{T} clearly
overestimates the value of the GDH integral. We find a smaller upper bound for
the contributions of kaon-hyperon final states to the proton's anomalous
magnetic moment in kaon photoproduction, and a positive contribution for the
square of the neutron's magnetic moment.Comment: 6 pages, revtex, 1 postscript figure, to appear in Phys. Rev.
Low-energy and low-momentum representation of the virtual Compton scattering amplitude
We perform an expansion of the virtual Compton scattering amplitude for low
energies and low momenta and show that this expansion covers the transition
from the regime to be investigated in the scheduled photon electroproduction
experiments to the real Compton scattering regime.
We discuss the relation of the generalized polarizabilities of virtual
Compton scattering to the polarizabilities of real Compton scattering.Comment: 13 pages, LaTeX2e/RevTeX, no figure
The role of chiral loops in
We consider the rare decay and calculate the
non-resonant contribution to the amplitude to one loop in Chiral Perturbation
Theory. We display our result as both a diphoton energy spectrum and a partial
decay rate as a function of the photon energy cut. It turns out that the
one-loop correction can be numerically very important and could be detected, at
sufficiently large center-of-mass photon energies, from a measurement of the
partial decay width.Comment: 10 pages, Latex. Discussion on the resonant background enlarged, sign
error corrected, basic results unchanged. To appear in Phys. Lett.
Measuring longitudinal amplitudes for electroproduction of pseudoscalar mesons using recoil polarization in parallel kinematics
We propose a new method for measuring longitudinal amplitudes for
electroproduction of pseudoscalar mesons that exploits a symmetry relation for
polarization observables in parallel kinematics. This polarization technique
does not require variation of electron scattering kinematics and avoids the
major sources of systematic errors in Rosenbluth separation.Comment: intended for Phys. Rev. C as a Brief Repor
Generalized polarizabilities and the spin-averaged amplitude in virtual Compton scattering off the nucleon
We discuss the low-energy behavior of the spin-averaged amplitude of virtual
Compton scattering (VCS) off a nucleon.
Based on gauge invariance, Lorentz invariance and the discrete symmetries, it
is shown that to first order in the frequency of the final real photon only two
generalized polarizabilities appear.
Different low-energy expansion schemes are discussed and put into
perspective.Comment: 13 pages, 1 postscript figure, Revtex using eps
Gerasimov-Drell-Hearn Sum Rule and the Discrepancy between the New CLAS and SAPHIR Data
Contribution of the K^+\Lambda channel to the Gerasimov-Drell-Hearn (GDH) sum
rule has been calculated by using the models that fit the recent SAPHIR or CLAS
differential cross section data. It is shown that the two data sets yield quite
different contributions. Contribution of this channel to the forward spin
polarizability of the proton has been also calculated. It is also shown that
the inclusion of the recent CLAS C_x and C_z data in the fitting data base does
not significantly change the result of the present calculation. Results of the
fit, however, reveal the role of the S_{11}(1650), P_{11}(1710), P_{13}(1720),
and P_{13}(1900) resonances for the description of the C_x and C_z data. A
brief discussion on the importance of these resonances is given. Measurements
of the polarized total cross section \sigma_{TT'} by the CLAS, LEPS, and MAMI
collaborations are expected to verify this finding.Comment: 15 pages, 8 figure
Quark model approach to the meson electroproduction on the proton
A recently developed quark model approach to pseudoscalar meson
photoproduction is extended to electroproduction process for the meson
in the kinematics of momentum transfer 4 (GeV/c) and total
center of mass energy 1.6 GeV. Existing data are well reproduced and
the roles of the and resonances are closely
investigated. In the study of the longitudinal excitation of the
resonance, a reliable constraint on the properties is obtained
by cleanly removing the electromagnetic transition from the amplitude. Thus, the fitted quantities can be
determined with an uncertainty of about 15%. This could be the first direct
constraint on the properties in theory.Comment: 31 pages, 9 figures; Substantially modified version, to appear on
Jour. of Phys.
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change