41 research outputs found
A meson-exchange model of the associated photoproduction of vector mesons and N*(1440) resonances
We discuss the photoproduction of omega- and rho0-mesons off protons in the
particular channel where the target proton is excited to a Roper resonance
N*(1440). We propose a simple meson-exchange model for these processes in order
to evaluate their cross sections near threshold and at low momentum transfers.
It is suggested in particular that the differential cross section for the
associated photoproduction of a rho0-meson and a Roper resonance in these
kinematic conditions could provide direct information on the strength of the
scalar-isoscalar excitation of the N*(1440) and hence on the magnitude of an
effective sigma N N*(1440) coupling. The latter quantity is poorly known and of
much interest for the nuclear many-body problem.Comment: 18 pages, 9 figures, accepted for publication in Nuclear Physics
Study of the threshold behavior of the N scattering amplitude through the associated photoproduction of - and -mesons
We suggest that the reaction cross section, in the
kinematics where the invariant mass in the final state lies between
the threshold value (m+m) and the N*(1535) resonance mass, is
largely determined by the scattering amplitude close to threshold. The
initial photon energy is chosen in the range GeV, in
order to reach low (absolute) values of the squared 4-momentum transfer from
the initial photon to the final -meson. In these conditions, we expect
the t-channel - and -meson exchanges to drive the dynamics
underlying the process. We show that the
-exchange is the dominating contribution to the cross section while the
-exchange is negligible. The - interference is of the order
of . The sign of this term is not known and alters significantly our
results. Data on the process would be therefore very
useful to help unravelling the behavior of the scattering amplitude
close to threshold and assessing the possibility of producing -nucleus
bound states.Comment: 8 pages, 6 figures, Invited talk at MESON 2006, June 9th-13th, 2006,
Cracow (Poland
e+e- pair production from nucleon targets in the resonance region
We present consistent theoretical descriptions of the
and reactions on proton and neutron targets for total
center of mass energies ranging between 1.50 GeV and 1.75 GeV. These
reactions are complementary to study the coupling of low-lying baryon
resonances to vector meson-nucleon channels. We show in particular how the
resonant structure of the amplitudes for both processes generates specific and
large quantum interferences between - and -meson decays into
pairs. Data on the and
reactions are expected in the near future from the HADES program at GSI and
from dilepton studies with CLAS at JLab.Comment: 6 pages, 6 figures, Paper presented at Hirschegg 2004, 'Probing
nuclei and nucleons with electrons and photons', January 12th-16th, 200
Dynamics of strong and radiative decays of Ds mesons
The positive parity scalar D(2317) and axial-vector D(2460)
charmed strange mesons are generated by coupled-channel dynamics through the
s-wave scattering of Goldstone bosons off the pseudoscalar and vector
D(D)-meson ground states. The attraction leading to the specific masses of
these states reflects the chiral symmetry breaking scale which characterizes
the Weinberg-Tomozawa interaction in the chiral Lagrangian. Chiral corrections
to order Q are calculated and found to be small. The D(2317)
and D(2460) mesons decay either strongly into the isospin-violating
D and D channels or electromagnetically. We show that
the - and (KD-KD) mixings act constructively to
generate strong widths of the order of 140 keV. The one-loop contribution to
the radiative decay amplitudes of scalar and axial-vector states is calculated
using the electromagnetic Lagrangian to chiral order Q. We show the
importance of taking into account processes involving light vector mesons
explicitly in the dynamics of electromagnetic decays to obtain a satisfactory
description of the available data.Comment: Contribution to the Meson 2008 Conference, June 6-10, Cracow, Polan
The reaction close to the vector meson production threshold
The and reaction cross sections
are calculated below and in the vicinity of the vector meson (,
) production threshold. These processes are largely responsible for the
emission of pairs in pion-nucleus reactions and contribute to the
dilepton spectra observed in relativistic heavy ion collisions. They are
dominated by the decay of low-lying baryon resonances into vector meson-nucleon
channels. The vector mesons materialize subsequently into pairs. Using
and amplitudes calculated in the
center of mass energy interval GeV, we compute the and reaction cross sections in these
kinematics. Below the vector meson production threshold, the
interference in the channel appears largely destructive for the
cross section and constructive for the cross section. The pion beam and the HADES detector at GSI offer a unique
possibility to measure these effects. Such data would provide strong
constraints on the coupling of vector meson-nucleon channels to low-lying
baryon resonances.Comment: Talk given at the Budapest'02 Workshop on Quark and Hadron Dynamics,
Budapest (Hungary), March 3rd-7th, 200
Recommended from our members
Hyperon Radiative Decay
The radiative decay widths of the low-lying strange baryons are calculated both within the relativistic quark bag model and the nonrelativistic potential model. These widths are found to depend sensitively upon the quark-model dynamics through multiplet mixing and q4¯q admixtures. The comparison between our calculated results and the very limited experimental data is discussed.Engineering and Applied Science
Study of the N scattering amplitude through the associated photoproduction of - and -mesons in the region of the N*(1535) resonance
The reaction is studied in the kinematic region
where the final state originates dominantly from the decay of the
N*(1535) resonance. The threshold laboratory photon energy for this reaction
(at the peak of the S11 resonance) is 3 GeV. We will discuss
it somewhat above threshold, at GeV, in order to
reach lower (absolute) values of the squared 4-momentum transfer from the
initial photon to the final -meson. In these conditions, we expect the
t-channel - and -meson exchanges to drive the dynamics underlying
the process. The initial photon dissociates into the
final -meson and a virtual pseudoscalar meson ( or ). The
virtual pseudoscalar meson scatters from the proton target to produce the final
state. The and amplitudes are
derived in the framework of a coupled-channel effective field theory of
meson-baryon scattering. We found the -meson exchange to be largely
dominant. The - interference is of the order of . The
sign of this term is not known and has a significant influence on the results.
The amplitude being largely determined by data on the reaction, we found that the reaction
cross section is rather directly related to the -nucleon scattering
amplitude in the N*(1535) resonance region. Accurate data on the process would therefore put additional constraints on this still
poorly known amplitude.Comment: 13 pages, 8 figure
Quantum interferences in the reaction close to the vector meson production threshold
The exclusive photoproduction of pairs from nucleons close to the
vector meson production threshold ( GeV) results from two
main processes: the emission of Bethe-Heitler pairs and the photoproduction of
- and -mesons decaying into pairs. The Bethe-Heitler
amplitudes are purely electromagnetic and reflect mostly the nucleon magnetic
structure. The amplitudes arising from vector meson
production and decay are derived from and amplitudes supplemented by the Vector Meson Dominance assumption. The
vector meson photoproduction amplitudes are calculated using a relativistic and
unitary coupled-channel approach to meson-nucleon scattering. They depend
sensitively on the coupling of vector fields to baryon resonances. The differential cross sections display interference patterns. The
interference of Bethe-Heitler pair production with vector meson decay
is quite small in the domain of validity of our model for all angles of the
emitted pair. The interference of - and -mesons in the
channel can be large. It is constructive for the reaction and destructive for the reaction. We
discuss the shape and magnitude of the pair spectra produced in the
and reactions as functions of
the pair emission angle and of the total center of mass energy .Comment: 28 pages, 12 figure
Photoproduction of vector mesons off nucleons near threshold
We propose a simple meson-exchange model of the photoproduction of -
and -mesons off protons near threshold ( less than 2 GeV).
This model provides a good description of the available data and implies a
large -nucleon interaction in the scalar channel (-exchange). We
use this phenomenological interaction to estimate the leading contribution to
the self-energy of -mesons in matter. The implications of our calculation
for experimental studies of the -meson mass in nuclei are discussed.Comment: 14 pages, 11 Postscript figures, uses elsart.sty, epsfig.sty, to
appear in Nucl. Phys.