221 research outputs found
Hypernuclear production by () reaction within a relativistic model
Within a fully covariant model based on an effective Lagrangian picture, we
investigate the hypernuclear production in photon-nucleus interaction on O target. The explicit kaon production vertex is described via creation,
propagation and decay into relevant channel of (1650), (1710), and
(1720) intermediate baryonic resonance states in the initial interaction
of the incident photon with one of the target protons. Bound state nucleon and
hyperon wave functions are obtained by solving the Dirac equation. Using vertex
parameters determined in the previous studies, contributions of the (1710)
baryonic resonance dominate the total production cross sections which are found
to peak at photon energies below 1 GeV. The results show that photoproduction
is the most appropriate means for studying the unnatural parity hypernuclear
states, thus accessing the spin dependence of the hyperon-nucleon interaction.Comment: 12 pages, 4 figures, Dicussions revised, version to appear in Phys.
Rev.C (rapid communications
Exclusive production in proton-nucleus collisions
The exclusive meson production in a proton-nucleus collision, leading
to two body final states, is investigated in a fully covariant two-nucleon
model based on the effective Lagrangian picture. The explicit kaon production
vertex is described via creation, propagation and decay into relevant channel
of (1650), (1710) and (1720) intermediate baryonic states in the
initial collision of the projectile nucleon with one of its target counterparts
which is modeled by the one-pion exchange process. The calculated cross
sections show strong sensitivity to the medium effects on pion propagator and
to the final hypernuclear state excited in the reaction.Comment: Two new figures, version accepted for publication by Phys. Rev.
Stiff knots
We report on the geometry and mechanics of knotted stiff strings. We discuss
both closed and open knots. Our two main results are: (i) Their equilibrium
energy as well as the equilibrium tension for open knots depend on the type of
knot as the square of the bridge number; (ii) Braid localization is found to be
a general feature of stiff strings entanglements, while angles and knot
localization are forbidden. Moreover, we identify a family of knots for which
the equilibrium shape is a circular braid. Two other equilibrium shapes are
found from Monte Carlo simulations. These three shapes are confirmed by
rudimentary experiments. Our approach is also extended to the problem of the
minimization of the length of a knotted string with a maximum allowed
curvature.Comment: Submitted to Phys. Rev.
Low energy onset of nuclear shadowing in photoabsorption
The early onset of nuclear shadowing in photoabsorption at low photon
energies has recently been interpreted as a possible signature of a decrease of
the rho meson mass in nuclei. We show that one can understand this early onset
within simple Glauber theory if one takes the negative real part of the rho N
scattering amplitudes into account, corresponding to a higher effective mass of
the rho meson in nuclear medium.Comment: REVTEX, 9 pages, including 4 eps figure
Two- and three-body color flux tubes in the Chromo Dielectric Model
Using the framework of the Chromo Dielectric Model we perform an analysis of
color electric flux tubes in meson-like and baryon-like quark
configurations. We discuss the Abelian color structure of the model and point
out a symmetry in color space as a remnant of the SU(3) symmetry of QCD. The
generic features of the model are discussed by varying the model parameters. We
fix these parameters by reproducing the string tension MeV/fm and
the transverse width fm of the flux tube obtained in
lattice calculations. We use a bag constant MeV, a glueball
mass MeV and a strong coupling constant . We show that the asymptotic string profile of an infinitely long flux
tube is already reached for separations fm. A connection
to the Dual Color Superconductor is made by extracting a magnetic current from
the model equations and a qualitative agreement between the two descriptions of
confinement is shown. In the study of the system we observe a
-like geometry for the color electric fields and a
\textsf{Y}-like geometry in the scalar fields both in the energy density
distribution and in the corresponding potentials. The resulting total
potential is described neither by the -picture nor by the
\textsf{Y}-picture alone.Comment: 32 pages, 35 eps-figures, revised version, some references + 1
eps-file added, to be published in Phys.Rev.
Hadronization of a Quark-Gluon Plasma in the Chromodielectric Model
We have carried out simulations of the hadronization of a hot, ideal but
effectively massive quark-gluon gas into color neutral clusters in the
framework of the semi-classical SU(3) chromodielectric model. We have studied
the possible quark-gluon compositions of clusters as well as the final mass
distribution and spectra, aiming to obtain an insight into relations between
hadronic spectral properties and the confinement mechanism in this model.Comment: 34 pages, 37 figure
Topological effects in the thermal properties of knotted polymer rings
The topological effects on the thermal properties of several knot
configurations are investigated using Monte Carlo simulations. In order to
check if the topology of the knots is preserved during the thermal fluctuations
we propose a method that allows very fast calculations and can be easily
applied to arbitrarily complex knots. As an application, the specific energy
and heat capacity of the trefoil, the figure-eight and the knots are
calculated at different temperatures and for different lengths. Short-range
repulsive interactions between the monomers are assumed. The knots
configurations are generated on a three-dimensional cubic lattice and sampled
by means of the Wang-Landau algorithm and of the pivot method. The obtained
results show that the topological effects play a key role for short-length
polymers. Three temperature regimes of the growth rate of the internal energy
of the system are distinguished.Comment: 7 pages, 12 figures, LaTeX + RevTeX. With respect to the first
version, in the second version the text has been improved and all figures are
now in black and whit
Anti-Kaon Induced Reactions on the Nucleon
Using a previously established effective Lagrangian model we describe
anti-kaon induced reactions on the nucleon. The dominantly contributing
channels in the cm-energy region from threshold up to 1.72 GeV are included (K
N, \pi \Sigma, \pi \Lambda). We solve the Bethe-Salpeter equation in an unitary
-matrix approximation.Comment: 21 pages, 13 figures, minor typos corrected, accepted for publication
in Phys. Rev.
e^+e^- Pair Production from A Reactions
We present a calculation of e^+e^- production in reactions at MAMI
and TJNAF energies within a semi-classical BUU transport model. Dilepton
invariant mass spectra for C, Ca and Pb are calculated
at 0.8, 1.5 and 2.2 GeV. We focus on observable effects of medium modifications
of the and mesons. The in-medium widths of these mesons are
taken into account in a dynamical, consistent way. We discuss the transport
theoretical treatment of broad resonances.Comment: 42 pages including 16 figure
A Precision Measurement of pp Elastic Scattering Cross Sections at Intermediate Energies
We have measured differential cross sections for \pp elastic scattering with
internal fiber targets in the recirculating beam of the proton synchrotron
COSY. Measurements were made continuously during acceleration for projectile
kinetic energies between 0.23 and 2.59 GeV in the angular range deg. Details of the apparatus and the data analysis are
given and the resulting excitation functions and angular distributions
presented. The precision of each data point is typically better than 4%, and a
relative normalization uncertainty of only 2.5% within an excitation function
has been reached. The impact on phase shift analysis as well as upper bounds on
possible resonant contributions in lower partial waves are discussed.Comment: 23 pages 29 figure
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