141.
Deuteron Photodisintegration in the Quark-Hadron Picture - Rossi, Patrizia; Collaboration, for the CLAS
The study of the two-body photodisintegration of the deuteron in the few GeV
region is the ideal reaction to clarify the transition from the nucleonic
degrees of freedom to the QCD picture of hadrons. The CLAS large angle
spectrometer of Hall B at Jlab allowed for the first time the complete
measurement of the angular distribution of the differential cross section at
photon energy between 0.5 and 3 GeV. Preliminary results of the E93-017
experiment from the analysis of the 30% of the total statistic accumulated show
persistent forward-backward asymmetry and are well described by the recent
calculation of the deuteron photodisintegration cross section derived in the
framework of...
142.
Nucleon Properties in the Perturbative Chiral Quark Model - Lyubovitskij, V. E.; Gutsche, Th.; Faessler, Amand
We apply the perturbative chiral quark model (PCQM) to analyse low-energy
nucleon properties: electromagnetic form factors, meson-nucleon sigma-terms and
pion-nucleon scattering. Baryons are described as bound states of valence
quarks surrounded by a cloud of Goldstone bosons (pi, K, eta) as required by
chiral symmetry. The model is based on the following guide lines: chiral
symmetry constraints, fulfilment of low-energy theorems and proper treatment of
sea-quarks, that is meson cloud contributions. Analytic expressions for nucleon
observables are obtained in terms of fundamental parameters of low-energy
pion-nucleon physics (weak pion decay constant, axial nucleon coupling
constant, strong pion-nucleon form factor) and of only one model parameter
(radius of the nucleonic three-quark...
143.
Quark-Hadron Picture of the Deuteron Photodisintegration - Mirazita, Marco
The high energy two-body photodisintegration of the deuteron is an ideal
reaction for the identification of quark effects in the nuclei. In particular,
the study of this reaction in the few GeV region can clarify the transition
from the nucleonic degrees of freedom to the QCD picture of hadrons. The CLAS
large angle spectrometer of Hall B at TJNAF allowed for the first time to
measure the differential cross section for photon energy between 0.5 and 3 GeV,
over all proton angles between 10 and 140 degrees in the laboratory frame.
Preliminary results from the analysis of 30% of the total statistic accumulated
is shown. These results are...
144.
How the nuclear Fermi motion plus a simple statistical model explains
the EMC effect - zynek, J. Ro\.; Wilk, G.
We present calculation of influence caused by nucleon Fermi motion on the
parton distributions in nuclei. Our approach is based on the model where
momenta of valence partons have some primordial distribution inside the hadron
at rest, which is either provided by a statistical considerations or calculated
using spherically symmetric Gaussian distribution with a width derived from the
Heisenberg uncertainty relation. The sea parton contribution emerges from the
similar Gaussian distribution with a width dictated by the presence of virtual
pions in hadron. We show that the influence of Fermi motion changes
substantially the nucleonic structure function inside the nucleus in the right
direction and therefore should be considered...
145.
A STRANGE MESONIC TRANSITION FORM FACTOR - Goity, J. L.; Musolf, M. J.
The strange-quark vector current $\rho$-to-$\pi$ meson transition form factor
is computed at one-loop order using strange meson intermediate states. A
comparison is made with a $\phi$-meson dominance model estimate. We find that
one-loop contributions are comparable in magnitude to those predicted by
$\phi$-meson dominance. It is possible that the one-loop contribution can make
the matrix element as large as those of the electromagnetic current mediating
vector meson radiative decays. However, due to the quadratic dependence of the
one-loop results on the hadronic form factor cut-off mass, a large uncertainty
in the estimate of the loops is unavoidable. These results indicate that
non-nucleonic strange quarks could contribute appreciably in moderate-$|Q^2|$
parity-violating...
146.
Subthreshold rho^0 photoproduction on 3He - Kagarlis, TAGX Collaboration: M. A.; Papandreou, Z.; Huber, G. M.; Lolos, G. J.; Shinozaki, A.; Brash, E. J.; Farzanpay, F.; Iurescu, M.; Weinerman, A.; Garino, G.; Maruyama, K.; Konno, O.; Maeda, K.; Terasawa, T.; Yamazaki, H.; Emura, T.; Hirosawa, H.; Niwa, K.; Yamashita, H.; Endo, S.; Miyamoto, K.; Sumi, Y.; Leone, A.; Perrino, R.; Maki, T.; Sasaki, A.; Kim, J. C.
A large reduction of the rho^0 mass in the nuclear medium is reported,
inferred from dipion photoproduction spectra in the 1 GeV region, for the
reaction 3He(gamma,pi+ pi-)X with a 10% duty factor tagged-photon beam and the
TAGX multi-particle spectrometer. The energy range covered (800 < E(gamma) <
1120 MeV) lies mostly below the free rho^0 production threshold, a region which
is believed sensitive to modifications of light vector-meson properties at
nuclear-matter densities. The rho^0 masses extracted from the MC fitting of the
data, m*(rho^0) = 642 +/- 40, 669 +/- 32, and 682 +/- 56 MeV/c^2 for E(gamma)
in the 800-880, 880-960, and 960-1040 MeV regions respectively,...
147.
Mean-Field and Pairing Properties of Exotic Nuclei: Exploring the
Nuclear Landscape - Dobaczewski, Jacek; Nazarewicz, Witold
In years to come, we shall see substantial progress in our understanding of
nuclear structure - a rich and many-faceted field. An important element in this
task will be to extend the study of nuclei into new domains. The journey to
'the limits' of isospin, angular momentum, and mass and charge is a quest for
new and unexpected phenomena which await us in uncharted territories. What is
extremely important from a theoretical point of view is that the new data are
also expected to bring qualitatively new information about the effective
nucleon-nucleon interaction and hence about the fundamental properties of the
nucleonic many-body system. The main objective of...
148.
Pseudovector vs. pseudoscalar coupling in one-boson exchange NN
potentials - Caia, G.; Durso, J. W.; Elster, Ch.; Haidenbauer, J.; Sibirtsev, A.; Speth, J.
We examine the effects of pseudoscalar and pseudovector coupling of the pi
and eta mesons in one-boson exchange models of the NN interaction using two
approaches: time-ordered perturbation theory unitarized with the relativistic
Lippmann-Schwinger equation, and a reduced Bethe-Salpeter equation approach
using the Thompson equation. Contact terms in the one-boson exchange amplitudes
in time-ordered perturbation theory lead naturally to the introduction of
s-channel nucleonic cutoffs for the interaction, which strongly suppresses the
far off-shell behavior of the amplitudes in both approaches. Differences
between the resulting NN predictions of the various models are found to be
small, and particularly so when coupling constants of the other mesons are
readjusted within reasonable...
149.
On the minimum and maximum mass of neutron stars and the delayed
collapse - Strobel, Klaus; Weigel, Manfred K.
The minimum and maximum mass of protoneutron stars and neutron stars are
investigated. The hot dense matter is described by relativistic (including
hyperons) and non-relativistic equations of state. We show that the minimum
mass ($\sim$ 0.88 - 1.28 $M_{\sun}$) of a neutron star is determined by the
earliest stage of its evolution and is nearly unaffected by the presence of
hyperons. The maximum mass of a neutron star is limited by the protoneutron
star or hot neutron star stage. Further we find that the delayed collapse of a
neutron star into a black hole during deleptonization is not only possible for
equations of state with softening components, as...
150.
The 2p-2h electromagnetic response in the quasielastic peak and beyond - De Pace, A.; Nardi, M.; Alberico, W. M.; Donnelly, T. W.; Molinari, A.
The contribution to the nuclear transverse response function R_T arising from
two particle-two hole (2p-2h) states excited through the action of
electromagnetic meson exchange currents (MEC) is computed in a fully
relativistic framework. The MEC considered are those carried by the pion and by
Delta degrees of freedom, the latter being viewed as a virtual nucleonic
resonance. The calculation is performed in the relativistic Fermi gas model in
which Lorentz covariance can be maintained. All 2p-2h many-body diagrams
containing two pionic lines that contribute to R_T are taken into account and
the relative impact of the various components of the MEC on R_T is addressed.
The non-relativistic limit of...
151.
Inclusive scattering of electrons from nuclear matter in the few GeV
range - Rinat, A. S.; Taragin, M. F.
We study Final State Interaction effects (FSI) in inclusive electron
scattering of several GeV from nuclear matter. We consider separately the cases
of 'elastic' and 'inelastic' knockout of respectively a nucleon and a
non-nucleonic particle. For the former category FSI have been calculated, using
the first cumulant approximation of a relativistic generalization of the theory
of Gersch et al, including intermediate NN states states with produced hadrons.
We also discuss a formal expression for the first cumulant approximation on the
basis of the PWIA. FSI between an inelasticlly knocked-out particle and core
nucleons are neglected. We present two scaling analyses of the data, from which
the above 'inelastic' contributions...
152.
Quasifree photoabsorption on neutron-proton pairs in 3He - Niskanen, J. A.; Wilhelm, P.; Arenhoevel, H.
Three-body photodisintegration of 3He is calculated in the photon energy
range 200 - 400 MeV assuming quasifree absorption on np pairs both in initial
quasideuteron and singlet configurations. The model includes the normal
nucleonic current, explicit meson exchange currents and the Delta(1232)-isobar
excitation. The total cross section is increased by a factor of about 1.5
compared with free deuteron photodisintegration. Well below and above the Delta
region also some spin observables differ significantly from the ones of free
deuteron disintegration due to the more compressed wave function of the
correlated np pairs in 3He compared to the deuteron. The initial singlet state
causes a significant change in the analyzing...
153.
Heavy Ion Collisions and the Density Dependence of the Local Mean Field - Kubo, T.; Lehmann, E.; Faessler, Amand; Puri, Rajeev K.; Ohtsuka, N.; Tsushima, K.; Zipprich, J.; Huang, S. W.
We study the effect of the density dependence of the scalar and the vector
part of the nucleonic self-energy in Relativistic Quantum Molecular Dynamics
(RQMD) on observables like the transversal flow and the rapidity distribution.
The stability of nuclei in RQMD is greatly improved if the density dependence
is included in the self-energies compared to a calculation assuming always
saturation density of nuclear matter. Different approaches are studied: The
main results are calculated with self-energies extracted from a
Dirac-Br\"uckner-Hartree-Fock G-matrix of a one boson exchange model, i.e. the
Bonn potential. These results are compared with those obtained by a
generalization of static Skyrme force, with calculations in the simple...
154.
Parity-violating longitudinal response - De Pace, A.
The parity-violating quasielastic electron scattering response is explored
within the context of a model that builds antisymmetrized random phase
approximation and Hartree-Fock correlations on a relativistic Fermi gas basis.
Particular emphasis is put on the weak-neutral longitudinal response function,
since this observable displays a strong sensitivity to isospin correlations:
specifically, it is shown how, through a diagrammatic cancellation/filtration
mechanism, this response acts as a magnifier of pionic correlations in the
nuclear medium. The parity-violating longitudinal response function also
displays appreciable sensitivity to the electric strangeness content of the
nucleon, thus making quasielastic electron scattering a possible candidate to
measure the nucleon electric strange form factor at relatively high momentum
transfers. Finally,...
155.
Isospin Breaking in the Pion-Nucleon Coupling from QCD Sum Rules - Meissner, T.; Henley, E. M.
We use QCD sum rules for the three point function of a pseudoscalar and two
nucleonic currents in order to estimate the charge dependence of the pion
nucleon coupling constant $g_{NN\pi}$ coming from isospin violation in the
strong interaction. The effect can be attributed primarily to the difference of
the quark condensates $<{\bar u}u>$ and $<{\bar d}d>$. For the splitting
$(g_{pp\pi_0} - g_{nn\pi_0}) / g_{NN\pi}$ we obtain an interval of $1.2 *
10^{-2}$ to $3.7 * 10^{-2}$, the uncertainties coming mainly from the input
parameters. The charged pion nucleon coupling is found to be the average of
$g_{pp\pi_0}$ and $g_{nn\pi_0}$. Electromagnetic effects are not included.
156.
New approach to $^4$He charge distribution - Wilets, L.; Alberg, M. A.; Pepin, S.; Stancu, Fl.; Carlson, J.; Koepf, W.
We present a study of the $^4$He charge distribution based on realistic
nucleonic wave functions and incorporation of the nucleon's quark substructure.
The central depression of the proton point density seen in modern four-body
calculations is too small by itself to lead to a correct description of the
charge distribution. We utilize six-quark structures calculated in the
Chromodielectric Model for N-N interactions, and we find a swelling of the
proton charge distribution as the internucleon distance decreases. These charge
distributions are combined with the $^4$He wave function using the Independent
Pair Approximation and two-body distributions generated from Green's Function
Monte Carlo calculations. We obtain a reasonably good fit to...
157.
Isospin Splitting in the Pion-Nucleon Couplings from QCD Sum Rules - Meissner, T.; Henley, E. M.
We use QCD sum rules for the three point function of a pseudoscalar and two
nucleonic currents in order to estimate the charge dependence of the pion
nucleon coupling constant coming from isospin violation in the strong
interaction. The effect can be attributed primarily to the difference of the
quark condensates and . Assuming that the pi0 is a pure
isostate we obtain for the splitting between the coupling of proton and neutron
to the neutral pion an interval of [0.008 ; 0.023], the uncertainties coming
mainly from the input parameters. In order to obtain the coupling to a physical
pi0 we have to take...
158.
Chiral Symmetry Restoration and Parity Mixing - Chanfray, G.; Delorme, J.; Ericson, M.
We derive the expressions of the vector and axial current from a chiral
Lagrangian restricted to nucleons and pions. They display mixing terms between
the axial and vector currents. We study the modifications in the nuclear medium
of the coupling constants of the axial current, namely the pion decay constant
and the nucleonic axial one due to the requirements of chiral symmetry. We
express the renormalizations in terms of the local scalar pion density. The
latter also governs the quark condensate evolu- tion and we discuss the link
between this evolution and the renormaliza- tions. In the case of the nucleon
axial coupling constant this renormali- zation corresponds...
159.
Formation of correlations and energy-conservation at short time scales - Morawetz, K.; Köhler, H. S.
The formation of correlations due to collisions in an interacting nucleonic
system is investigated. Results from one-time kinetic equations are compared
with the Kadanoff and Baym two-time equation with collisions included in Born
approximation. A reasonable agreement is found for a proposed approximation of
the memory effects by a finite duration of collisions. This form of collision
integral is in agreement with intuitive estimates from Fermi's golden rule. The
formation of correlations and the build up time is calculated analytically for
the high temperature and the low temperature limit. Different approximate
expressions are compared with the numerical results. We present analytically
the time dependent interaction energy and the formation...
160.
Formation of correlations at short time scales and consequences on
interferometry methods - Morawetz, K.; Köhler, H. S.
The formation of correlations due to collisions in an interacting nucleonic
system is investigated shortly after a disturbance. Results from one-time
kinetic equations are compared with the Kadanoff and Baym two-time equation
with collisions included in second order Born approximation. A reasonable
agreement is found for a proposed approximation of the memory effects by a
finite duration of collisions. The formation of correlations and the build up
time is calculated analytically for the high temperature and the low
temperature limit. This translates into a time dependent increase of the
effective temperature on time scales which interfere with standard fire ball
scenarios of heavy ion collisions. The consequences of the...
141.
