1,500 research outputs found
Videoconferencing via satellite. Opening Congress to the people: Technical report
The feasibility of using satellite videoconferencing as a mechanism for informed dialogue between Congressmen and constituents to strengthen the legislative process was evaluated. Satellite videoconferencing was defined as a two-way interactive television with the TV signals transmitted by satellite. With videoconferencing, one or more Congressmen in Washington, D. C. can see, hear and talk with groups of citizens at distant locations around the country. Simultaneously, the citizens can see, hear and talk with the Congressmen
Spin distribution of nuclear levels using static path approximation with random-phase approximation
We present a thermal and quantum-mechanical treatment of nuclear rotation
using the formalism of static path approximation (SPA) plus random-phase
approximation (RPA). Naive perturbation theory fails because of the presence of
zero-frequency modes due to dynamical symmetry breaking. Such modes lead to
infrared divergences. We show that composite zero-frequency excitations are
properly treated within the collective coordinate method. The resulting
perturbation theory is free from infrared divergences. Without the assumption
of individual random spin vectors, we derive microscopically the spin
distribution of the level density. The moment of inertia is thereby related to
the spin-cutoff parameter in the usual way. Explicit calculations are performed
for 56^Fe; various thermal properties are discussed. In particular, we
demonstrate that the increase of the moment of inertia with increasing
temperature is correlated with the suppression of pairing correlations.Comment: 12 pages, 8 figures, accepted for publication in Physical Review
Quark Condensate in the Deuteron
We study the changes produced by the deuteron on the QCD quark condensate by
means the Feynman-Hellmann theorem and find that the pion mass dependence of
the pion-nucleon coupling could play an important role. We also discuss the
relation between the many body effect of the condensate and the meson exchange
currents, as seen by photons and pions. For pion probes, the many-body term in
the physical amplitude differs significantly from that of soft pions, the one
linked to the condensate. Thus no information about the many-body term of the
condensate can be extracted from the pion-deuteron scattering length. On the
other hand, in the Compton amplitude, the relationship with the condensate is a
more direct one.Comment: to appear in Physics Review C (19 pages, 3 figures
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.
Determination of the pion-nucleon coupling constant and scattering lengths
We critically evaluate the isovector GMO sum rule for forward pion-nucleon
scattering using the recent precision measurements of negatively charged
pion-proton and pion-deuteron scattering lengths from pionic atoms. We deduce
the charged-pion-nucleon coupling constant, with careful attention to
systematic and statistical uncertainties. This determination gives, directly
from data a pseudoscalar coupling constant of
14.11+-0.05(statistical)+-0.19(systematic) or a pseudovector one of 0.0783(11).
This value is intermediate between that of indirect methods and the direct
determination from backward neutron-proton differential scattering cross
sections. We also use the pionic atom data to deduce the coherent symmetric and
antisymmetric sums of the negatively charged pion-proton and pion-neutron
scattering lengths with high precision. The symmetric sum gives
0.0012+-0.0002(statistical)+-0.0008 (systematic) and the antisymmetric one
0.0895+-0.0003(statistical)+-0.0013(systematic), both in units of inverse
charged pion-mass. For the need of the present analysis, we improve the
theoretical description of the pion-deuteron scattering length.Comment: 27 pages, 5 figures, submitted to Phys. Rev. C, few modifications and
clarifications, no change in substance of the pape
Multiscale fluctuations in nuclear response
The nuclear collective response is investigated in the framework of a doorway
picture in which the spreading width of the collective motion is described as a
coupling to more and more complex configurations. It is shown that this
coupling induces fluctuations of the observed strength. In the case of a
hierarchy of overlapping decay channels, we observe Ericson fluctuations at
different scales. Methods for extracting these scales and the related lifetimes
are discussed. Finally, we show that the coupling of different states at one
level of complexity to some common decay channels at the next level, may
produce interference-like patterns in the nuclear response. This quantum effect
leads to a new type of fluctuations with a typical width related to the level
spacing.Comment: 34 Latex pages including 6 figures (submitted to Phys. Rev. C
Charge-Asymmetry of the Nucleon-Nucleon Interaction
Based upon the Bonn meson-exchange model for the nucleon-nucleon ()
interaction, we study systematically the charge-symmetry-breaking (CSB) of the
interaction due to nucleon mass splitting. Particular attention is payed
to CSB generated by the -exchange contribution to the interaction,
diagrams, and other multi-meson-exchanges. We calculate the CSB
differences in the effective range parameters as well as phase shift
differences in , and higher partial waves up to 300 MeV lab. energy. We
find a total CSB difference in the singlet scattering length of 1.6 fm which
explains the empirical value accurately. The corresponding CSB phase-shift
differences are appreciable at low energy in the state. In the other
partial waves, the CSB splitting of the phase shifts is small and increases
with energy, with typical values in the order of 0.1 deg at 300 MeV in and
waves.Comment: 11 pages, RevTex, 14 figure
Unitarity constraint for threshold coherent pion photoproduction on the deuteron and chiral perturbation theory
The contribution of the two-step process gamma + d -> p + n -> pi0 + d to the
imaginary part of the amplitude for coherent pion production on the deuteron is
calculated exploiting unitarity constraints. The result shows that this
absorptive process is not negligible and has to be considered in an extraction
of the elementary neutron production amplitude from the gamma + d -> pi0 + d
cross section at threshold. In addition, it is argued that a consistent
calculation of gamma + d -> pi0 + d in baryon chiral perturbation theory beyond
next-to-leading order requires the inclusion of this absorptive process.Comment: 11 pages revtex including 2 postscript figure
Isoscalar off-shell effects in threshold pion production from pd collisions
We test the presence of pion-nucleon isoscalar off-shell effects in the
reaction around the threshold region. We find that these
effects significantly modify the production cross section and that they may
provide the missing strength needed to reproduce the data at threshold.Comment: 6 pages, REVTeX, twocolumn, including 3 figures (Postscript), uses
psfig, updated and extended versio
Correlation Between the Deuteron Characteristics and the Low-energy Triplet np Scattering Parameters
The correlation relationship between the deuteron asymptotic normalization
constant, , and the triplet np scattering length, , is
investigated. It is found that 99.7% of the asymptotic constant is
determined by the scattering length . It is shown that the linear
correlation relationship between the quantities and
provides a good test of correctness of various models of nucleon-nucleon
interaction. It is revealed that, for the normalization constant and
for the root-mean-square deuteron radius , the results obtained with the
experimental value recommended at present for the triplet scattering length
are exaggerated with respect to their experimental counterparts. By
using the latest experimental phase shifts of Arndt et al., we obtain, for the
low-energy scattering parameters (, , ) and for the
deuteron characteristics (, ), results that comply well with
experimental data.Comment: 19 pages, 1 figure, To be published in Physics of Atomic Nucle
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