267 research outputs found
New Forms of Deuteron Equations and Wave Function Representations
A recently developed helicity basis for nucleon-nucleon (NN) scattering is
applied to th e deuteron bound state. Here the total spin of the deuteron is
treated in such a helicity representation. For the bound state, two sets of two
coupled eigenvalue equations are developed, where the amplitudes depend on two
and one variable, respectively. Numerical illustrations based on the realistic
Bonn-B NN potential are given. In addition, an `operator form' of the deuteron
wave function is presented, and several momentum dependent spin densities are
derived and shown, in which the angular dependence is given analytically.Comment: 19 pages (Revtex), 9 fig
The alpha-particle in nuclear matter
Among the light nuclear clusters the alpha-particle is by far the strongest
bound system and therefore expected to play a significant role in the dynamics
of nuclei and the phases of nuclear matter. To systematically study the
properties of the alpha-particle we have derived an effective four-body
equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant
medium effects, i.e. self energy corrections and Pauli-blocking in a consistent
way. The equation is solved utilizing the energy dependent pole expansion for
the sub system amplitudes. We find that the Mott transition of an
alpha-particle at rest differs from that expected from perturbation theory and
occurs at approximately 1/10 of nuclear matter densities.Comment: 9 pages RevTex file, 1 figure, submitted to Phys. Lett.
Light clusters in nuclear matter of finite temperature
We investigate properties and the distribution of light nuclei (A<4) in
symmetric nuclear matter of finite temperature within a microscopic framework.
For this purpose we have solved few-body Alt-Grassberger-Sandhas type equations
for quasi-nucleons that include self-energy corrections and Pauli blocking in a
systematic way. In a statistical model we find a significant influence in the
composition of nuclear matter if medium effects are included in the microscopic
calculation of nuclei. If multiplicities are frozen out at a certain time (or
volume), we expect significant consequences for the formation of light
fragments in a heavy ion collision. As a consequence of the systematic
inclusion of medium effects the ordering of multiplicities becomes opposite to
the law of mass action of ideal components. This is necessary to explain the
large abundance of -particles in a heavy ion collision that are
otherwise largely suppressed in an ideal equilibrium scenario.Comment: 9 pages, 9 figures, epja-style file
Low-momentum effective interaction in the three-dimensional approach
The formulation of the low-momentum effective interaction in the model space
Lee-Suzuki and the renormalization group methods is implemented in the
three-dimensional approach. In this approach the low-momentum effective
interaction V_{low k} has been formulated as a function of the magnitude of
momentum vectors and the angle between them. As an application the spin-isospin
independent Malfliet-Tjon potential has been used into the model space
Lee-Suzuki method and it has been shown that the low-momentum effective
interaction V_{low k} reproduces the same two-body observables obtained by the
bare potential V_{NN}.Comment: 15 pages, 5 eps figure
New Exact Solutions of a Generalized Shallow Water Wave Equation
In this work an extended elliptic function method is proposed and applied to
the generalized shallow water wave equation. We systematically investigate to
classify new exact travelling wave solutions expressible in terms of
quasi-periodic elliptic integral function and doubly-periodic Jacobian elliptic
functions. The derived new solutions include rational, periodic, singular and
solitary wave solutions. An interesting comparison with the canonical procedure
is provided. In some cases the obtained elliptic solution has singularity at
certain region in the whole space. For such solutions we have computed the
effective region where the obtained solution is free from such a singularity.Comment: A discussion about singularity and some references are added. To
appear in Physica Script
Is there a Pronounced Giant Dipole Resonance in ^4He?
A four-nucleon calculation of the total ^4He photodisintegration cross
section is performed. The full final-state interaction is taken into account
for the first time. This is achieved via the method of the Lorentz integral
transform. Semi-realistic NN interactions are employed. Different from the
known partial two-body ^4He(\gamma,n)^3He and ^4He(\gamma,p)^3H cross sections
our total cross section exhibits a pronounced giant resonance. Thus, in
contrast to older data, we predict quite a strong contribution of
the channel at the giant resonance peak energy.Comment: 10 pages, Latex (REVTEX), 4 Postscript figures, to appear in Phys.
Rev. Let
Total Photoabsorption Cross Sections of A=6 Nuclei with Complete Final State Interaction
The total photoabsorption cross sections of 6He and 6Li are calculated
microscopically with full inclusion of the six-nucleon final state interaction
using semirealistic nucleon-nucleon potentials. The Lorentz Integral Transform
(LIT) method and the effective interaction approach for the hyperspherical
formalism are employed. While 6Li has a single broad giant resonance peak,
there are two well separated peaks for 6He corresponding to the breakup of the
neutron halo and the alpha core, respectively. The comparison with the few
available experimental data is discussed.Comment: LaTeX, 8 pages, 3 ps figure
Sub-threshold resonances in few-neutron systems
Three- and four-neutron systems are studied within the framework of the
hyperspherical approach with a local S-wave nn-potential. Possible bound and
resonant states of these systems are sought as zeros of three- and four-body
Jost functions in the complex momentum plane. It is found that zeros closest to
the origin correspond to sub-threshold (nnn) (1/2-) and (nnnn) (0+) resonant
states. The positions of these zeros turned out to be sensitive to the choice
of the --potential. For the Malfliet- Tjon potential they are
E(nnn)=-4.9-i6.9 (MeV) and E(nnnn)=-2.6-i9.0 (MeV). Movement of the zeros with
an artificial increase of the potential strength also shows an extreme
sensitivity to the choice of potential. Thus, to generate ^3n and ^4n bound
states, the Yukawa potential needs to be multiplied by 2.67 and 2.32
respectively, while for the Malfliet-Tjon potential the required multiplicative
factors are 4.04 and 3.59.Comment: Latex, 22 pages, no PS-figures, submitted to J.Phys.
Cerebral blood flow and heart rate variability in chronic fatigue syndrome : a randomized cross-over study
Background: Pain, fatigue, and concentration difficulties are typical features of chronic fatigue syndrome (CFS). The exact underlying mechanisms of these symptoms are still unknown, but available evidence suggests an important role for impaired pain modulation. As evidence also suggests that pain modulation is related to cardiovascular mechanisms, it seems logical to investigate whether cerebral blood flow (CBF) and heart rate variability (HRV) are altered in these patients.
Objectives: We aimed to investigate the role of the cardiovascular system in pain modulation and symptoms of CFS; the response of CBF and HRV to physical stress and their relation to the change in temporal summation (TS) of pressure pain and self-reported symptoms was evaluated.
Study Design: A controlled, randomized cross-over trial.
Setting: University Hospital Brussels.
Methods: Twenty CFS patients and 20 sedentary healthy controls were included in this study. In both of the groups, the change in TS of pressure pain, CBF (using transcranial Doppler), and HRV (using finger plethysmography) was examined during physical and emotional stress (to control for potential bias), as well as their association mutually and with self-reported symptoms of pain, fatigue, and concentrations difficulties.
Results: There was no significant interaction or group (F-values ranging from .100 to 1.862, P-values ranging from .754 to .181) effect in CBF or HRV parameters. HRV and CBF did change during physical exercise, but the changes did not differ between patients and controls. While pain scores during TS at the trapezius site reduced in the control group after the physical exercise protocol (P=.037), they did not change in the CFS group (P=.108), suggesting impaired pain modulation. There were no significant correlations between CBF, HRV, TS, and self-reported symptoms (all P-values of correlation analyses > .01).
Limitations: Although effect sizes were medium to large, the study sample was relatively low. Also, the mild nature of the exercise bout is discussable. Nonetheless, this mild exercise was able to provoke endogenous pain modulation in the control group, which endorsed a proper execution of the cycling exercise. Moreover, mild exercises are more applicable to daily physical activities in CFS patients than vigorous exercises.
Conclusion: These results seem to refute the previously suggested alterations of CBF/HRV in CFS patients. These cardiovascular parameters appear not to explain pain before, during, and following exercise
Instant Two-Body Equation in Breit Frame
A quasipotential formalism for elastic scattering from relativistic bound
states is based on applying an instant constraint to both initial and final
states in the Breit frame. This formalism is advantageous for the analysis of
electromagnetic interactions because current conservation and four momentum
conservation are realized within a three-dimensional formalism. Wave functions
are required in a frame where the total momentum is nonzero, which means that
the usual partial wave analysis is inapplicable. In this work, the
three-dimensional equation is solved numerically, taking into account the
relevant symmetries. A dynamical boost of the interaction also is needed for
the instant formalism, which in general requires that the boosted interaction
be defined as the solution of a four-dimensional equation. For the case of a
scalar separable interaction, this equation is solved and the Lorentz
invariance of the three-dimensional formulation using the boosted interaction
is verified. For more realistic interactions, a simple approximation is used to
characterize the boost of the interaction.Comment: 20 pages in revtex 3, 3 figures. Fixed reform/tex errors
- …