21,833 research outputs found
Single-particle potential in a chiral approach to nuclear matter including short range NN-terms
We extend a recent chiral approach to nuclear matter of Lutz et al. [Phys.
Lett. B474 (2000) 7] by calculating the underlying (complex-valued)
single-particle potential U(p,k_f) + i W(p,k_f). The potential for a nucleon at
the bottom of the Fermi-sea, U(0,k_{f0})= - 20.0 MeV, comes out as much too
weakly attractive in this approach. Even more seriously, the total
single-particle energy does not rise monotonically with the nucleon momentum p,
implying a negative effective nucleon mass at the Fermi-surface. Also, the
imaginary single-particle potential, W(0,k_{f0}) = 51.1 MeV, is too large. More
realistic single-particle properties together with a good nuclear matter
equation of state can be obtained if the short range contributions of
non-pionic origin are treated in mean-field approximation (i.e. if they are not
further iterated with 1pi-exchange). We also consider the equation of state of
pure neutron matter and the asymmetry energy A(k_f) in that
approach. The downward bending of these quantities above nuclear matter
saturation density seems to be a generic feature of perturbative chiral
pion-nucleon dynamics.Comment: 12 pages, 7 figures, submitted to Physical Review
From Caenorhabditis elegans to the Human Connectome: A Specific Modular Organisation Increases Metabolic, Functional, and Developmental Efficiency
The connectome, or the entire connectivity of a neural system represented by
network, ranges various scales from synaptic connections between individual
neurons to fibre tract connections between brain regions. Although the
modularity they commonly show has been extensively studied, it is unclear
whether connection specificity of such networks can already be fully explained
by the modularity alone. To answer this question, we study two networks, the
neuronal network of C. elegans and the fibre tract network of human brains
yielded through diffusion spectrum imaging (DSI). We compare them to their
respective benchmark networks with varying modularities, which are generated by
link swapping to have desired modularity values but otherwise maximally random.
We find several network properties that are specific to the neural networks and
cannot be fully explained by the modularity alone. First, the clustering
coefficient and the characteristic path length of C. elegans and human
connectomes are both higher than those of the benchmark networks with similar
modularity. High clustering coefficient indicates efficient local information
distribution and high characteristic path length suggests reduced global
integration. Second, the total wiring length is smaller than for the
alternative configurations with similar modularity. This is due to lower
dispersion of connections, which means each neuron in C. elegans connectome or
each region of interest (ROI) in human connectome reaches fewer ganglia or
cortical areas, respectively. Third, both neural networks show lower
algorithmic entropy compared to the alternative arrangements. This implies that
fewer rules are needed to encode for the organisation of neural systems
Chiral -exchange NN-potentials: Two-loop contributions
We calculate in heavy baryon chiral perturbation theory the local
NN-potentials generated by the two-pion exchange diagrams at two-loop order. We
give explicit expressions for the mass-spectra (or imaginary parts) of the
corresponding isoscalar and isovector central, spin-spin and tensor
NN-amplitudes. We find from two-loop two-pion exchange a sizeable isoscalar
central repulsion which amounts to MeV at fm. There is a
similarly strong isovector central attraction which however originates mainly
from the third order low energy constants entering the chiral -scattering amplitude. We also evaluate the one-loop -exchange diagram
with two second order chiral -vertices proportional to the low
energy constants as well as the first relativistic 1/M-correction
to the -exchange diagrams with one such vertex. The diagrammatic results
presented here are relevant components of the chiral NN-potential at
next-to-next-to-next-to-leading order.Comment: 6 pages, 2 figure
Nuclear energy density functional from chiral pion-nucleon dynamics
We calculate the nuclear energy density functional relevant for N=Z even-even
nuclei in the systematic framework of chiral perturbation theory. The
calculation includes the one-pion exchange Fock diagram and the iterated
one-pion exchange Hartree and Fock diagrams. From these few leading order
contributions in the small momentum expansion one obtains already a very good
equation of state of isospin symmetric nuclear matter. We find that in the
region below nuclear matter saturation density the effective nucleon mass
deviates by at most 15% from its free space value ,
with for and
for higher densities. The parameterfree strength of
the -term, , is at saturation density
comparable to that of phenomenological Skyrme forces. The magnitude of
accompanying the squared spin-orbit density comes out
somewhat larger. The strength of the nuclear spin-orbit interaction,
, as given by iterated one-pion exchange is about half as large as
the corresponding empirical value, however, with the wrong negative sign. The
novel density dependencies of and
as predicted by our parameterfree calculation should be examined in nuclear
structure calculations (after introducing an additional short range spin-orbit
contribution constant in density).Comment: 16 pages, 5 figure
Quality of life in first-admitted schizophrenia patients: a follow-up study
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Chiral -exchange NN-potentials: Results for diagrams proportional to g_A^4 and g_A^6
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams proportional to g_A^4 and g_A^6.
Surprisingly, we find that the total isoscalar central -exchange
potential vanishes identically. The individually largest -exchange
potentials are of isoscalar spin-spin, isovector central and isoscalar tensor
type. For these potentials simple analytical expressions can be given. The
strength of these dominant -exchange potentials at r=1.0 fm is 4.6 MeV,
2.9 MeV and 1.4 MeV, respectively. Furthermore, we observe that the spin-spin
and tensor potentials due to the diagrams proportional to g_A^6 do not exist in
the infinite nucleon mass limit.Comment: 8 pages, 5 figure
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