391 research outputs found
Primitive roles for inhibitory interneurons in developing frog spinal cord
Understanding the neuronal networks in the mammal spinal cord is hampered by the diversity of neurons and their connections. The simpler networks in developing lower vertebrates may offer insights into basic organization. To investigate the function of spinal inhibitory interneurons in Xenopus tadpoles, paired whole-cell recordings were used. We show directly that one class of interneuron, with distinctive anatomy, produces glycinergic, negative feedback inhibition that can limit firing in motoneurons and interneurons of the central pattern generator during swimming. These same neurons also produce inhibitory gating of sensory pathways during swimming. This discovery raises the possibility that some classes of interneuron, with distinct functions later in development, may differentiate from an earlier class in which these functions are shared. Preliminary evidence suggests that these inhibitory interneurons express the transcription factor engrailed, supporting a probable homology with interneurons in developing zebrafish that also express engrailed and have very similar anatomy and functions
Chiral phase transition at high temperature in the QCD-like gauge theory
The chiral phase transition at high temperature is investigated using the
effect ive potential in the framework of the QCD-like gauge theory with a
variational a pproach. We have a second order phase transition at MeV.
We also investigate numerically the temperature dependence of condensate,
a nd (coefficient of the quadratic term in the effective
potential) and es timate the critical exponents of these quantities.Comment: 12 pages,7 figure
Non-trivial Infrared Structure in (2+1)-dimensional Quantum Electrodynamics
We show that the gauge-fermion interaction in multiflavour
-dimensional quantum electrodynamics with a finite infrared cut-off is
responsible for non-fermi liquid behaviour in the infrared, in the sense of
leading to the existence of a non-trivial fixed point at zero momentum, as well
as to a significant slowing down of the running of the coupling at intermediate
scales as compared with previous analyses on the subject. Both these features
constitute deviations from fermi-liquid theory. Our discussion is based on the
leading- resummed solution for the wave-function renormalization of the
Schwinger-Dyson equations . The present work completes and confirms the
expectations of an earlier work by two of the authors (I.J.R.A. and N.E.M.) on
the non-trivial infrared structure of the theory.Comment: 10 pages (LaTex), 5 figures (Postscript
Current quark mass effects on chiral phase transition of QCD in the improved ladder approximation
Current quark mass effects on the chiral phase transition of QCD is studied
in the improved ladder approximation. An infrared behavior of the gluon
propagator is modified in terms of an effective running coupling. The analysis
is based on a composite operator formalism and a variational approach. We use
the Schwinger-Dyson equation to give a ``normalization condition'' for the
Cornwall-Jackiw-Tomboulis effective potential and to isolate the ultraviolet
divergence which appears in an expression for the quark-antiquark condensate.
We study the current quark mass effects on the order parameter at zero
temperature and density. We then calculate the effective potential at finite
temperature and density and investigate the current quark mass effects on the
chiral phase transition. We find a smooth crossover for , and a
first-order phase transition for , T=0. Critical exponents are also
studied and our model gives the classical mean-field values. We also study the
temperature dependence of masses of scalar and pseudoscalar bosons. A critical
end point in the - plane is found at MeV,
MeV.Comment: 19 pages, 13 figure
Quantum cosmology in the models of 2d and 4d dilatonic supergravity with WZ matter
We consider N=1 two-dimensional (2d) dilatonic supergravity (SG), 2d
dilatonic SG obtained by dimensional reduction from N=1 four-dimensional (4d)
SG, N=2 2d dilatonic SG and string-inspired 4d dilatonic SG. For all the
theories, the corresponding action on a bosonic background is constructed and
the interaction with (dilatonic) Wess-Zumino (WZ) multiplets is presented.
Working in the large-N approximation, it is enough to consider the trace
anomaly induced effective action due to dilaton-coupled conformal matter as a
quantum correction (for 2d models s-waves approximation is additionally used).
The equations of motion for all such models with quantum corrections are
written in a form convenient for numerical analysis. Their solutions are
numerically investigated for 2d and 4d Friedmann-Robertson-Walker (FRW) or 4d
Kantowski-Sacks Universes with a time-dependent dilaton via exponential dilaton
coupling. The evolution of the corresponding quantum cosmological models is
given for different choices of initial conditions and theory parameters. In
most cases we find quantum singular Universes. Nevertheless, there are examples
of Universe non-singular at early times. Hence, it looks unlikely that quantum
matter back reaction on dilatonic background (at least in large
approximation) may really help to solve the singularity problem.Comment: LaTeX file of the text (36 pages) and 3 ps files of 14 figures, few
misprints are corrected and references adde
QCD S Parameter from Inhomogeneous Bethe-Salpeter Equation
We calculate the low-energy parameter S in QCD, which is also known as
L_{10}, and the pion decay constant f_\pi using inhomogeneous Bethe-Salpeter
equation in improved ladder approximation. To extract these quantities we
calculate the ``V-A'' two-point function, \Pi_{VV}(q^2) - \Pi_{AA}(q^2), in
space-like region. We obtain S = 0.43 \sim 0.48, which is about 30% larger than
the experimental value. The calculated f_\pi is well consistent with the result
by solving the homogeneous Bethe-Salpeter equation for pion. We also evaluate
parameter in SU(3) gauge theory with N_D doublets of fermions in connection
with walking technicolor model, and find that the value of S/N_D hardly depends
on N_D.Comment: 22 pages (LaTeX), 6 PostScript figures are included as
uuencoded-compressed-tar file at the end (need 'epsf.tex' macro package),
KUNS-1270 HE(TH)94/0
D-branes in Generalized Geometry and Dirac-Born-Infeld Action
The purpose of this paper is to formulate the Dirac-Born-Infeld (DBI) action
in a framework of generalized geometry and clarify its symmetry. A D-brane is
defined as a Dirac structure where scalar fields and gauge field are treated on
an equal footing in a static gauge. We derive generalized Lie derivatives
corresponding to the diffeomorphism and B-field gauge transformations and show
that the DBI action is invariant under non-linearly realized symmetries for all
types of diffeomorphisms and B-field gauge transformations. Consequently, we
can interpret not only the scalar field but also the gauge field on the D-brane
as the generalized Nambu-Goldstone boson.Comment: 32 pages, 4 figures, ver2:typos corrected, references adde
Chiral Symmetry Restoration at Finite Temperature and Chemical Potential in the Improved Ladder Approximation
The chiral symmetry of QCD is studied at finite temperature and chemical
potential using the Schwinger-Dyson equation in the improved ladder
approximation. We calculate three order parameters; the vacuum expectation
value of the quark bilinear operator, the pion decay constant and the quark
mass gap. We have a second order phase transition at the temperature
MeV along the zero chemical potential line, and a first order phase transition
at the chemical potential MeV along the zero temperature line. We
also calculate the critical exponents of the three order parameters.Comment: 16 pages + 10 uuencoded eps figures, LaTe
Calculation of the Chiral Lagrangian Coefficients from the Underlying Theory of QCD: A Simple Approach
We calculate the coefficients in the chiral Lagrangian approximately from QCD
based on a previous study of deriving the chiral Lagrangian from the first
principles of QCD in which the chiral Lagrangian coefficients are defined in
terms of certain Green's functions in QCD. We first show that, in the large
N(c)-limit, the anomaly part contributions to the coefficients are exactly
cancelled by certain terms in the normal part contributions, and the final
results of the coefficients only concern the remaining normal part
contributions depending on QCD interactions. We then do the calculation in a
simple approach with the approximations of taking the large-N(c) limit, the
leading order in dynamical perturbation theory, and the improved ladder
approximation, thereby the relevant Green's functions are expressed in terms of
the quark self energy. By solving the Schwinger-Dyson equation for the quark
self energy, we obtain the approximate QCD predicted coefficients and the quark
condensate which are consistent with the experimental values.Comment: Further typos corrected, to appear in Phys. Rev.
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator?
We study a model Dyson-Schwinger equation for the quark propagator closed
using an {\it Ansatz} for the gluon propagator of the form \mbox{} and two {\it Ans\"{a}tze} for the quark-gluon vertex: the
minimal Ball-Chiu and the modified form suggested by Curtis and Pennington.
Using the quark condensate as an order parameter, we find that there is a
critical value of such that the model does not support dynamical chiral
symmetry breaking for . We discuss and apply a confinement test which
suggests that, for all values of , the quark propagator in the model {\bf is
not} confining. Together these results suggest that this Ansatz for the gluon
propagator is inadequate as a model since it does not yield the expected
behaviour of QCD.Comment: 21 Pages including 4 PostScript figures uuencoded at the end of the
file. Replacement: slight changes of wording and emphasis. ADP-93-215/T133,
ANL-PHY-7599-TH-93, FSU-SCRI-93-108, REVTEX 3.
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