644 research outputs found
Periaxonal K+ regulation in the small squid Alloteuthis. Studies on isolated and in situ axons
A novel giant axon preparation from the squid Alloteuthis is described. Properties of in situ and isolated axons are similar. Periaxonal K+ accumulation is a function of the physiological state of the animal and of the axon and its sheathing layers. Carefully dissected isolated axons, and axons in situ in a healthy mantle, show much less K+ accumulation than previously reported in squid. It is suggested that the Schwann cells are involved in the observed K+ regulation
Point singularities and suprathreshold stochastic resonance in optimal coding
Motivated by recent studies of population coding in theoretical neuroscience,
we examine the optimality of a recently described form of stochastic resonance
known as suprathreshold stochastic resonance, which occurs in populations of
noisy threshold devices such as models of sensory neurons. Using the mutual
information measure, it is shown numerically that for a random input signal,
the optimal threshold distribution contains singularities. For large enough
noise, this distribution consists of a single point and hence the optimal
encoding is realized by the suprathreshold stochastic resonance effect.
Furthermore, it is shown that a bifurcational pattern appears in the optimal
threshold settings as the noise intensity increases. Fisher information is used
to examine the behavior of the optimal threshold distribution as the population
size approaches infinity.Comment: 11 pages, 3 figures, RevTe
Searching for Leptoquarks in electron-photon Collisions
We study the production of composite scalar leptoquarks in
colliders, and we show that an machine operating in its mode
is the best way to look for these particles in collisions, due to the
hadronic content of the photon.Comment: 12 pages in REVTeX3. 6 figures appended as postcript files. Report:
IFT-P.014/93 and IFUSP-P 104
Heavy Quark production at the TEVATRON and HERA using k_t-factorization with CCFM evolution
The application of k_t-factorization supplemented with the CCFM small-x
evolution equation to heavy quark production at the TEVATRON and at HERA is
discussed. The bb_bar production cross sections at the TEVATRON can be
consistently described using the k_t-factorization formalism together with the
unintegrated gluon density obtained within the CCFM evolution approach from a
fit to HERA F_2 data. Special attention is drawn to the comparison with
measured visible cross sections, which are compared to the hadron level Monte
Carlo generator CASCADEComment: 13 pages, updated references and updated Fig. 9, Fig. 2 replace
The Sachs-Wolfe Effect: Gauge Independence and a General Expression
In this paper we address two points concerning the Sachs-Wolfe effect: (i)
the gauge independence of the observable temperature anisotropy, and (ii) a
gauge-invariant expression of the effect considering the most general situation
of hydrodynamic perturbations. The first result follows because the gauge
transformation of the temperature fluctuation at the observation event only
contributes to the isotropic temperature change which, in practice, is absorbed
into the definition of the background temperature. Thus, we proceed without
fixing the gauge condition, and express the Sachs-Wolfe effect using the
gauge-invariant variables.Comment: 5 pages, closer to published versio
Prospects for CP-violation searches in the future experiment with RF-separated K+/- beam at U-70
The first description of the experimental program "OKA" with RF-separated
K+/- beam at Protvino U-70 PS is presented. The parameters of the beam as well
as the potential for CP-violation searches are discussed.Comment: 8 pages, 6 figures, submitted to proceedings of International
Conference on CP Violation, Sep 18-22, 2000, Ital
Gravitons and Lightcone Fluctuations
Gravitons in a squeezed vacuum state, the natural result of quantum creation
in the early universe or by black holes, will introduce metric fluctuations.
These metric fluctuations will introduce fluctuations of the lightcone. It is
shown that when the various two-point functions of a quantized field are
averaged over the metric fluctuations, the lightcone singularity disappears for
distinct points. The metric averaged functions remain singular in the limit of
coincident points. The metric averaged retarded Green's function for a massless
field becomes a Gaussian which is nonzero both inside and outside of the
classical lightcone. This implies some photons propagate faster than the
classical light speed, whereas others propagate slower. The possible effects of
metric fluctuations upon one-loop quantum processes are discussed and
illustrated by the calculation of the one-loop electron self-energy.Comment: 18pp, LATEX, TUTP-94-1
Effective lagrangian for the tbH^+ interaction in the MSSM and charged Higgs phenomenology
In the framework of a 2HDM effective lagrangian for the MSSM, we analyse
important phenomenological aspects associated with quantum soft SUSY-breaking
effects that modify the relation between the bottom mass and the bottom Yukawa
coupling. We derive a resummation of the dominant supersymmetric corrections
for large values of \tb to all orders in perturbation theory. With the help of
the operator product expansion we also perform the resummation of the leading
and next-to-leading logarithms of the standard QCD corrections. We use these
resummation procedures to compute the radiative corrections to the \tbH, \Htb
decay rates. In the large \tb regime, we derive simple formulae embodying all
the dominant contributions to these decay rates and we compute the
corresponding branching ratios. We show, as an example, the effect of these new
results on determining the region of the \mH--\tb plane excluded by the
Tevatron searches for a supersymmetric charged Higgs boson in top quark decays,
as a function of the MSSM parameter space.Comment: 33 pages, LaTeX, 17 figures, revised version submitted to Nuc. Phys.
Improved tests of extra-dimensional physics and thermal quantum field theory from new Casimir force measurements
We report new constraints on extra-dimensional models and other physics
beyond the Standard Model based on measurements of the Casimir force between
two dissimilar metals for separations in the range 0.2--1.2 m. The Casimir
force between an Au-coated sphere and a Cu-coated plate of a
microelectromechanical torsional oscillator was measured statically with an
absolute error of 0.3 pN. In addition, the Casimir pressure between two
parallel plates was determined dynamically with an absolute error of mPa. Within the limits of experimental and theoretical errors, the results
are in agreement with a theory that takes into account the finite conductivity
and roughness of the two metals. The level of agreement between experiment and
theory was then used to set limits on the predictions of extra-dimensional
physics and thermal quantum field theory. It is shown that two theoretical
approaches to the thermal Casimir force which predict effects linear in
temperture are ruled out by these experiments. Finally, constraints on Yukawa
corrections to Newton's law of gravity are strengthened by more than an order
of magnitude in the range 56 nm to 330 nm.Comment: Revtex 4, 35 pages, 14 figures in .gif format, accepted for
publication in Phys. Rev.
Nuclear transparency from quasielastic A(e,e'p) reactions uo to Q^2=8.1 (GeV/c)^2
The quasielastic (e,ep) reaction was studied on targets of
deuterium, carbon, and iron up to a value of momentum transfer of 8.1
(GeV/c). A nuclear transparency was determined by comparing the data to
calculations in the Plane-Wave Impulse Approximation. The dependence of the
nuclear transparency on and the mass number was investigated in a
search for the onset of the Color Transparency phenomenon. We find no evidence
for the onset of Color Transparency within our range of . A fit to the
world's nuclear transparency data reflects the energy dependence of the free
proton-nucleon cross section.Comment: 11 pages, 6 figure
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