2,451 research outputs found
Model Checking Tap Withdrawal in C. Elegans
We present what we believe to be the first formal verification of a
biologically realistic (nonlinear ODE) model of a neural circuit in a
multicellular organism: Tap Withdrawal (TW) in \emph{C. Elegans}, the common
roundworm. TW is a reflexive behavior exhibited by \emph{C. Elegans} in
response to vibrating the surface on which it is moving; the neural circuit
underlying this response is the subject of this investigation. Specifically, we
perform reachability analysis on the TW circuit model of Wicks et al. (1996),
which enables us to estimate key circuit parameters. Underlying our approach is
the use of Fan and Mitra's recently developed technique for automatically
computing local discrepancy (convergence and divergence rates) of general
nonlinear systems. We show that the results we obtain are in agreement with the
experimental results of Wicks et al. (1995). As opposed to the fixed parameters
found in most biological models, which can only produce the predominant
behavior, our techniques characterize ranges of parameters that produce (and do
not produce) all three observed behaviors: reversal of movement, acceleration,
and lack of response
Collisional Energy Loss of Non Asymptotic Jets in a QGP
We calculate the collisional energy loss suffered by a heavy (charm) quark
created at a finite time within a Quark Gluon Plasma (QGP) in the classical
linear response formalism as in Peigne {\it et al.} \cite{peigne}. We pay close
attention to the problem of formulating a suitable current and the isolation of
binding and radiative energy loss effects. We find that unrealistic large
binding effects arising in previous formulations must be subtracted. The finite
time correction is shown to be important only for very short length scales on
the order of a Debye length. The overall energy loss is similar in magnitude to
the energy loss suffered by a charge created in the asymptotic past. This
result has significant implications for the relative contribution to energy
loss from collisional and radiative sources and has important ramifications for
the ``single electron puzzle'' at RHIC.Comment: 15 Pages, 11 figures, revte
Anisotropy of Imbalanced Alfvenic Turbulence in Fast Solar Wind
We present the first measurement of the scale-dependent power anisotropy of
Elsasser variables in imbalanced fast solar wind turbulence. The dominant
Elsasser mode is isotropic at lower spacecraft frequencies but becomes
increasingly anisotropic at higher frequencies. The sub-dominant mode is
anisotropic throughout, but in a scale-independent way (at higher frequencies).
There are two distinct subranges exhibiting different scalings within what is
normally considered the inertial range. The low Alfven ratio and shallow
scaling of the sub-dominant Elsasser mode suggest an interpretation of the
observed discrepancy between the velocity and magnetic field scalings. The
total energy is dominated by the latter. These results do not appear to be
fully explained by any of the current theories of incompressible imbalanced MHD
turbulence.Comment: 5 pages, 2 figure
Open heavy flavor production at RHIC
The study of heavy flavor production in relativistic heavy ion collisions is
an extreme experimental challenge but provides important information on the
properties of the Quark-Gluon Plasma (QGP) created in Au+Au collisions at RHIC.
Heavy-quarks are believed to be produced in the initial stages of the
collision, and are essential on the understanding of parton energy loss in the
dense medium created in such environment. Moreover, heavy-quarks can help to
investigate fundamental properties of QCD in elementary p+p collisions. In this
work we review recent results on heavy flavor production and their interaction
with the hot and dense medium at RHIC.Comment: Quark Matter 2006 proceedings, 8 pages, 5 figure
Scaling anisotropy of the power in parallel and perpendicular components of the solar wind magnetic field
Power spectra of the components of the magnetic field parallel (Pzz) and perpendicular (Pzz+Pyy) to the local mean magnetic field direction were determined by wavelet methods from Ulysses’ MAG instrument data during eighteen 10-day segments of its first North Polar pass at high latitude at solar minimum in 1995. The power depends on frequency f and the angle θ between the solar wind direction and the local mean field, and with distance from the Sun. This data includes the solar wind whose total power (Pxx + Pyy + Pzz) in magnetic fluctuations we previously reported depends on f and the angle θ nearly as predicted by the GS95 critical balance model of strong incompressible MHD turbulence. Results at much wider range of frequencies during six evenly-spaced 10-day periods are presented here to illustrate the variability and evolution with distance from the Sun. Here we investigate the aniso tropic scaling of Pzz(f,θ) in particular because it is a reduced form of the Poloidal (pseudo-Alfvenic) component of the (incompressible) fluctuations. We also report the much larger Pxx(f,θ)+Pyy(f,θ) which is (mostly) reduced from the Toroidal (Alfvenic, i.e., perpendicular to both B and k) fluctuations, and comprises most of the total power. These different components of the total power evolve and scale differently in the inertial range. We compare these elements of the magnetic power spectral tensor with “critical balance” model predictions
Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe
A polytropic process describes the transition of a fluid from one state to
another through a specific relationship between the fluid density and
temperature. The value of the polytropic index that governs this relationship
determines the heat transfer and the effective degrees of freedom during the
process. In this study, we analyze solar wind proton plasma measurements,
obtained by the Faraday cup instrument on-board Parker Solar Probe. We examine
the large-scale variations of the proton plasma density and temperature within
the inner heliosphere explored by the spacecraft. We also address a polytropic
behavior in the density and temperature fluctuations in short-time intervals,
which we analyze in order to derive the effective polytropic index of small
time-scale processes. The large-scale variations of the solar wind proton
density and temperature which are associated with the plasma expansion through
the heliosphere, follow a polytropic model with a polytropic index ~5/3. On the
other hand, the short time-scale fluctuations which may be associated with
turbulence, follow a model with a larger polytropic index. We investigate
possible correlations between the polytropic index of short time-scale
fluctuations and the plasma speed, plasma beta, and the magnetic field
direction. We discuss the scenario of mechanisms including energy transfer or
mechanisms that restrict the particle effective degrees of freedom.Comment: 20 pages, 9 figure
THREE-DIMENSIONAL STRUCTURE OF SOLAR WIND TURBULENCE
We present a measurement of the scale-dependent, three-dimensional structure of the magnetic field fluctuations in inertial range solar wind turbulence with respect to a local, physically motivated coordinate system. The Alfvenic fluctuations are three-dimensionally anisotropic, with the sense of this anisotropy varying from large to small scales. At the outer scale, the magnetic field correlations are longest in the local fluctuation direction, consistent with Alfven waves. At the proton gyroscale, they are longest along the local mean field direction and shortest in the direction perpendicular to the local mean field and the local field fluctuation. The compressive fluctuations are highly elongated along the local mean field direction, although axially symmetric perpendicular to it. Their large anisotropy may explain why they are not heavily damped in the solar wind
The Weights of Young African Children in a Township in Rhodesia
A CAJM article on the standard weight of Zimbabwean (Rhodesian) children vis-a-vis the avarage incomes of their parents.No standards of normal weight for young children exist for the African population in Rhodesia. Ford (1964) suggested that the Havard Standards (Stuart and Stevenson, 1959) could be used as a guide in assessing malnutrition in different ethnic groups when no local standards of reference are available. This idea has been further elaborated by Jelliffe (1966), who suggested a General Standard of Reference, which was derived from the Havard data. This study establishes a standard for African children in Rhodesia and also compares the weight-for-age curve of these young children of mixed genetic background with that derived from the Havard data
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