1,282 research outputs found
Interference Phenomena in Medium Induced Radiation
We consider the interference pattern for the medium-induced gluon radiation
produced by a color singlet quark-antiquark antenna embedded in a QCD medium
with size and `jet quenching' parameter . Within the BDMPS-Z
regime, we demonstrate that, for a dipole opening angle , the interference between the
medium--induced gluon emissions by the quark and the antiquark is suppressed
with respect to the direct emissions. This is so since direct emissions are
delocalized throughout the medium and thus yield contributions proportional to
while interference occurs only between emissions at early times, when both
sources remain coherent. Thus, for \tqq \gg\theta_c, the medium-induced
radiation is the sum of the two spectra individually produced by the quark and
the antiquark, without coherence effects like angular ordering. For \tqq
\ll\theta_c, the medium--induced radiation vanishes.Comment: 4 pages, 2 figures; Proceedings of the "Quark Matter 2011" conferenc
A New Mechanism of Quark Energy Loss
We show that a heavy quark moving sufficiently fast through a quark-gluon
plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this
takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The
energy loss is exactly calculable in these models despite being an
O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion
collision experiments.Comment: 4 pages, 4 figures; v2: plot modified, conclusions unchange
Pair correlation function of short-ranged square-well fluids
We have performed extensive Monte Carlo simulations in the canonical (NVT)
ensemble of the pair correlation function for square-well fluids with well
widths ranging from 0.1 to 1.0, in units of the diameter
of the particles. For each one of these widths, several densities and
temperatures in the ranges and
, where is the
critical temperature, have been considered. The simulation data are used to
examine the performance of two analytical theories in predicting the structure
of these fluids: the perturbation theory proposed by Tang and Lu [Y. Tang and
B. C.-Y. Lu, J. Chem. Phys. {\bf 100}, 3079, 6665 (1994)] and the
non-perturbative model proposed by two of us [S. B. Yuste and A. Santos, J.
Chem. Phys. {\bf 101}, 2355 (1994)]. It is observed that both theories
complement each other, as the latter theory works well for short ranges and/or
moderate densities, while the former theory does for long ranges and high
densities.Comment: 10 pages, 10 figure
Closing the circle of germline and stem cells: the Primordial Stem Cell hypothesis
BACKGROUND: Germline determination is believed to occur by either preformation or epigenesis. Animals that undergo germ cell specification by preformation have a continuous germline. However, animals with germline determination by epigenesis have a discontinuous germline, with somatic cells intercalated. This vision is contrary to August Weismann's Germ Plasm Theory and has led to several controversies. Recent data from metazoans as diverse as planarians, annelids and sea urchins reveal the presence of pluripotent stem cell populations that express germ plasm components, despite being considered to be somatic. These data also show that germ plasm is continuous in some of these animals, despite their discontinuous germline. PRESENTATION OF THE HYPOTHESIS: Here, based on recent molecular data on germ plasm components, I revise the germline concept. I introduce the concept of primordial stem cells, which are evolutionarily conserved stem cells that carry germ plasm components from the zygote to the germ cells. These cells, delineated by the classic concept of the Weismann barrier, can contribute to different extents to somatic tissues or be present in a rudimentary state. The primordial stem cells are a part of the germline that can drive asexual reproduction. TESTING THE HYPOTHESIS: Molecular information on the expression of germ plasm components is needed during early development of non-classic model organisms, with special attention to those capable of undergoing asexual reproduction and regeneration. The cell lineage of germ plasm component-containing cells will also shed light on their position with respect to the Weismann barrier. This information will help in understanding the germline and its associated stem cells across metazoan phylogeny. IMPLICATIONS OF THE HYPOTHESIS: This revision of the germline concept explains the extensive similarities observed among stem cells and germline cells in a wide variety of animals, and predicts the expression of germ plasm components in many others. The life history of these animals can be simply explained by changes in the extent of self-renewal, proliferation and developmental potential of the primordial stem cells. The inclusion of the primordial stem cells as a part of the germline, therefore, solves many controversies and provides a continuous germline, just as originally envisaged by August Weismann
Heat capacity of square-well fluids of variable width
We have obtained by Monte Carlo NVT simulations the constant-volume excess
heat capacity of square-well fluids for several temperatures, densities and
potential widths. Heat capacity is a thermodynamic property much more sensitive
to the accuracy of a theory than other thermodynamic quantities, such as the
compressibility factor. This is illustrated by comparing the reported
simulation data for the heat capacity with the theoretical predictions given by
the Barker-Henderson perturbation theory as well as with those given by a
non-perturbative theoretical model based on Baxter's solution of the
Percus-Yevick integral equation for sticky hard spheres. Both theories give
accurate predictions for the equation of state. By contrast, it is found that
the Barker-Henderson theory strongly underestimates the excess heat capacity
for low to moderate temperatures, whereas a much better agreement between
theory and simulation is achieved with the non-perturbative theoretical model,
particularly for small well widths, although the accuracy of the latter worsens
for high densities and low temperatures, as the well width increases.Comment: 11 pages, 4 figures; figures now include additional perturbation
data; to be published in Mol. Phy
Fourier Harmonics of High-pT Particles Probing the Fluctuating Intitial Condition Geometries in Heavy-Ion Collisions
Second Fourier harmonics of jet quenching have been thoroughly explored in
the literature and shown to be sensitive to the underlying jet path-length
dependence of energy loss and the differences between the mean eccentricity
predicted by Glauber and CGC/KLN models of initial conditions. We compute the
jet path-length dependence of energy-loss for higher azimuthal harmonics of
jet-fragments in a generalized model of energy-loss for RHIC energies and find,
however, that even the high- second moment is most sensitive to the poorly
known early-time evolution during the first fm/c. Moreover, we demonstrate that
higher-jet harmonics are remarkably insensitive to the initial conditions,
while the different vs. correlations
between the moments of monojet and dijet nuclear modifications factors remain a
most sensitive probe to differentiate between Glauber and CGC/KLN initial state
sQGP geometries.Comment: 6 pages, 7 figures, updated figures, version accepted for publication
in Phys. Rev.
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