91,097 research outputs found
Production at High in Central Au+Au and collisions at GeV in STAR
he production at high- (5.0 10.0 GeV/)
measured in minimum bias , Au+Au and central Au+Au collisions in the STAR
detector are presented. The ratio measured in is compared to
PYTHIA calculations as a test of perturbative quantum chromodynamics (pQCD)
that describes reasonably well particle production from hard processes. The
nuclear modification factor are also presented. In collisions,
charged pions and (anti-)protons are measured in the range 5.0
15.0 GeV/ and the anti-particle to particle ratio and the baryon to meson
ratios of these hadrons are discussed.Comment: 4 pages, 8 figures, proccedings for QM200
Perturbation theorems for Hele-Shaw flows and their applications
In this work, we give a perturbation theorem for strong polynomial solutions
to the zero surface tension Hele-Shaw equation driven by injection or suction,
so called the Polubarinova-Galin equation. This theorem enables us to explore
properties of solutions with initial functions close to but are not polynomial.
Applications of this theorem are given in the suction or injection case. In the
former case, we show that if the initial domain is close to a disk, most of
fluid will be sucked before the strong solution blows up. In the later case, we
obtain precise large-time rescaling behaviors for large data to Hele-Shaw flows
in terms of invariant Richardson complex moments. This rescaling behavior
result generalizes a recent result regarding large-time rescaling behavior for
small data in terms of moments. As a byproduct of a theorem in this paper, a
short proof of existence and uniqueness of strong solutions to the
Polubarinova-Galin equation is given.Comment: 25 page
Twisted and Nontwisted Bifurcations Induced by Diffusion
We discuss a diffusively perturbed predator-prey system. Freedman and
Wolkowicz showed that the corresponding ODE can have a periodic solution that
bifurcates from a homoclinic loop. When the diffusion coefficients are large,
this solution represents a stable, spatially homogeneous time-periodic solution
of the PDE. We show that when the diffusion coefficients become small, the
spatially homogeneous periodic solution becomes unstable and bifurcates into
spatially nonhomogeneous periodic solutions.
The nature of the bifurcation is determined by the twistedness of an
equilibrium/homoclinic bifurcation that occurs as the diffusion coefficients
decrease. In the nontwisted case two spatially nonhomogeneous simple periodic
solutions of equal period are generated, while in the twisted case a unique
spatially nonhomogeneous double periodic solution is generated through
period-doubling.
Key Words: Reaction-diffusion equations; predator-prey systems; homoclinic
bifurcations; periodic solutions.Comment: 42 pages in a tar.gz file. Use ``latex2e twisted.tex'' on the tex
files. Hard copy of figures available on request from
[email protected]
Optical Spectroscopy of K-selected Extremely Red Galaxies
We have obtained spectroscopic redshifts for 24 red galaxies from a sample
with median Ks=18.7 and F814W - Ks > 4, using the Keck telescope. These
EROshave high resolution morphologies from HST (Yan & Thompson 2003). Among the
24 redshifts, the majority (92%) are at . We derived the
rest-frame J-band luminosity function at . Our result
suggests that the luminosity evolution between bright EROs at and the
present-day L massive galaxies is at most about 0.7 magnitude. Combining
the morphologies and deep spectroscopy revealed the following properties: (1)
86% of the spectra have absorption features from old stars, suggesting that the
dominant stellar populations seen in the rest-frame UV are old stars. 50% of
the sources have pure absorption lines, while the remaining 50% have emission
lines, indicating recent star formation. We conclude that the color criterion
for EROs is very effective in selecting old stellar populations at ,
and a large fraction of these systems with prominent old stellar populations
also have recent star formation. (2) The 12 emission line systems have the same
number of disk and bulge galaxies as in the remaining 12 pure absorption line
systems. We conclude that spectral classes do not have a simple, direct
correspondence with morphological types. (3) Three EROs could be isolated, pure
passively evolving early-type galaxies at . This implies that only a
small fraction (10%--15%) of early-type galaxies are formed in a rapid burst of
star formation at high redshifts and evolved passively since then. (Abridged).Comment: 27 pages, 8 figures. Accepted for publication in Astronomical
Journal, issue March 200
Dynamical Self-assembly during Colloidal Droplet Evaporation Studied by in situ Small Angle X-ray Scattering
The nucleation and growth kinetics of highly ordered nanocrystal
superlattices during the evaporation of nanocrystal colloidal droplets was
elucidated by in situ time resolved small-angle x-ray scattering. We
demonstrated for the first time that evaporation kinetics can affect the
dimensionality of the superlattices. The formation of two-dimensional
nanocrystal superlattices at the liquid-air interface of the droplet has an
exponential growth kinetics that originates from interface "crushing".Comment: 4 pages, 4 figure
Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of force
balance governing the underlying equilibria. Using a toroidal flux rope in an
external bipolar or quadrupolar field as a model for the current-carrying flux,
we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for
several representative evolutionary sequences in the stable domain of parameter
space. We verify that this catastrophe and the torus instability occur at the
same point; they are thus equivalent descriptions for the onset condition of
solar eruptions.Comment: V2: update to conform to the published article; new choice for
internal inductance of torus; updated Fig. 2; new Figs. 3, 5, and
The Giant Flare of December 27, 2004 from SGR 1806-20
The giant flare of December 27, 2004 from SGR 1806-20 represents one of the
most extraordinary events captured in over three decades of monitoring the
gamma-ray sky. One measure of the intensity of the main peak is its effect on
X- and gamma-ray instruments. RHESSI, an instrument designed to study the
brightest solar flares, was completely saturated for ~0.5 s following the start
of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the
time of the giant flare, however, allowed RHESSI a unique view of the giant
flare event, including the precursor, the main peak decay, and the pulsed tail.
Since RHESSI was saturated during the main peak, we augment these observations
with Wind and RHESSI particle detector data in order to reconstruct the main
peak as well. Here we present detailed spectral analysis and evolution of the
giant flare. We report the novel detection of a relatively soft fast peak just
milliseconds before the main peak, whose timescale and sizescale indicate a
magnetospheric origin. We present the novel detection of emission extending up
to 17 MeV immediately following the main peak, perhaps revealing a
highly-extended corona driven by the hyper-Eddington luminosities. The spectral
evolution and pulse evolution during the tail are presented, demonstrating
significant magnetospheric twist and evolution during this phase. Blackbody
radii are derived for every stage of the flare, which show remarkable agreement
despite the range of luminosities and temperatures covered. Finally, we place
significant upper limits on afterglow emission in the hundreds of seconds
following the giant flare.Comment: 32 pages, 14 figures, submitted to Ap
Quantum tunneling on graphs
We explore the tunneling behavior of a quantum particle on a finite graph, in
the presence of an asymptotically large potential. Surprisingly the behavior is
governed by the local symmetry of the graph around the wells.Comment: 18 page
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