13,426 research outputs found
Acceptance Dependence of Fluctuation in Particle Multiplicity
The effect of limiting the acceptance in rapidity on event-by-event
multiplicity fluctuations in nucleus-nucleus collisions has been investigated.
Our analysis shows that the multiplicity fluctuations decrease when the
rapidity acceptance is decreased. We explain this trend by assuming that the
probability distribution of the particles in the smaller acceptance window
follows binomial distribution. Following a simple statistical analysis we
conclude that the event-by-event multiplicity fluctuations for full acceptance
are likely to be larger than those observed in the experiments, since the
experiments usually have detectors with limited acceptance. We discuss the
application of our model to simulated data generated using VENUS, a widely used
event generator in heavy-ion collisions. We also discuss the results from our
calculations in presence of dynamical fluctuations and possible observation of
these in the actual data.Comment: To appear in Int. J. Mod. Phys.
Dephasing Effect in Photon-Assisted Resonant Tunneling through Quantum Dots
We analyze dephasing in single and double quantum dot systems. The
decoherence is introduced by the B\"{u}ttiker model with current conserving
fictitious voltage leads connected to the dots. By using the non-equilibrium
Green function method, we investigate the dephasing effect on the tunneling
current. It is shown that a finite dephasing rate leads to observable effects.
The result can be used to measure dephasing rates in quantum dots.Comment: 4 pages, 3 figures, to be published in Rapid Communications of Phys.
Rev.
A Spin-Mechanical Device for Detection and Control of Spin Current by Nanomechanical Torque
We propose a spin-mechanical device to control and detect spin currents by
mechanical torque. Our hybrid nano-electro-mechanical device, which contains a
nanowire with a ferromagnetic-nonmagnetic interface, is designed to measure or
induce spin polarized currents. Since spin carries angular momentum, a spin
flip or spin transfer process involves a change in angular momentum--and hence,
a torque--which enables mechanical measurement of spin flips. Conversely, an
applied torque can result in spin polarization and spin current.Comment: 6 pages, 2 figure
Quantum Circuits for the Unitary Permutation Problem
We consider the Unitary Permutation problem which consists, given unitary
gates and a permutation of , in
applying the unitary gates in the order specified by , i.e. in
performing . This problem has been
introduced and investigated by Colnaghi et al. where two models of computations
are considered. This first is the (standard) model of query complexity: the
complexity measure is the number of calls to any of the unitary gates in
a quantum circuit which solves the problem. The second model provides quantum
switches and treats unitary transformations as inputs of second order. In that
case the complexity measure is the number of quantum switches. In their paper,
Colnaghi et al. have shown that the problem can be solved within calls in
the query model and quantum switches in the new model. We
refine these results by proving that quantum switches
are necessary and sufficient to solve this problem, whereas calls
are sufficient to solve this problem in the standard quantum circuit model. We
prove, with an additional assumption on the family of gates used in the
circuits, that queries are required, for any
. The upper and lower bounds for the standard quantum circuit
model are established by pointing out connections with the permutation as
substring problem introduced by Karp.Comment: 8 pages, 5 figure
Scaling of the low temperature dephasing rate in Kondo systems
We present phase coherence time measurements in quasi-one-dimensional Ag
wires doped with Fe Kondo impurities of different concentrations . Due to
the relatively high Kondo temperature of this system, we
are able to explore a temperature range from above down to below . We show that the magnetic contribution to the dephasing rate
per impurity is described by a single, universal curve when plotted as a
function of . For , the dephasing rate is remarkably well
described by recent numerical results for spin impurities. At lower
temperature, we observe deviations from this theory. Based on a comparison with
theoretical calculations for , we discuss possible explanations for the
observed deviations.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Quark-hadron phase transition with surface fluctuation
The effect of surface fluctuation on the observables of quark-hadron phase
transition is studied. The Ginzburg-Landau formalism is extended by the
inclusion of an extra term in the free energy that depends on the vertical
displacements from a flat surface. The probability that a bin has a particular
net displacement is determined by lattice simulation, where the physics input
is color confinement. The surface fluctuation from bin to bin is related to
multiplicity fluctuation, which in turn is measured by the factorial moments.
It is found that both the F-scaling behavior and the scaling exponent are
essentially unaffected by the inclusion of surface fluctuation.Comment: 9 pages, LaTex, 7 figures in a single postscript file, submitted to
Phys. Rev.
Hierarchical search strategy for the detection of gravitational waves from coalescing binaries: Extension to post-Newtonian wave forms
The detection of gravitational waves from coalescing compact binaries would
be a computationally intensive process if a single bank of template wave forms
(i.e., a one step search) is used. In an earlier paper we had presented a
detection strategy, called a two step search}, that utilizes a hierarchy of
template banks. It was shown that in the simple case of a family of Newtonian
signals, an on-line two step search was about 8 times faster than an on-line
one step search (for initial LIGO). In this paper we extend the two step search
to the more realistic case of zero spin 1.5 post-Newtonian wave forms. We also
present formulas for detection and false alarm probabilities which take
statistical correlations into account. We find that for the case of a 1.5
post-Newtonian family of templates and signals, an on-line two step search
requires about 1/21 the computing power that would be required for the
corresponding on-line one step search. This reduction is achieved when signals
having strength S = 10.34 are required to be detected with a probability of
0.95, at an average of one false event per year, and the noise power spectral
density used is that of advanced LIGO. For initial LIGO, the reduction achieved
in computing power is about 1/27 for S = 9.98 and the same probabilities for
detection and false alarm as above.Comment: 30 page RevTeX file and 17 figures (postscript). Submitted to PRD Feb
21, 199
X-Shooter study of accretion in Chamaeleon I: II. A steeper increase of accretion with stellar mass for very low mass stars?
The dependence of the mass accretion rate on the stellar properties is a key
constraint for star formation and disk evolution studies. Here we present a
study of a sample of stars in the Chamaeleon I star forming region carried out
using the VLT/X-Shooter spectrograph. The sample is nearly complete down to
M~0.1Msun for the young stars still harboring a disk in this region. We derive
the stellar and accretion parameters using a self-consistent method to fit the
broad-band flux-calibrated medium resolution spectrum. The correlation between
the accretion luminosity to the stellar luminosity, and of the mass accretion
rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3,
respectively. These slopes and the accretion rates are consistent with previous
results in various star forming regions and with different theoretical
frameworks. However, we find that a broken power-law fit, with a steeper slope
for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller
than ~ 0.3 Msun, is slightly preferred according to different statistical
tests, but the single power-law model is not excluded. The steeper relation for
lower mass stars can be interpreted as a faster evolution in the past for
accretion in disks around these objects, or as different accretion regimes in
different stellar mass ranges. Finally, we find two regions on the mass
accretion versus stellar mass plane empty of objects. One at high mass
accretion rates and low stellar masses, which is related to the steeper
dependence of the two parameters we derived. The second one is just above the
observational limits imposed by chromospheric emission. This empty region is
located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be
effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with
the one expected for photoevaporation to rapidly dissipate the inner disk.Comment: Accepted for publication on Astronomy & Astrophysics. Abstract
shortened for arxiv constraints. Revised version after language editin
Teaching introductory undergraduate Physics using commercial video games
Commercial video games are increasingly using sophisticated physics
simulations to create a more immersive experience for players. This also makes
them a powerful tool for engaging students in learning physics. We provide some
examples to show how commercial off-the-shelf games can be used to teach
specific topics in introductory undergraduate physics. The examples are
selected from a course taught predominantly through the medium of commercial
video games.Comment: Accepted to Physics Education, Fig1 does not render properly in this
versio
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