852 research outputs found
Comparison of Vlasov-Uehling-Uhlenbeck model with 4 Ï Heavy Ion Data
Streamer chamber data for collisions of Ar + KCl and Ar + BaI2 at 1.2 GeV/nucleon are compared with microscopic model predictions based on the Vlasov-Uehling-Uhlenbeck equation, for various density-dependent nuclear equations of state. Multiplicity distributions and inclusive rapidity and transverse momentum spectra are in good agreement. Rapidity spectra show evidence of being useful in determining whether the model uses the correct cross sections for binary collisions in the nuclear medium, and whether momentum-dependent interactions are correctly incorporated. Sideward flow results do not favor the same nuclear stiffness parameter at all multiplicities
Transport spectroscopy in a time-modulated open quantum dot
We have investigated the time-modulated coherent quantum transport phenomena
in a ballistic open quantum dot. The conductance and the electron dwell
time in the dots are calculated by a time-dependent mode-matching method. Under
high-frequency modulation, the traversing electrons are found to exhibit three
types of resonant scatterings. They are intersideband scatterings: into
quasibound states in the dots, into true bound states in the dots, and into
quasibound states just beneath the subband threshold in the leads. Dip
structures or fano structures in are their signatures. Our results show
structures due to 2 intersideband processes. At the above
scattering resonances, we have estimated, according to our dwell time
calculation, the number of round-trip scatterings that the traversing electrons
undertake between the two dot openings.Comment: 8 pages, 5 figure
A Knob for Changing Light Propagation from Subluminal to Superluminal
We show how the application of a coupling field connecting the two lower
metastable states of a lambda-system can produce a variety of new results on
the propagation of a weak electromagnetic pulse. In principle the light
propagation can be changed from subluminal to superluminal. The negative group
index results from the regions of anomalous dispersion and gain in
susceptibility.Comment: 6 pages,5 figures, typed in RevTeX, accepted in Phys. Rev.
Superluminal optical pulse propagation in nonlinear coherent media
The propagation of light-pulse with negative group-velocity in a nonlinear
medium is studied theoretically. We show that the necessary conditions for
these effects to be observable are realized in a three-level -system
interacting with a linearly polarized laser beam in the presence of a static
magnetic field. In low power regime, when all other nonlinear processes are
negligible, the light-induced Zeeman coherence cancels the resonant absorption
of the medium almost completely, but preserves the dispersion anomalous and
very high. As a result, a superluminal light pulse propagation can be observed
in the sense that the peak of the transmitted pulse exits the medium before the
peak of the incident pulse enters. There is no violation of causality and
energy conservation. Moreover, the superluminal effects are prominently
manifested in the reshaping of pulse, which is caused by the
intensity-dependent pulse velocity. Unlike the shock wave formation in a
nonlinear medium with normal dispersion, here, the self-steepening of the pulse
trailing edge takes place due to the fact that the more intense parts of the
pulse travel slower. The predicted effect can be easily observed in the well
known schemes employed for studying of nonlinear magneto-optical rotation. The
upper bound of sample length is found from the criterion that the pulse
self-steepening and group-advance time are observable without pulse distortion
caused by the group-velocity dispersion.Comment: 16 pages, 7 figure
Phenomenology of Particle Production and Propagation in String-Motivated Canonical Noncommutative Spacetime
We outline a phenomenological programme for the search of effects induced by
(string-motivated) canonical noncommutative spacetime. The tests we propose are
based, in analogy with a corresponding programme developed over the last few
years for the study of Lie-algebra noncommutative spacetimes, on the role of
the noncommutativity parameters in the dispersion relation. We focus on
the role of deformed dispersion relations in particle-production collision
processes, where the noncommutativity parameters would affect the threshold
equation, and in the dispersion of gamma rays observed from distant
astrophysical sources. We emphasize that the studies here proposed have the
advantage of involving particles of relatively high energies, and may therefore
be less sensitive to "contamination" (through IR/UV mixing) from the UV sector
of the theory. We also explore the possibility that the relevant deformation of
the dispersion relations could be responsible for the experimentally-observed
violations of the GZK cutoff for cosmic rays and could have a role in the
observation of hard photons from distant astrophysical sources.Comment: With respect to the experimental information available at the time of
writing version 1 of this manuscript (hep-th/0109191v1) the situation has
evolved significantly. Our remarks on the benefits of high-energy
observations found additional encouragement from the results reported in
hep-th/020925
Intracranial Arterial Calcification Relates to Long-Term Risk of Recurrent Stroke and Post-stroke Mortality
Background: Intracranial arterial calcification (IAC) is highly prevalent in ischemic stroke
patients. However, data on the association of IAC with stroke recurrence and mortality
remains limited. We examined the effect of IAC on the long-term recurrence of stroke
and the risk of post-stroke mortality.
Methods: Using a prospective stroke registry, we recruited 694 patients (mean age 71.6
± 12.4; male sex 50.3%) since December 2004. IAC was visualized using the computed
tomography exam that was made at hospital admission and was quantified with the
Agatston method. All patients were regularly followed up till July 2016. The impacts of
IAC on stroke recurrence and mortality were assessed using Cox-regression models with
adjustments for age, sex, and relevant cardiovascular risk factors.
Results: During a median follow-up period of 8.8 years, 156 patients (22.5%) suffered
a recurrent stroke and 84 died (12.1%). We found that a higher IAC Agatston score
related to a higher risk of stroke recurrence (HR per 1-SD increase in IAC: 1.30; 95%
CI, 1.08â1.56, p = 0.005) and a higher risk of post-stroke mortality (HR per 1-SD
increase, 1.44; 95% CI, 1.06â1.96, p = 0.019). After investigating etiology-specific risks
of stroke-recurrence, we found that a higher IAC Agatston score specifically associated
with small-vessel occlusive stroke.
Conclusions: IAC is a strong risk factor for recurrent stroke and post-stroke
mortality. Among stroke subtypes, IAC relates to higher risk of stroke recurrence among
patients with small-vessel disease, which indicates chronic calcification detected in large
cerebral arteries may have potential effects on the cerebrovascular beds extending to
small vessels
The Exact Correspondence between Phase Times and Dwell Times in a Symmetrical Quantum Tunneling Configuration
The general and explicit relation between the phase time and the dwell time
for quantum tunneling or scattering is investigated. Considering a symmetrical
collision of two identical wave packets with an one-dimensional barrier, here
we demonstrate that these two distinct transit time definitions give connected
results where, however, the phase time (group delay) accurately describes the
exact position of the scattered particles. The analytical difficulties that
arise when the stationary phase method is employed for obtaining phase
(traversal) times are all overcome. Multiple wave packet decomposition allows
us to recover the exact position of the reflected and transmitted waves in
terms of the phase time, which, in addition to the exact relation between the
phase time and the dwell time, leads to right interpretation for both of them.Comment: 11 pages, 2 figure
Small Corrections to the Tunneling Phase Time Formulation
After reexamining the above barrier diffusion problem where we notice that
the wave packet collision implies the existence of {\em multiple} reflected and
transmitted wave packets, we analyze the way of obtaining phase times for
tunneling/reflecting particles in a particular colliding configuration where
the idea of multiple peak decomposition is recovered. To partially overcome the
analytical incongruities which frequently rise up when the stationary phase
method is adopted for computing the (tunneling) phase time expressions, we
present a theoretical exercise involving a symmetrical collision between two
identical wave packets and a unidimensional squared potential barrier where the
scattered wave packets can be recomposed by summing the amplitudes of
simultaneously reflected and transmitted wave components so that the conditions
for applying the stationary phase principle are totally recovered. Lessons
concerning the use of the stationary phase method are drawn.Comment: 14 pages, 3 figure
Superstrings on PP-Wave Backgrounds and Symmetric Orbifolds
We study the superstring theory on pp-wave background with NSNS-flux that is
realized as the Penrose limit of AdS_3 x S^3 x M^4, where M^4 is T^4 or
T^4/Z_2(~ K3). Quantizing this system in the covariant gauge, we explicitly
construct the space-time supersymmetry algebra and the complete set of DDF
operators. We analyse the spectrum of physical states by using the spectrally
flowed representations of current algebra. This spectrum is classified by the
``short string sectors'' and the ``long string sectors'' as in AdS_3 string
theory. The states of the latter propagate freely along the transverse plane of
pp-wave background, but the states of the former do not. We compare the short
string spectrum with the BPS and almost BPS states which have large R-charges
in the symmetric orbifold conformal theory, which is known as the candidate of
dual theory of superstrings on AdS_3 x S^3 x M^4. We show that every short
string states can be embedded successfully in the single particle Hilbert space
of symmetric orbifold conformal theory.Comment: Latex, 35 pages, minor change
Transverse Fresnel-Fizeau drag effects in strongly dispersive media
A light beam normally incident upon an uniformly moving dielectric medium is
in general subject to bendings due to a transverse Fresnel-Fizeau light drag
effect. In conventional dielectrics, the magnitude of this bending effect is
very small and hard to detect. Yet, it can be dramatically enhanced in strongly
dispersive media where slow group velocities in the m/s range have been
recently observed taking advantage of the electromagnetically induced
transparency (EIT) effect. In addition to the usual downstream drag that takes
place for positive group velocities, we predict a significant anomalous
upstream drag to occur for small and negative group velocities. Furthermore,
for sufficiently fast speeds of the medium, higher order dispersion terms are
found to play an important role and to be responsible for peculiar effects such
as light propagation along curved paths and the restoration of the spatial
coherence of an incident noisy beam. The physics underlying this new class of
slow-light effects is thoroughly discussed
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