18,598 research outputs found
The Rise and Fall of Pentaquarks in Experiments
Experimental evidence for and against the existence of pentaquarks has
accumulated rapidly in the last three years. If they exist, they would be
dramatic examples of hadronic states beyond our well-tested and successful
particle models. The positive evidence suggests existence of baryonic objects
with widths of at most a few MeV, some displaying exotic quantum numbers, such
as baryons with strangeness . The non-observations of these states have
often come from reaction channels very different from the positive evidence
channels, making comparisons difficult. The situation has now been largely
clarified, however, by high-statistics repetitions of the positive sightings,
with the result that none of the positive sightings have been convincingly
reproduced. The most recent unconfirmed positive sightings suffer again from
low statistics and large backgrounds. It seems that a kind of ``bandwagon''
effect led to the overly-optimistic interpretation of numerous experiments in
the earlier reports of exotic pentaquarks.Comment: Presented at Particles and Nuclei International Conference
(PANIC'05), Santa Fe, N.M. October 28, 2005; 9 pages, 1 figur
Strangeness Production Experiments at Jefferson Lab
Experimental results for photo- and electro-production of open strangeness
from the Thomas Jefferson National Accelerator Facility are discussed. The
results are from work completed by mid-2003 on elementary production,
nuclear targets, and the exotic state. It is shown how the increases
in intensity and precision of JLab experiments over earlier work have allowed
new phenomena to become measurable.Comment: 14 pages, 10 figures, For proceedings of SENDAI03:
Electrophoto-production of Strangeness on Nucleons and Nucle
Quantum privacy and quantum coherence
We derive a simple relation between a quantum channel's capacity to convey
coherent (quantum) information and its usefulness for quantum cryptography.Comment: 6 pages RevTex; two short comments added 7 October 199
Relation between shear parameter and Reynolds number in statistically stationary turbulent shear flows
Studies of the relation between the shear parameter S^* and the Reynolds
number Re are presented for a nearly homogeneous and statistically stationary
turbulent shear flow. The parametric investigations are in line with a
generalized perspective on the return to local isotropy in shear flows that was
outlined recently [Schumacher, Sreenivasan and Yeung, Phys. Fluids, vol.15, 84
(2003)]. Therefore, two parameters, the constant shear rate S and the level of
initial turbulent fluctuations as prescribed by an energy injection rate
epsilon_{in}, are varied systematically. The investigations suggest that the
shear parameter levels off for larger Reynolds numbers which is supported by
dimensional arguments. It is found that the skewness of the transverse
derivative shows a different decay behavior with respect to Reynolds number
when the sequence of simulation runs follows different pathways across the
two-parameter plane. The study can shed new light on different interpretations
of the decay of odd order moments in high-Reynolds number experiments.Comment: 9 pages, 9 Postscript figure
On statistically stationary homogeneous shear turbulence
A statistically stationary turbulence with a mean shear gradient is realized
in a flow driven by suitable body forces. The flow domain is periodic in
downstream and spanwise directions and bounded by stress free surfaces in the
normal direction. Except for small layers near the surfaces the flow is
homogeneous. The fluctuations in turbulent energy are less violent than in the
simulations using remeshing, but the anisotropy on small scales as measured by
the skewness of derivatives is similar and decays weakly with increasing
Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files
Information transmission through a noisy quantum channel
Noisy quantum channels may be used in many information-carrying applications. We show that different applications may result in different channel capacities. Upper bounds on several of these capacities are proved. These bounds are based on the coherent information, which plays a role in quantum information theory analogous to that played by the mutual information in classical information theory. Many new properties of the coherent information and entanglement fidelity are proved. Two nonclassical features of the coherent information are demonstrated: the failure of subadditivity, and the failure of the pipelining inequality. Both properties arise as a consequence of quantum entanglement, and give quantum information new features not found in classical information theory. The problem of a noisy quantum channel with a classical observer measuring the environment is introduced, and bounds on the corresponding channel capacity proved. These bounds are always greater than for the unobserved channel. We conclude with a summary of open problems
Sub-Kolmogorov-Scale Fluctuations in Fluid Turbulence
We relate the intermittent fluctuations of velocity gradients in turbulence
to a whole range of local dissipation scales generalizing the picture of a
single mean dissipation length. The statistical distribution of these local
dissipation scales as a function of Reynolds number is determined in numerical
simulations of forced homogeneous isotropic turbulence with a spectral
resolution never applied before which exceeds the standard one by at least a
factor of eight. The core of the scale distribution agrees well with a
theoretical prediction. Increasing Reynolds number causes the generation of
ever finer local dissipation scales. This is in line with a less steep decay of
the large-wavenumber energy spectra in the dissipation range. The energy
spectrum for the highest accessible Taylor microscale Reynolds number
R_lambda=107 does not show a bottleneck.Comment: 8 pages, 5 figures (Figs. 1 and 3 in reduced quality
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