156,587 research outputs found
Surprises in the RHIC Data
The data from RHIC have produced many unanticipated results. I will describe
a few of the surprises that occur in the soft spectra while my colleagues at
this conference will summarize the hard spectra. One particularly important
discovery is that properties of the initial state have an impact on the final
state in relativistic heavy ion collisions. Another important discovery is that
the collision zone is opaque to the passage of hadrons and perhaps even
partons. And finally, the data tell us very precisely where the colliding
systems hadronize on the phase diagram for nuclear matter.Comment: 13 pages, 10 figures, Proceedings of the Seventh Workshop on Quantum
Chromodynamics, Villefranche sur Mer, 6-10 Jan 200
Quantum spill out in few-nanometer metal gaps: Effect on gap plasmons and reflectance from ultrasharp groove arrays
Plasmons in ultranarrow metal gaps are highly sensitive to the electron
density profile at the metal surfaces. Using a fully quantum mechanical
approach, we study the effects of electron spill-out on gap plasmons and
reflectance from ultrasharp metal grooves. We demonstrate that the mode index
of ultranarrow gap plasmons converges to the bulk refractive index in the limit
of vanishing gap and, thereby, rectify the unphysical divergence found in
classical models. Surprisingly, spill-out also significantly increases the
plasmonic absorption for few-nanometer gaps and lowers the reflectance from
arrays of ultrasharp metal grooves. These findings are explained in terms of
enhanced gap plasmon absorption taking place inside the gap 1-2 {\AA} from the
walls and delocalization near the groove bottom. Reflectance calculations
taking spill-out into account are shown to be in much better agreement with
measurements compared with classical models
Quantum spill-out in nanometer-thin gold slabs: Effect on plasmon mode index and plasmonic absorption
A quantum mechanical approach and local response theory are applied to study
plasmons propagating in nanometer-thin gold slabs sandwiched between different
dielectrics. The metal slab supports two different kinds of modes, classified
as long-range and short-range plasmons. Quantum spill-out is found to
significantly increase the imaginary part of their mode indices, and,
surprisingly, even for slabs wide enough to approach bulk the increase is 20%.
This is explained in terms of enhanced plasmonic absorption, which mainly takes
place at narrow peaks located near the slab surface
The X-ray Variability of AGN and its Implications for Observations of Galaxy Clusters
The detection of new clusters of galaxies or the study of known clusters of
galaxies in X-rays can be complicated by the presence of X-ray point sources,
the majority of which will be active galactic nuclei (AGN). This can be
addressed by combining observations from a high angular resolution observatory
(such as Chandra) with deeper data from a more sensitive observatory that may
not be able to resolve the AGN (like XMM). However, this approach is undermined
if the AGN varies in flux between the epochs of the observations. To address
this we measure the characteristic X-ray variability of serendipitously
detected AGN in 70 pairs of Chandra observations, separated by intervals of
between one month and thirteen years. After quality cuts, the full sample
consists of 1511 sources, although the main analysis uses a subset of 416
sources selected on the geometric mean of their flux in the pairs of
observations, which eliminates selection biases. We find a fractional
variability that increases with increasing interval between observations, from
about 0.25 for observations separated by tens of days up to about 0.45 for
observations separated by years. As a rule of thumb, given the
precise X-ray flux of a typical AGN at one epoch, its flux at a second epoch
some years earlier or later can be predicted with a precision of about
due to its variability (ignoring any statistical noise). This is larger than
the characteristic variability of the population by a factor of due
to the uncertainty on the mean flux of the AGN due to a single prior
measurement. The precision can thus be improved with multiple prior flux
measurements (reducing the factor), or by reducing the interval
between observations to reduce the characteristic variability.Comment: 13 pages, 7 figures; accepted for publication in the Open Journal of
Astrophysics; full data table included with source files; comments welcom
Two-Body T-Matrices without Angular Momentum Decomposition: Energy and Momentum Dependencies
The two-body t-matrix is calculated directly as function of two vector
momenta for different Malfliet-Tjon type potentials. At a few hundred MeV
projectile energy the total amplitude is quite a smooth function showing only a
strong peak in forward direction. In contrast the corresponding partial wave
contributions, whose number increases with increasing energy, become more and
more oscillatory with increasing energy. The angular and momentum dependence of
the full amplitude is studied and displayed on as well as off the energy shell
as function of positive and negative energies. The behavior of the t-matrix in
the vicinity of bound state poles and resonance poles in the second energy
sheet is studied. It is found that the angular dependence of T exhibits a very
characteristic behavior in the vicinity of those poles, which is given by the
Legendre function corresponding to the quantum number either of the bound state
or the resonance (or virtual) state. This behavior is illustrated with
numerical examples.Comment: 19 pages (revtex), 15 figure
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