47,370 research outputs found
Is the centrality dependence of the elliptic flow and of the average in RHIC experiments more than a Core-Corona Effect?
Recently we have shown that the centrality dependence of the multiplicity of
different hadron species observed in RHIC and SPS experiments can be well
understood in a simple model, dubbed core-corona model. There it is assumed
that those incoming nucleons which scatter only once produce hadrons as in pp
collisions whereas those which scatter more often form an equilibrated source
which decays according to phase space. In this article we show that also
kinematical variables like as well as
and
between peripheral heavy ion collisions and pp collisions for different
hadrons, reproduced in this model, questions whether hydrodynamical
calculations are the proper tool to describe non-central heavy ion collision.
The model explains as well the centrality dependence of
of charged particles, considered up to now as an observable which allows to
determine the viscosity of the quark gluon plasma. The observed dependence of
on the particle species is a simple
consequence of the different ratios of core to corona particles.Comment: Figure added, text partially rewritten, interpretation of v2 of
identified particle
Identification of NeVIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars
For the first time, we have identified NeVIII absorption lines in far-UV
spectra of the hottest known (Teff>150,000 K) hydrogen-deficient (pre-) white
dwarfs of spectral type PG1159. They are of photospheric origin and can be
matched by synthetic non-LTE line profiles. We also show that a number of UV
and optical emission lines in these stars can be explained as being
photospheric NeVIII features and not, as hitherto suspected, as ultrahigh
ionised OVIII lines created along shock-zones in the stellar wind.
Consequently, we argue that the long-standing identification of the same
emission lines in hot [WR]-type central stars as being due to ultrahigh-ionised
species (OVII-VIII, CV-VI) must be revised. These lines can be entirely
attributed to thermally excited species (NeVII-VIII, NV, OVI). Photospheric
NeVIII lines are also identified in the hottest known He-rich white dwarf
KPD0005+5106 some of which were also attributed to OVIII previously. This is a
surprise because it must be concluded that KPD0005+5106 is much hotter
(Teff=200,000 K) than hitherto assumed (Teff=120,000 K). This is confirmed by a
re-assessment of the HeII line spectrum. We speculate that the temperature is
high enough to explain the mysterious, hard X-ray emission (1 keV) as being of
photospheric origin.Comment: Accepted for publication in A&
Maximal Subrings and Covering Numbers of Finite Semisimple Rings
We classify the maximal subrings of the ring of nx n matrices over a finite field, and
show that these subrings may be divided into three types. We also describe all of the
maximal subrings of a finite semisimple ring, and categorize them into two classes. As
an application of these results, we calculate the covering number of a finite semisimple
ring
Magnitude Uncertainties Impact Seismic Rate Estimates, Forecasts and Predictability Experiments
The Collaboratory for the Study of Earthquake Predictability (CSEP) aims to
prospectively test time-dependent earthquake probability forecasts on their
consistency with observations. To compete, time-dependent seismicity models are
calibrated on earthquake catalog data. But catalogs contain much observational
uncertainty. We study the impact of magnitude uncertainties on rate estimates
in clustering models, on their forecasts and on their evaluation by CSEP's
consistency tests. First, we quantify magnitude uncertainties. We find that
magnitude uncertainty is more heavy-tailed than a Gaussian, such as a
double-sided exponential distribution, with scale parameter nu_c=0.1 - 0.3.
Second, we study the impact of such noise on the forecasts of a simple
clustering model which captures the main ingredients of popular short term
models. We prove that the deviations of noisy forecasts from an exact forecast
are power law distributed in the tail with exponent alpha=1/(a*nu_c), where a
is the exponent of the productivity law of aftershocks. We further prove that
the typical scale of the fluctuations remains sensitively dependent on the
specific catalog. Third, we study how noisy forecasts are evaluated in CSEP
consistency tests. Noisy forecasts are rejected more frequently than expected
for a given confidence limit. The Poisson assumption of the consistency tests
is inadequate for short-term forecast evaluations. To capture the
idiosyncrasies of each model together with any propagating uncertainties, the
forecasts need to specify the entire likelihood distribution of seismic rates.Comment: 35 pages, including 15 figures, agu styl
Comment on "Analysis of the Spatial Distribution between Successive Earthquakes" by Davidsen and Paczuski
By analyzing a southern California earthquake catalog, Davidsen and Paczuski
[Phys. Rev. Lett. 94, 048501 (2005)] claim to have found evidence contradicting
the theory of aftershock zone scaling in favor of scale-free statistics. We
present four elements showing that Davidsen and Paczuski's results may be
insensitive to the existence of physical length scales associated with
aftershock zones or mainshock rupture lengths, so that their claim is
unsubstantiated. (i) Their exponent smaller than 1 for a pdf implies that the
power law statistics they report is at best an intermediate asymptotic; (ii)
their power law is not robust to the removal of 6 months of data around Landers
earthquake within a period of 17 years; (iii) the same analysis for Japan and
northern California shows no evidence of robust power laws; (iv) a statistical
model of earthquake triggering that explicitely obeys aftershock zone scaling
can reproduce the observed histogram of Davidsen and Paczuski, demonstrating
that their statistic may not be sensitive to the presence of characteristic
scales associated with earthquake triggering
Dynamical Screening in Correlated Electron Materials
We present an efficient method for incorporating the dynamical effects of the
screening of the Hubbard U by electronic degrees of freedom in the solid into
the single site dynamical mean field approximation. The formalism is
illustrated by model system calculations which capture the essential features
of the frequency dependent interactions proposed for Gd, Ni, SrVO_3 and other
compounds. Screening leads to shifts in the metal-insulator phase boundary,
changes in the spectral function near the Mott-Hubbard gap edge and to a
renormalization of the quasiparticle weight. Hubbard bands are generically
neither separated by the screened nor the unscreened interaction energy,
implying that the common practice of extracting the Hubbard U from the energies
of features in photoemission and inverse photoemission spectra requires
reexamination
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