37,602 research outputs found
Tensor networks and the numerical renormalization group
The full-density-matrix numerical renormalization group (NRG) has evolved as
a systematic and transparent setting for the cal- culation of thermodynamical
quantities at arbitrary temperatures within the NRG framework. It directly
evaluates the relevant Lehmann representations based on the complete basis sets
intro- duced by Anders and Schiller (2005). In addition, specific attention is
given to the possible feedback from low energy physics to high energies by the
explicit and careful construction of the full thermal density matrix, naturally
generated over a distribution of energy shells. Specific examples are given in
terms of spectral functions (fdmNRG), time-dependent NRG (tdmNRG), Fermi-Golden
rule calculations (fgrNRG), as well as the calculation of plain thermodynamic
expectation values. Furthermore, based on the very fact that, by its iterative
nature, the NRG eigenstates are naturally described in terms of matrix product
states, the language of tensor networks has proven enormously convenient in the
description of the underlying algorithmic procedures. This paper therefore also
provides a detailed introduction and discussion of the prototypical NRG
calculations in terms of their corresponding tensor networks.Comment: 20 pages, 11 figures (adapted from habilitation thesis
Degree-dependent intervertex separation in complex networks
We study the mean length of the shortest paths between a vertex of
degree and other vertices in growing networks, where correlations are
essential. In a number of deterministic scale-free networks we observe a
power-law correction to a logarithmic dependence, in a wide range of network
sizes. Here is the number of vertices in the network, is the
degree distribution exponent, and the coefficients and depend on a
network. We compare this law with a corresponding dependence obtained
for random scale-free networks growing through the preferential attachment
mechanism. In stochastic and deterministic growing trees with an exponential
degree distribution, we observe a linear dependence on degree, . We compare our findings for growing networks with those for
uncorrelated graphs.Comment: 8 pages, 3 figure
Experience with the Open Source based implementation for ATLAS Conditions Data Management System
Conditions Data in high energy physics experiments is frequently seen as
every data needed for reconstruction besides the event data itself. This
includes all sorts of slowly evolving data like detector alignment, calibration
and robustness, and data from detector control system. Also, every Conditions
Data Object is associated with a time interval of validity and a version.
Besides that, quite often is useful to tag collections of Conditions Data
Objects altogether. These issues have already been investigated and a data
model has been proposed and used for different implementations based in
commercial DBMSs, both at CERN and for the BaBar experiment. The special case
of the ATLAS complex trigger that requires online access to calibration and
alignment data poses new challenges that have to be met using a flexible and
customizable solution more in the line of Open Source components. Motivated by
the ATLAS challenges we have developed an alternative implementation, based in
an Open Source RDBMS. Several issues were investigated land will be described
in this paper:
-The best way to map the conditions data model into the relational database
concept considering what are foreseen as the most frequent queries.
-The clustering model best suited to address the scalability problem.
-Extensive tests were performed and will be described.
The very promising results from these tests are attracting the attention from
the HEP community and driving further developments.Comment: 8 pages, 4 figures, 3 tables, conferenc
The lattice gluon propagator in renormalizable gauges
We study the SU(3) gluon propagator in renormalizable gauges
implemented on a symmetric lattice with a total volume of (3.25 fm) for
values of the guage fixing parameter up to . As expected, the
longitudinal gluon dressing function stays constant at its tree-level value
. Similar to the Landau gauge, the transverse gauge gluon
propagator saturates at a non-vanishing value in the deep infrared for all
values of studied. We compare with very recent continuum studies and
perform a simple analysis of the found saturation with a dynamically generated
effective gluon mass.Comment: 6 pages, 4 figure
Collapse of Primordial Clouds
We present here studies of collapse of purely baryonic Population III objects
with masses ranging from to . A spherical Lagrangian
hydrodynamic code has been written to study the formation and evolution of the
primordial clouds, from the beginning of the recombination era () until the redshift when the collapse occurs. All the relevant processes
are included in the calculations, as well as, the expansion of the Universe. As
initial condition we take different values for the Hubble constant and for the
baryonic density parameter (considering however a purely baryonic Universe), as
well as different density perturbation spectra, in order to see their influence
on the behavior of the Population III objects evolution. We find, for example,
that the first mass that collapses is for ,
and with the mass scale . For
we obtain for the first
mass that collapses. The cooling-heating and photon drag processes have a key
role in the collapse of the clouds and in their thermal history. Our results
show, for example, that when we disregard the Compton cooling-heating, the
collapse of the objects with masses occurs earlier. On
the other hand, disregarding the photon drag process, the collapse occurs at a
higher redshift.Comment: 10 pages, MN plain TeX macros v1.6 file, 9 PS figures. Also available
at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES").
MNRAS in pres
Collapse of Primordial Clouds II. The Role of Dark Matter
In this article we extend the study performed in our previous article on the
collapse of primordial objects. We here analyze the behavior of the physical
parameters for clouds ranging from to . We
studied the dynamical evolution of these clouds in two ways: purely baryonic
clouds and clouds with non-baryonic dark matter included. We start the
calculations at the beginning of the recombination era, following the evolution
of the structure until the collapse (that we defined as the time when the
density contrast of the baryonic matter is greater than ). We analyze the
behavior of the several physical parameters of the clouds (as, e.g., the
density contrast and the velocities of the baryonic matter and the dark matter)
as a function of time and radial position in the cloud. In this study all
physical processes that are relevant to the dynamical evolution of the
primordial clouds, as for example photon-drag (due to the cosmic background
radiation), hydrogen molecular production, besides the expansion of the
Universe, are included in the calculations. In particular we find that the
clouds, with dark matter, collapse at higher redshift when we compare the
results with the purely baryonic models. As a general result we find that the
distribution of the non-baryonic dark matter is more concentrated than the
baryonic one. It is important to stress that we do not take into account the
putative virialization of the non-baryonic dark matter, we just follow the time
and spatial evolution of the cloud solving its hydrodynamical equations. We
studied also the role of the cooling-heating processes in the purely baryonic
clouds.Comment: 8 pages, MN plain TeX macros v1.6 file, 13 PS figures. Also available
at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES").
MNRAS in pres
Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
The early Solar System contained short-lived radionuclides such as 60Fe (t1/2
= 1.5 Myr) whose most likely source was a nearby supernova. Previous models of
Solar System formation considered a supernova shock that triggered the collapse
of the Sun's nascent molecular cloud. We advocate an alternative hypothesis,
that the Solar System's protoplanetary disk had already formed when a very
close (< 1 pc) supernova injected radioactive material directly into the disk.
We conduct the first numerical simulations designed to answer two questions
related to this hypothesis: will the disk be destroyed by such a close
supernova; and will any of the ejecta be mixed into the disk? Our simulations
demonstrate that the disk does not absorb enough momentum from the shock to
escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss
occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the
disk, which also mix into the disk about 1% of the intercepted ejecta. These
low efficiencies of destruction and injectation are due to the fact that the
high disk pressures prevent the ejecta from penetrating far into the disk
before stalling. Injection of gas-phase ejecta is too inefficient to be
consistent with the abundances of radionuclides inferred from meteorites. On
the other hand, the radionuclides found in meteorites would have condensed into
dust grains in the supernova ejecta, and we argue that such grains will be
injected directly into the disk with nearly 100% efficiency. The meteoritic
abundances of the short-lived radionuclides such as 60Fe therefore are
consistent with injection of grains condensed from the ejecta of a nearby (< 1
pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure
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