761 research outputs found
Tuning FORM with large calculations
Some recent additions to FORM are discussed. In particular large file support
and the tablebases are presented.Comment: 5 pages, LaTeX. Talk given at RADCOR02, Kloster Ban
Introduction to the GiNaC Framework for Symbolic Computation within the C++ Programming Language
The traditional split-up into a low level language and a high level language
in the design of computer algebra systems may become obsolete with the advent
of more versatile computer languages. We describe GiNaC, a special-purpose
system that deliberately denies the need for such a distinction. It is entirely
written in C++ and the user can interact with it directly in that language. It
was designed to provide efficient handling of multivariate polynomials,
algebras and special functions that are needed for loop calculations in
theoretical quantum field theory. It also bears some potential to become a more
general purpose symbolic package
First Results with a new Method for calculating 2-loop Box-Functions
We describe a first attempt to calculate scalar 2-loop box-functions with
arbitrary internal masses, applying a novel method proposed in hep-ph/9407234.
Four of the eight integrals are accessible to integration by means of the
residue theorem, leaving a rational function in the remaining variables. The
result of the procedure is a three- or sometimes two-dimensional integral
representation over a finite volume that can be further evaluated using
numerical methods.Comment: 10 pages, LaTeX2e, 11 eps-figures, needs epsfig.st
First Time-dependent Study of H2 and H3+ Ortho-Para Chemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions
The chemistry in the diffuse interstellar medium initiates the gradual
increase of molecular complexity during the life cycle of matter. A key
molecule that enables build-up of new molecular bonds and new molecules via
proton-donation is H3+. Its evolution is tightly related to molecular hydrogen
and thought to be well understood. However, recent observations of ortho and
para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in
nuclear spin excitation temperatures and populations between these two key
species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to
the predictions of modern astrochemical models. We conduct the first
time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM
and compare our results to a set of line-of-sight observations, including new
measurements presented in this study. We isolate a set of key reactions for H3+
and find that the destruction of the lowest rotational states of H3+ by
dissociative recombination largely control its ortho/para ratio. A plausible
agreement with observations cannot be achieved unless a ratio larger than 1:5
for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally,
an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas
variations of other individual physical parameters, such as density and
chemical age, have only a minor effect on the predicted ortho/para ratios. Thus
our study calls for new laboratory measurements of the dissociative
recombination rate and branching ratio of the key ion H3+ under interstellar
conditions.Comment: 27 pages, 6 figures, 3 table
Simulated void galaxies in the standard cold dark matter model
We analyze a (120 h^{-1} Mpc)^3 adaptive mesh refinement hydrodynamic
simulation that contains a higher-resolution 31 x 31 x 35 h^{-3} Mpc subvolume
centered on a ~30 Mpc diameter void. Our detailed ~1 kpc resolution allows us
to identify 1300 galaxies within this void to a limiting halo mass of ~10^{10}
M_sun. Nearly 1000 galaxies are found to be in underdense regions, with 300
galaxies residing in regions less than half the mean density of the simulation
volume. We construct mock observations of the stellar and gas properties of
these systems, and reproduce the range of colors and luminosities observed in
the SDSS for nearby (z < 0.03) galaxies. We find no trends with density for the
most luminous (M_r
-16), though they are less reliably resolved, typically appear bluer, with
higher rates of star formation and specific star formation and lower mean
stellar ages than galaxies in average density environments. We find a
significant population of low luminosity (M_r ~ -14) dwarf galaxies that is
preferentially located in low density regions and specifically in the void
center. This population may help to reduce, but not remove, the discrepancy
between the predicted and observed number of void galaxies.Comment: 23 pages, 14 figures, submitted to Ap
Spectroscopy and dissociative recombination of the lowest rotational states of H3+
The dissociative recombination of the lowest rotational states of H3+ has
been investigated at the storage ring TSR using a cryogenic 22-pole
radiofrequency ion trap as injector. The H3+ was cooled with buffer gas at ~15
K to the lowest rotational levels, (J,G)=(1,0) and (1,1), which belong to the
ortho and para proton-spin symmetry, respectively. The rate coefficients and
dissociation dynamics of H3+(J,G) populations produced with normal- and para-H2
were measured and compared to the rate and dynamics of a hot H3+ beam from a
Penning source. The production of cold H3+ rotational populations was
separately studied by rovibrational laser spectroscopy using chemical probing
with argon around 55 K. First results indicate a ~20% relative increase of the
para contribution when using para-H2 as parent gas. The H3+ rate coefficient
observed for the para-H2 source gas, however, is quite similar to the H3+ rate
for the normal-H2 source gas. The recombination dynamics confirm that for both
source gases, only small populations of rotationally excited levels are
present. The distribution of 3-body fragmentation geometries displays a broad
part of various triangular shapes with an enhancement of ~12% for events with
symmetric near-linear configurations. No large dependences on internal state or
collision energy are found.Comment: 10 pages, 9 figures, to be published in Journal of Physics:
Conference Proceeding
Characterizing spiral arm and interarm star formation
Interarm star formation contributes significantly to a galaxy's star
formation budget, and provides an opportunity to study stellar birthplaces
unperturbed by spiral arm dynamics. Using optical integral field spectroscopy
of the nearby galaxy NGC 628 with VLT/MUSE, we construct Halpha maps including
detailed corrections for dust extinction and stellar absorption to identify 391
HII regions at 35pc resolution over 12 kpc^2. Using tracers sensitive to the
underlying gravitational potential, we associate HII regions with either arm
(271) or interarm (120) environments. Using our full spectral coverage of each
region, we find that most HII region physical properties (luminosity, size,
metallicity, ionization parameter) are independent of environment. We calculate
the fraction of Halpha luminosity due to the diffuse ionized gas (DIG)
background contaminating each HII region, and find the DIG surface brightness
to be higher within HII regions compared to the surroundings, and slightly
higher within arm HII regions. Use of the temperature sensitive [SII]/Halpha
line ratio map instead of the Halpha surface brightness to identify HII region
boundaries does not change this result. Using the dust attenuation as a tracer
of the gas, we find depletion times consistent with previous work (2 x 10^9 yr)
with no differences between the arm and interarm, however this is very
sensitive to the DIG correction. Unlike molecular clouds, which can be
dynamically affected by the galactic environment, we see fairly consistent HII
region properties in both arm and interarm environments. This suggests either a
difference in arm star formation and feedback, or a decoupling of dense star
forming clumps from the more extended surrounding molecular gas.Comment: 10 pages, 4 figures, 1 table, accepted for publication in Ap
On the Invariance of Residues of Feynman Graphs
We use simple iterated one-loop graphs in massless Yukawa theory and QED to
pose the following question: what are the symmetries of the residues of a graph
under a permutation of places to insert subdivergences. The investigation
confirms partial invariance of the residue under such permutations: the highest
weight transcendental is invariant under such a permutation. For QED this
result is gauge invariant, ie the permutation invariance holds for any gauge.
Computations are done making use of the Hopf algebra structure of graphs and
employing GiNaC to automate the calculations.Comment: 24 pages, latex generated figures. Minor changes in revised versio
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