3,251 research outputs found
The hbar Expansion in Quantum Field Theory
We show how expansions in powers of Planck's constant hbar = h/2\pi can give
new insights into perturbative and nonperturbative properties of quantum field
theories. Since hbar is a fundamental parameter, exact Lorentz invariance and
gauge invariance are maintained at each order of the expansion. The physics of
the hbar expansion depends on the scheme; i.e., different expansions are
obtained depending on which quantities (momenta, couplings and masses) are
assumed to be independent of hbar. We show that if the coupling and mass
parameters appearing in the Lagrangian density are taken to be independent of
hbar, then each loop in perturbation theory brings a factor of hbar. In the
case of quantum electrodynamics, this scheme implies that the classical charge
e, as well as the fine structure constant are linear in hbar. The connection
between the number of loops and factors of hbar is more subtle for bound states
since the binding energies and bound-state momenta themselves scale with hbar.
The hbar expansion allows one to identify equal-time relativistic bound states
in QED and QCD which are of lowest order in hbar and transform dynamically
under Lorentz boosts. The possibility to use retarded propagators at the Born
level gives valence-like wave-functions which implicitly describe the sea
constituents of the bound states normally present in its Fock state
representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.
Interpretation of High Energy String Scattering in terms of String Configurations
High energy string scattering at fixed momentum transfer, known to be
dominated by Regge trajectory exchange, is interpreted by identifying families
of string states which induce each type of trajectory exchange. These include
the usual leading trajectory and its daughters as
well as the ``sister'' trajectories and their
daughters. The contribution of the sister to high energy scattering
is dominated by string excitations in the mode. Thus, at large ,
string scattering is dominated by wee partons, consistently with a picture of
string as an infinitely composite system of ``constituents'' which carry zero
energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-
Complete spectral energy distribution of the hot, helium-rich white dwarf RX J0503.9-2854
In the line-of-sight toward the DO-type white dwarf RX J0503.9-2854, the
density of the interstellar medium (ISM) is very low, and thus the
contamination of the stellar spectrum almost negligible. This allows us to
identify many metal lines in a wide wavelength range from the extreme
ultraviolet to the near infrared. In previous spectral analyses, many metal
lines in the ultraviolet spectrum of RX J0503.9-2854 have been identified. A
complete line list of observed and identified lines is presented here. We
compared synthetic spectra that had been calculated from model atmospheres in
non-local thermodynamical equilibrium, with observations. In total, we
identified 1272 lines (279 of them were newly assigned) in the wavelength range
from the extreme ultraviolet to the near infrared. 287 lines remain
unidentified. A close inspection of the EUV shows that still no good fit to the
observed shape of the stellar continuum flux can be achieved although He, C, N,
O, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Cr, Ni Zn, Ga, Ge, As, Kr, Zr, Mo,
Sn, Xe, and Ba are included in the stellar atmosphere models. There are two
possible reasons for the deviation between observed and synthetic flux in the
EUV. Opacities from hitherto unconsidered elements in the model-atmosphere
calculation may be missing and/or the effective temperature is slightly lower
than previously determined.Comment: 92 pages, 45 figure
Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium
(NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is for
the calculation of such model atmospheres.
We aim to calculate new Sr IV - VII oscillator strengths to identify for the
first time Sr spectral lines in hot white dwarf (WD) stars and to determine the
photospheric Sr abundances. o measure the abundances of Se, Te, and I in hot
WDs, we aim to compute new Se V, Te VI, and I VI oscillator strengths.
To consider radiative and collisional bound-bound transitions of Se V, Sr IV
- VII, Te VI, and I VI in our NLTE atmosphere models, we calculated oscillator
strengths for these ions.
We newly identified four Se V, 23 Sr V, 1 Te VI, and three I VI lines in the
ultraviolet (UV) spectrum of RE0503-289. We measured a photospheric Sr
abundance of 6.5 +3.8/-2.4 x 10**-4 (mass fraction, 9500 - 23800 times solar).
We determined the abundances of Se (1.6 +0.9/-0.6 x 10**-3, 8000 - 20000), Te
(2.5 +1.5/-0.9 x 10**-4, 11000 - 28000), and I (1.4 +0.8/-0.5 x 10**-5, 2700 -
6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we
could determine only upper abundance limits of approximately 100 times solar.
All identified Se V, Sr V, Te VI, and I VI lines in the UV spectrum of
RE0503-289 were simultaneously well reproduced with our newly calculated
oscillator strengths.Comment: 26 pages, 5 figure
Photoluminescence and Terahertz Emission from Femtosecond Laser-Induced Plasma Channels
Luminescence as a mechanism for terahertz emission from femtosecond
laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb
scattering between electrons and ions is shown to lead to luminescence even for
a spatially homogeneous plasma. The spectral features introduced by the rod
geometry of laser-induced plasma channels in air are discussed on the basis of
a generalized mode-function analysis.Comment: 4 pages with 2 figures
Stability of NLO Global Analysis and Implications for Hadron Collider Physics
The phenomenology of Standard Model and New Physics at hadron colliders
depends critically on results from global QCD analysis for parton distribution
functions (PDFs). The accuracy of the standard next-to-leading-order (NLO)
global analysis, nominally a few percent, is generally well matched to the
expected experimental precision. However, serious questions have been raised
recently about the stability of the NLO analysis with respect to certain
inputs, including the choice of kinematic cuts on the data sets and the
parametrization of the gluon distribution. In this paper, we investigate this
stability issue systematically within the CTEQ framework. We find that both the
PDFs and their physical predictions are stable, well within the few percent
level. Further, we have applied the Lagrange Multiplier method to explore the
stability of the predicted cross sections for W production at the Tevatron and
the LHC, since W production is often proposed as a standard candle for these
colliders. We find the NLO predictions on sigma_W to be stable well within
their previously-estimated uncertainty ranges.Comment: 24 pages, 11 figures. Minor changes in response to JHEP referee
repor
Search for trans-iron elements in hot, helium-rich white dwarfs with the HST Cosmic Origins Spectrograph
The metal abundances in the atmospheres of hot white dwarfs (WDs) entering
the cooling sequence are determined by the preceding Asymptotic Giant Branch
(AGB) evolutionary phase and, subsequently, by the onset of gravitational
settling and radiative levitation. In this paper, we investigate three hot
He-rich WDs, which are believed to result from a late He-shell flash. During
such a flash, the He-rich intershell matter is dredged up and dominates the
surface chemistry. Hence, in contrast to the usual H-rich WDs, their spectra
allow direct access to s-process element abundances in the intershell that were
synthesized during the AGB stage. In order to look for trans-iron group
elements (atomic number Z>29), we performed a non-local thermodynamic
equilibrium model atmosphere analysis of new ultraviolet spectra taken with the
Cosmic Origins Spectrograph aboard the Hubble Space Telescope. One of our
program stars is of PG1159 spectral type; this star, PG1707+427, has effective
temperature Teff=85,000 K, and surface gravity logg=7.5. The two other stars
are DO white dwarfs: WD0111+002 has Teff=58,000 K and logg=7.7, and PG0109+111
has Teff=70,000 K and logg=8.0. These stars trace the onset of element
diffusion during early WD evolution. While zinc is the only trans-iron element
we could detect in the PG1159 star, both DOs exhibit lines from Zn, Ga, Ge, Se;
one additionally exhibits lines from Sr, Sn, Te, and I and the other from As.
Generally, the trans-iron elements are very abundant in the DOs, meaning that
radiative levitation must be acting. Most extreme is the almost six orders of
magnitude oversolar abundance of tellurium in PG0109+111. In terms of mass
fraction, it is the most abundant metal in the atmosphere. The two DOs join the
hitherto unique hot DO RE0503-289, in which 14 trans-iron elements had even
been identified.Comment: A&A accepte
An infrared study of the double nucleus in NGC3256
We present new resolved near and mid-IR imaging and N-band spectroscopy of
the two nuclei in the merger system NGCA3256, the most IR luminous galaxy in
the nearby universe. The results from the SED fit to the data are consistent
with previous estimates of the amount of obscuration towards the nuclei and the
nuclear star formation rates. However, we also find substantial differences in
the infrared emission from the two nuclei which cannot be explained by
obscuration alone. We conclude that the northern nucleus requires an additional
component of warm dust in order to explain its properties. This suggests that
local starforming conditions can vary significantly within the environment of a
single system.Comment: Accepted for publication (MNRAS
A Participatory Process to Develop a Landslide Warning System: Paradoxes of Responsibility Sharing in a Case Study in Upper Austria
During a participatory process in Gmunden, Austria, the organizational and responsibility-sharing arrangements for a landslide warning system proved to be contested issues. While questions on the warning system technology and the distribution of information, including the alarm for evacuation, could be resolved with the support of experts, controversies arose on the financial and legal responsibilities that ensure long-term and effective monitoring for the protection of the landslide-prone community. This paper examines how responsibilities can be shared among the residents, experts, and public authorities during the design and operation of landslide warning systems. In particular, we discuss the outcome and implications of three stakeholder workshops where participants deliberated on warning-system options that, in turn, were based on a discourse analysis of extensive stakeholder interviews. The results of the case study show that an end-user orientation requires the consideration of stakeholder worldviews, interests, and conflicts. Paradoxically, the public did not fully support their own involvement in the maintenance and control of the warning system, but the authorities promoted shared responsibility. Deliberative planning does not then necessarily lead to responsibility sharing, but it proved effective as a platform for information and for shared ownership in the warning syste
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