1,021 research outputs found
The conservation of energy-momentum and the mass for the graviton
In this work we give special attention to the bimetric theory of gravitation
with massive gravitons proposed by Visser in 1998. In his theory, a prior
background metric is necessary to take in account the massive term. Although in
the great part of the astrophysical studies the Minkowski metric is the best
choice to the background metric, it is not possible to consider this metric in
cosmology. In order to keep the Minkowski metric as background in this case, we
suggest an interpretation of the energy-momentum conservation in Visser's
theory, which is in accordance with the equivalence principle and recovers
naturally the special relativity in the absence of gravitational sources.
Although we do not present a general proof of our hypothesis we show its
validity in the simple case of a plane and dust-dominated universe, in which
the `massive term' appears like an extra contribution for the energy density.Comment: 9 pages, accepted for publishing in GR
Implications of Scaling Violations of F2 at HERA for Perturbative QCD
We critically examine the QCD predictions for the dependence of the
electron-proton deep-inelastic structure function in the small
region, which is being probed at HERA. The standard results based on
next-to-leading order Altarelli-Parisi evolution are compared with those that
follow from the BFKL equation, which corresponds to the resummation of the
leading log terms. The effects of parton screening are also quantified.
The theoretical predictions are confronted with each other, and with existing
data from HERA. (3 Postscript figures included).Comment: (8 Latex Pages) IFJ 1653/P
Dyson-Schwinger Equations - aspects of the pion
The contemporary use of Dyson-Schwinger equations in hadronic physics is
exemplified via applications to the calculation of pseudoscalar meson masses,
and inclusive deep inelastic scattering with a determination of the pion's
valence-quark distribution function.Comment: 4 pages. Contribution to the Proceedings of ``DPF 2000,'' the Meeting
of the Division of Particles and Fields of the American Physical Society,
August 9-12, 2000, Department of Physics, the Ohio State University,
Columbus, Ohi
Low wave-functions of pions and kaons and their parton distribution functions
We study the low wave-functions of pions and kaons as an expansion in
terms of hadron-like Fock state fluctuations. In this formalism, pion and kaon
wave-functions are related one another. Consequently, the knowledge of the pion
structure allows the determination of parton distributions in kaons. In
addition, we show that the intrinsic (low ) sea of pions and kaons are
different due to their different valence quark structure. Finally, we analize
the feasibility of a method to extract kaon's parton distribution functions
within this approach and compare with available experimental data.Comment: 13 pages, 3 postscript figures include
Meson Structure Functions in Valon Model
Parton distributions in a {\it{valon}} in the next-to-leading order is used
to determine the patron distributions in pion and kaon. The validity of the
valon model is tested and shown that the partonic content of the valon is
universal and independent of the valon type. We have evaluated the valon
distribution in pion and kaon, and in particular it is shown that the results
are in good agreement with the experimental data on pion structure in a wide
range of Comment: 13 pages with 7 figures included, The manuscript is revised, figures
are added and some errors are corrected. Accepted for publication in Physical
Review
Higher twists in the pion structure function
We calculate the QCD moments of the pion structure function using Drell-Yan
data on the quark distributions in the pion and a phenomenological model for
the resonance region. The extracted higher twist corrections are found to be
larger than those for the nucleon, contributing around 50% of the lowest moment
at Q^2=1 GeV^2.Comment: 8 pages, 3 figures, to appear in Phys. Rev.
The pion transition form factor and the pion distribution amplitude
Recent BaBaR data on the pion transition form factor, whose Q^2 dependence is
much steeper then predicted by asymptotic Quantum Chromodynamics (QCD), have
caused a renewed interest in its theoretical description. We present here a
formalism based on a model independent low energy description and a high energy
description based on QCD, which match at a scale Q_0. The high energy
description incorporates a flat pion distribution amplitude, phi(x)=1, at the
matching scale Q_0 and QCD evolution from Q_0 to Q>Q_0. The flat pion
distribution is connected, through soft pion theorems and chiral symmetry, to
the pion valance parton distribution at the same low scale Q_0. The procedure
leads to a good description of the data, and incorporating additional twist
three effects, to an excellent description of the data.Comment: 11 pages, 5 postscript figures, uses epsfig.sty and 1 appendi
Did transit through the galactic spiral arms seed crust production on the early Earth?
Although there is evidence for periodic geological perturbations driven by regular or semi-regular extra-terrestrial bombardment, the production of Earth’s continental crust is generally regarded as a function of planetary differentiation driven by internal processes. We report time series analysis of the Hf isotopic composition of zircon grains from the North Atlantic and Pilbara cratons, the archetypes of Archean plate tectonic and non-plate tectonic settings, respectively. An ~170–200 m.y. frequency is recognized in both cratons that matches the transit of the solar system through the galactic spiral arms, where the density of stars is high. An increase in stellar density is consistent with an enhanced rate of Earth bombardment by comets, the larger of which would have initiated crustal nuclei production via impact-driven decompression melting of the mantle. Hence, the production and preservation of continental crust on the early Earth may have been fundamentally influenced by exogenous processes. A test of this model using oxygen isotopes in zircon from the Pilbara craton reveals correlations between crust with anomalously light isotopic signatures and exit from the Perseus spiral arm and entry into the Norma spiral arm, the latter of which matches the known age of terrestrial spherule beds. Our data support bolide impact, which promoted the growth of crustal nuclei, on solar system transit into and out of the galactic spiral arms
Pion Content of the Nucleon as seen in the NA51 Drell-Yan experiment
In a recent CERN Drell-Yan experiment the NA51 group found a strong asymmetry
of and densities in the proton at . We interpret
this result as a decisive confirmation of the pion-induced sea in the nucleon.Comment: 10 pages + 3 figures, Preprint KFA-IKP(TH)-1994-14 .tex file. After
\enddocument a uu-encodeded Postscript file comprising the figures is
appende
Measurement of Creep Deformation across Welds in 316H Stainless Steel Using Digital Image Correlation
Spatially resolved measurement of creep deformation across weldments at high temperature cannot be achieved using standard extensometry approaches. In this investigation, a Digital Image Correlation (DIC) based system has been developed for long-term high-temperature creep strain measurement in order to characterise the material deformation behaviour of separate regions of a multi-pass weld. The optical system was sufficiently stable to allow a sequence of photographs to be taken suitable for DIC analysis of creep specimens tested at a temperature of 545 °C for over 2000 h. The images were analysed to produce local creep deformation curves from two cross-weld samples cut from contrasting regions of a multi-pass V-groove weld joining thick-section AISI Type 316H austenitic stainless steel. It is shown that for this weld, the root pass is the weakest region of the structure in creep, most likely due to the large number of thermal cycles it has experienced during the fabrication process. The DIC based measurement method offers improved spatial resolution over conventional methods and greatly reduces the amount of material required for creep characterisation of weldments
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