14,177 research outputs found
String Organization of Field Theories: Duality and Gauge Invariance
String theories should reduce to ordinary four-dimensional field theories at
low energies. Yet the formulation of the two are so different that such a
connection, if it exists, is not immediately obvious. With the Schwinger
proper-time representation, and the spinor helicity technique, it has been
shown that field theories can indeed be written in a string-like manner, thus
resulting in simplifications in practical calculations, and providing novel
insights into gauge and gravitational theories. This paper continues the study
of string organization of field theories by focusing on the question of local
duality. It is shown that a single expression for the sum of many diagrams can
indeed be written for QED, thereby simulating the duality property in strings.
The relation between a single diagram and the dual sum is somewhat analogous to
the relation between a old- fashioned perturbation diagram and a Feynman
diagram. Dual expressions are particularly significant for gauge theories
because they are gauge invariant while expressions for single diagrams are not.Comment: 20 pages in Latex, including seven figures in postscrip
Unification of bulk and interface electroresistive switching in oxide systems
We demonstrate that the physical mechanism behind electroresistive switching
in oxide Schottky systems is electroformation, as in insulating oxides.
Negative resistance shown by the hysteretic current-voltage curves proves that
impact ionization is at the origin of the switching. Analyses of the
capacitance-voltage and conductance-voltage curves through a simple model show
that an atomic rearrangement is involved in the process. Switching in these
systems is a bulk effect, not strictly confined at the interface but at the
charge space region.Comment: 4 pages, 3 figures, accepted in PR
Implementing Unitarity in Perturbation Theory
Unitarity cannot be perserved order by order in ordinary perturbation theory
because the constraint UU^\dagger=\1 is nonlinear. However, the corresponding
constraint for , being , is linear so it can be
maintained in every order in a perturbative expansion of . The perturbative
expansion of may be considered as a non-abelian generalization of the
linked-cluster expansion in probability theory and in statistical mechanics,
and possesses similar advantages resulting from separating the short-range
correlations from long-range effects. This point is illustrated in two QCD
examples, in which delicate cancellations encountered in summing Feynman
diagrams of are avoided when they are calculated via the perturbative expansion
of . Applications to other problems are briefly discussed.Comment: to appear in Phys. Rev.
Novel Bose-Einstein Interference in the Passage of a Fast Particle in a Dense Medium
When an energetic particle collides coherently with many medium particles at
high energies, the Bose-Einstein symmetry with respect to the interchange of
the exchanged virtual bosons leads to a destructive interference of the Feynman
amplitudes in most regions of the phase space but a constructive interference
in some other regions of the phase space. As a consequence, the recoiling
medium particles have a tendency to come out collectively along the direction
of the incident fast particle, each carrying a substantial fraction of the
incident longitudinal momentum. Such an interference appearing as collective
recoils of scatterers along the incident particle direction may have been
observed in angular correlations of hadrons associated with a high-
trigger in high-energy AuAu collisions at RHIC.Comment: 10 pages, 2 figures, invited talk presented at the 35th Symposium on
Nuclear Physics, Cocoyoc, Mexico, January 3, 2012, to be published in IOP
Conference Serie
THE NON-CONSTANT CTOD/CTOA IN CRACK PROPAGATION
Abstract Unlike the common belief that crack propagation behavior can be predicted successfully by employing fracture criteria based on a constant crack tip opening displacement or angle (CTOD/CTOA), this paper shows that the initially non-constant portion of the CTOD/CTOA plays an essential role in predicting the fracture load for a growing crack. Three-and two-dimensional finite element analyses indicate that a severe underestimate of the experimental load vs. crack extension curve would occur if a constant CTOD/CTOA criterion is used. However, the use of a simplified, bilinear CTOD/CTOA criterion including its non-constant portion will closely duplicate the test data. Furthermore, as a result of using the experimental data from J-integral test with various crack length to specimen width ratios (a/W), it is demonstrated that the CTOD/CTOA is crack tip constraint dependent. The initially higher values of the CTOD/CTOA are in fact a natural consequence of crack growth process which is reflected by the J-resistance (J-R) curve and its slope (tearing modulus)
Reevaluating electron-phonon coupling strengths: Indium as a test case for ab initio and many-body-theory methods
Using indium as a test case, we investigate the accuracy of the
electron-phonon coupling calculated with state-of-the-art ab initio and
many-body theory methods. The ab initio calculations -- where electrons are
treated in the local-density approximation, and phonons and the electron-phonon
interaction are treated within linear response -- predict an electron-phonon
spectral function alpha^2 F(omega) which translates into a relative tunneling
conductance that agrees with experiment to within one part in 1000. The
many-body theory calculations -- where alpha^2 F(omega) is extracted from
tunneling data by means of the McMillan-Rowell tunneling inversion method --
provide spectral functions that depend strongly on details of the inversion
process. For the the most important moment of alpha^2 F(omega), the
mass-renormalization parameter lambda, we report 0.9 +/- 0.1, in contrast to
the value 0.805 quoted for nearly three decades in the literature. The ab
initio calculations also provide the transport electron-phonon spectral
function alpha_{tr}^2 F(omega), from which we calculate the resistivity as a
function of temperature in good agreement with experiment.Comment: 16 pages, 5 figure
Tribimaximal Neutrino Mixing and a Relation Between Neutrino- and Charged Lepton-Mass Spectra
Brannen has recently pointed out that the observed charged lepton masses
satisfy the relation m_e +m_\mu +m_\tau = {2/3}
(\sqrt{m_e}+\sqrt{m_\mu}+\sqrt{m_\tau})^2, while the observed neutrino masses
satisfy the relation m_{\nu 1} +m_{\nu 2} +m_{\nu 3} = {2/3} (-\sqrt{m_{\nu
1}}+\sqrt{m_{\nu 2}}+\sqrt{m_{\nu 3}})^2. It is discussed what neutrino Yukawa
interaction form is favorable if we take the fact pointed out by Brannen
seriously.Comment: 13 pages, presentation modifie
Full potential LAPW calculation of electron momentum density and related properties of Li
Electron momentum density and Compton profiles in Lithium along , and directions are calculated using Full-Potential Linear
Augmented Plane Wave basis within generalized gradient approximation. The
profiles have been corrected for correlations with Lam-Platzman formulation
using self-consistent charge density. The first and second derivatives of
Compton profiles are studied to investigate the Fermi surface breaks. Decent
agreement is observed between recent experimental and our calculated values.
Our values for the derivatives are found to be in better agreement with
experiments than earlier theoretical results. Two-photon momentum density and
one- and two-dimensional angular correlation of positron annihilation radiation
are also calculated within the same formalism and including the
electron-positron enhancement factor.Comment: 11 pages, 7 figures TO appear in Physical Review
Drosophila happyhour modulates JNK-dependent apoptosis
Mitogen-activated protein kinase kinase kinase kinase-3 (MAP4K3) is a Ste20 kinase family member that modulates multiple signal transduction pathways. We recently identified MAP4K3 as proapoptotic kinase using an RNA interference screening approach. In mammalian cells, MAP4K3 enhances the mitochondrial apoptosis pathway through the post-transcriptional modulation of selected proapoptotic Bcl-2 homology domain 3-only proteins. Recent data suggest that MAP4K3 mutations contribute to pancreatic cancer, which highlights the importance of studying the in vivo function of this kinase. To determine whether the cell death function is conserved in vivo and which downstream signalling pathways are involved, we generated transgenic flies expressing happyhour (hppy), the Drosophila MAP4K3 orthologue. Here, we show that the overexpression of hppy promotes caspase-dependent apoptosis and that the hypothetical kinase domain is essential for inducing cell death. In addition, we show that hppy expression triggers the activation of both the c-Jun N-terminal kinase (JNK) and target of rapamycin (TOR) signalling pathways; however, only JNK signalling is required for apoptosis. Together, our results show that hppy has a JNK-dependent proapoptotic function in Drosophila, which reinforces the hypothesis that MAP4K3 might act as tumour suppressor by regulating apoptosis in higher eukaryotes
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