320 research outputs found
Local gauge invariance implies Siegert's hypothesis
The nonrelativistic Ward-Takahashi identity, a consequence of local gauge
invariance in quantum mechanics, shows the necessity of exchange current
contributions in case of nonlocal and/or isospin-dependent potentials. It also
implies Siegert's hypothesis: in the nonrelativistic limit, two-body charge
densities identically vanish. Neither current conservation, which follows from
global gauge invariance, nor the constraints of (lowest order) relativity are
sufficient to arrive at this result. Furthermore, a low-energy theorem for
exchange contributions is established.Comment: 5 pages, REVTE
The electron-nucleon cross section in reactions
We examine commonly used approaches to deal with the scattering of electrons
from a bound nucleon. Several prescriptions are shown to be related by gauge
transformations. Nevertheless, due to current non-conservation, they yield
different results. These differences reflect the size of the uncertainty that
persists in the interpretation of experiments.Comment: 6 pp (10 in preprint form), ReVTeX, (+ 4 figures, uuencoded
Semi-relativistic charge-current density operator
The charge-current density and two-photon operators consistent with a
single-particle semi-relativistic Hamiltonian are derived within a suitable
functional derivative formalism which preserves gauge invariance. An
application to electron scattering is presented and results are compared with a
fully relativistic case and the non-relativistic cases corrected through fourth
order in M^{-1}.Comment: 20 pages, 3 postscript figures, typos correcte
Rayleigh scattering in the transit spectrum of HD 189733b
The transit spectrum of the exoplanet HD 189733b has recently been obtained
between 0.55 and 1.05 microns. Here we present an analysis of this spectrum. We
develop first-order equations to interpret absorption spectra. In the case of
HD 189733b, we show that the observed slope of the absorption as a function of
wavelength is characteristic of extinction proportional to the inverse of the
fourth power of the wavelength (lambda^-4). Assuming an extinction dominated by
Rayleigh scattering, we derive an atmospheric temperature of 1340+/-150 K. If
molecular hydrogen is responsible for the Rayleigh scattering, the atmospheric
pressure at the planetary characteristic radius of 0.1564 stellar radius must
be 410+/-30 mbar. However the preferred scenario is scattering by condensate
particles. Using the Mie approximation, we find that the particles must have a
low value for the imaginary part of the refraction index. We identify MgSiO3 as
a possible abundant condensate whose particle size must be between 0.01 and 0.1
microns. For this condensate, assuming solar abundance, the pressure at 0.1564
stellar radius is found to be between a few microbars and few millibars, and
the temperature is found to be in the range 1340-1540 K, and both depend on the
particle size.Comment: Accepted for publication in A&A Lette
Realistic Interactions and Dilepton production off pp-collisions
We present a model for dilepton production of proton-proton collisions using
a realist T-matrix that by incorporating Delta-isobar degrees of freedom fits
the NN-scattering data up to 2 GeV. The results we find differ in details from
earlier work that use less sophisticated interactions but the overall agreement
with these calculations is good.Comment: 11 pages Revtex, 2 postscript figures include
Targeting MAPK phosphorylation of Connexin43 provides neuroprotection in stroke
Connexin43 (Cx43) function is influenced by kinases that phosphorylate specific serine sites located near its C-terminus. Stroke is a powerful inducer of kinase activity, but its effect on Cx43 is unknown. We investigated the impact of wild-type (WT) and knock-in Cx43 with serine to alanine mutations at the protein kinase C (PKC) site Cx43(S368A), the casein kinase 1 (CK1) sites Cx43(S325A/328Y/330A), and the mitogen-activated protein kinase (MAPK) sites Cx43(S255/262/279/282A) (MK4) on a permanent middle cerebral artery occlusion (pMCAO) stroke model. We demonstrate that MK4 transgenic animals exhibit a significant decrease in infarct volume that was associated with improvement in behavioral performance. An increase in astrocyte reactivity with a concomitant decrease in microglial reactivity was observed in MK4 mice. In contrast to WT, MK4 astrocytes displayed reduced Cx43 hemichannel activity. Pharmacological blockade of Cx43 hemichannels with TAT-Gap19 also significantly decreased infarct volume in WT animals. This study provides novel molecular insights and charts new avenues for therapeutic intervention associated with Cx43 function
Quantum Electrodynamics in the Light-Front Weyl Gauge
We examine QED(3+1) quantised in the `front form' with finite `volume'
regularisation, namely in Discretised Light-Cone Quantisation. Instead of the
light-cone or Coulomb gauges, we impose the light-front Weyl gauge . The
Dirac method is used to arrive at the quantum commutation relations for the
independent variables. We apply `quantum mechanical gauge fixing' to implement
Gau{\ss}' law, and derive the physical Hamiltonian in terms of unconstrained
variables. As in the instant form, this Hamiltonian is invariant under global
residual gauge transformations, namely displacements. On the light-cone the
symmetry manifests itself quite differently.Comment: LaTeX file, 30 pages (A4 size), no figures. Submitted to Physical
review D. January 18, 1996. Originally posted, erroneously, with missing
`Weyl' in title. Otherwise, paper is identica
QCD near the Light Cone
Starting from the QCD Lagrangian, we present the QCD Hamiltonian for near
light cone coordinates. We study the dynamics of the gluonic zero modes of this
Hamiltonian. The strong coupling solutions serve as a basis for the complete
problem. We discuss the importance of zero modes for the confinement mechanism.Comment: 32 pages, ReVTeX, 2 Encapsulated PostScript figure
Transverse QCD Dynamics Near the Light Cone
Starting from the QCD Hamiltonian in near-light cone coordinates, we study
the dynamics of the gluonic zero modes. Euclidean 2+1 dimensional lattice
simulations show that the gap at strong coupling vanishes at intermediate
coupling. This result opens the possibility to synchronize the continuum limit
with the approach to the light cone.Comment: 15 pages, LaTeX, 3 figures (7 PS files
Gauge-invariant theory of pion photoproduction with dressed hadrons
Based on an effective field theory of hadrons in which quantum chromodynamics
is assumed to provide the necessary bare cutoff functions, a gauge-invariant
theory of pion photoproduction with fully dressed nucleons is developed. The
formalism provides consistent dynamical descriptions of pi-N --> pi-N
scattering and Gamma-N --> pi-N production mechanisms in terms of nonlinear
integral equations for fully dressed hadrons. Defining electromagnetic currents
via the gauging of hadronic n-point Green's functions, dynamically detailed
currents for dressed nucleons are introduced. The dressed hadron currents and
the pion photoproduction current are explicitly shown to satisfy gauge
invariance in a self-consistent manner. Approximations are discussed that make
the nonlinear formalism manageable in practice and yet preserve gauge
invariance. This is achieved by recasting the gauge conditions for all
contributing interaction currents as continuity equations with ``surface''
terms for the individual particle legs coming into or going out of the hadronic
interaction region. General procedures are given that approximate any type of
(global) interaction current in a gauge-invariance preserving manner as a sum
of single-particle ``surface'' currents. It is argued that these prescriptions
carry over to other reactions, irrespective of the number or type of
contributing hadrons or hadronic systems.Comment: 33 pages, RevTeX; includes 8 postscript figures (requires psfig.sty).
This version corrects some minor errors, etc.; contains updated references.
Accepted for publication in Phys. Rev. C56 (Oct. 97
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