2,901 research outputs found
On the kinetic equation approach to pair production by time-dependent electric field
We investigate the quantum kinetic approach to pair production from vacuum by
time-dependent electric field. Equivalence between this approach and the more
familiar S-matrix approach is explicitly established for both scalar and
fermion cases. For the particular case of a constant electric field exact
solution for kinetic equations is provided and the accuracy of low-density
approximation is estimated.Comment: 8 pages, 4 figure
Electroexcitation of nucleon resonances at Q^2=0.65 GeV/c^2 from a combined analysis of single- and double-pion electroproduction data
Data on single- and double-charged pion electroproduction off protons are
successfully described in the second and third nucleon resonance regions with
common N* photocouplings. The analysis was carried out using separate isobar
models for both reactions. From the combined analysis of two exclusive
channels, the gamma* p --> N*+ helicity amplitudes are obtained for the
resonances P11(1440), D13(1520), S31(1620), S11(1650), F15(1680), D33(1700),
D13(1700), and P13(1720) at Q2=0.65 GeV/c^2.Comment: 12 pages, 12 figures (eps), Published in PHYSICAL REVIEW C 72, 045201
(2005
Model Analysis of the Electroproduction Reaction on the Proton
Recent CLAS data on the electroproduction off protons at
1.3W1.57 GeV and 0.250.6 GeV have been analyzed using
a meson-baryon phenomenological model. By fitting nine 1-fold differential
cross section data for each and bin, the charged double pion
electroproduction mechanisms are identified from their manifestations in the
observables. We have extracted the cross sections from amplitudes of each of
the considered isobar channels as well as from their coherent sum. We also
obtained non-resonant partial wave amplitudes of all contributing isobar
channels which could be useful for advancing a complete coupled-channel
analysis of all meson electroproduction data.Comment: Experiment Numbers: E93-006, E94-005 Group: Hall
Phenomenological analysis of the CLAS data on double charged pion photo and electro- production
First comprehensive data on the evolution of nucleon resonance photocouplings
with photon virtuality Q^2 are presented for excited proton states in the mass
range from 1.4 to 2.0 GeV. N^* photocouplings were determined in
phenomenological analysis of CLAS data on 2 pi photo and electroproduction
within the framework of the JLAB-MSU phenomenological model.Comment: 10 pages, 7 figures (encapsulated postscript
Consistent alpha-cluster description of the 12C (0^+_2) resonance
The near-threshold 12C (0^+_2) resonance provides unique possibility for fast
helium burning in stars, as predicted by Hoyle to explain the observed
abundance of elements in the Universe. Properties of this resonance are
calculated within the framework of the alpha-cluster model whose two-body and
three-body effective potentials are tuned to describe the alpha - alpha
scattering data, the energies of the 0^+_1 and 0^+_2 states, and the
0^+_1-state root-mean-square radius. The extremely small width of the 0^+_2
state, the 0_2^+ to 0_1^+ monopole transition matrix element, and transition
radius are found in remarkable agreement with the experimental data. The
0^+_2-state structure is described as a system of three alpha-particles
oscillating between the ground-state-like configuration and the elongated chain
configuration whose probability exceeds 0.9
Electromagnetic wave diffraction by periodic planar metamaterials with nonlinear constituents
We present a theory which explains how to achieve an enhancement of nonlinear
effects in a thin layer of nonlinear medium by involving a planar periodic
structure specially designed to bear a trapped-mode resonant regime. In
particular, the possibility of a nonlinear thin metamaterial to produce the
bistable response at a relatively low input intensity due to a large quality
factor of the trapped-mode resonance is shown. Also a simple design of an
all-dielectric low-loss silicon-based planar metamaterial which can provide an
extremely sharp resonant reflection and transmission is proposed. The designed
metamaterial is envisioned for aggregating with a pumped active medium to
achieve an enhancement of quantum dots luminescence and to produce an
all-dielectric analog of a 'lasing spaser'.Comment: 18 pages, 13 figure
On the kinetic equation approach to pair production by time-dependent electric field
We investigate the quantum kinetic approach to pair production from vacuum by
time-dependent electric field. Equivalence between this approach and the more
familiar S-matrix approach is explicitly established for both scalar and
fermion cases. For the particular case of a constant electric field exact
solution for kinetic equations is provided and the accuracy of low-density
approximation is estimated.Comment: 8 pages, 4 figure
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