32,482 research outputs found
Planar radiation zeros in five-parton QCD amplitudes
We demonstrate the existence of `planar', or `type-II', radiation zeros in
5-parton QCD scattering processes. That is, the Born amplitudes are shown to
completely vanish for particular kinematic configurations, when all the
particle 3-momenta lie in a plane. This result is shown to follow particularly
simply from the known BCJ relations between the colour-ordered tree amplitudes,
and the MHV formalism is used to express the additional kinematic constraint as
a relatively simple expression in terms of only rapidity differences between
the final-state partons. In addition, we find that zeros exist for non-planar
configurations of the final-state partons, but for which the normal `type-I'
conditions on the particle four-momenta do not generally apply. We present
numerical results and comment on the possibility of observing planar radiation
zeros in hadronic collisions, via central exclusive three-jet production.Comment: 22 pages, 5 figures. Version accepted for publication in JHE
Effect of the Spatial Dispersion on the Shape of a Light Pulse in a Quantum Well
Reflectance, transmittance and absorbance of a symmetric light pulse, the
carrying frequency of which is close to the frequency of interband transitions
in a quantum well, are calculated. Energy levels of the quantum well are
assumed discrete, and two closely located excited levels are taken into
account. A wide quantum well (the width of which is comparable to the length of
the light wave, corresponding to the pulse carrying frequency) is considered,
and the dependance of the interband matrix element of the momentum operator on
the light wave vector is taken into account. Refractive indices of barriers and
quantum well are assumed equal each other. The problem is solved for an
arbitrary ratio of radiative and nonradiative lifetimes of electronic
excitations. It is shown that the spatial dispersion essentially affects the
shapes of reflected and transmitted pulses. The largest changes occur when the
radiative broadening is close to the difference of frequencies of interband
transitions taken into account.Comment: 7 pages, 5 figure
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Transmission of a Symmetric Light Pulse through a Wide QW
The reflection, transmission and absorption of a symmetric electromagnetic
pulse, which carrying frequency is close to the frequency of an interband
transition in a QW (QW), are obtained. The energy levels of a QW are assumed
discrete, one exited level is taken into account. The case of a wide QW is
considered when a length of the pulse wave, appropriate to the carrying
frequency, is comparable to the QW's width. In figures the time dependencies of
the dimensionless reflection, absorption are transmission are represented. It
is shown, that the spatial dispersion and a distinction in refraction indexes
influence stronger reflection.Comment: 8 pages,8 figures with caption
Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations
The bases of the theory of light reflection and absorption by low-dimensional
semiconductor objects (quantum wells, wires and dots) at both monochromatic and
pulse irradiations and at any form of light pulses are developed. The
semiconductor object may be placed in a stationary quantizing magnetic field.
As an example the case of normal light incidence on a quantum well surface is
considered. The width of the quantum well may be comparable to the light wave
length and number of energy levels of electronic excitations is arbitrary. For
Fourier-components of electric fields the integral equation (similar to the
Dyson-equation) and solutions of this equation for some individual cases are
obtained.Comment: 14 page
Effects of the low lying Dirac modes on excited hadrons in lattice QCD
Chiral symmetry breaking in Quantum Chromodynamics is associated with the low
lying spectral modes of the Dirac operator according to the Banks-Casher
relation. Here we study how removal of a variable number of low lying modes
from the valence quark sector affects the masses of the ground states and first
excited states of baryons and mesons in two flavor lattice QCD.Comment: 6 pages, 2 figures. Contribution to proceedings of "Excited QCD
2012", May 6-12, 2012, Peniche, Portuga
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