3,385 research outputs found
Instabilities for a relativistic electron beam interacting with a laser irradiated plasma
The effects of a radiation field (RF) on the unstable modes developed in
relativistic electron beam--plasma interaction are investigated assuming that
, where is the frequency of the RF and
is the plasma frequency. These unstable modes are parametrically
coupled to each other due to the RF and are a mix between two--stream and
parametric instabilities. The dispersion equations are derived by the
linearization of the kinetic equations for a beam--plasma system as well as the
Maxwell equations. In order to highlight the effect of the radiation field we
present a comparison of our analytical and numerical results obtained for
nonzero RF with those for vanishing RF. Assuming that the drift velocity
of the beam is parallel to the wave vector of the
excitations two particular transversal and parallel configurations of the
polarization vector of the RF with respect to are
considered in detail. It is shown that in both geometries resonant and
nonresonant couplings between different modes are possible. The largest growth
rates are expected at the transversal configuration when is
perpendicular to . In this case it is demonstrated that in general
the spectrum of the unstable modes in -- plane is split into two
distinct domains with long and short wavelengths, where the unstable modes are
mainly sensitive to the beam or the RF parameters, respectively. In parallel
configuration, , and at short wavelengths
the growth rates of the unstable modes are sensitive to both beam and RF
parameters remaining insensitive to the RF at long wavelengths.Comment: 23 pages, 5 figure
Two-Surface Wave Decay
Using an analytical model we discuss the parametric excitation of pairs of
electron surface waves (ESW) in the interaction of an ultrashort, intense laser
pulse with an overdense plasma which has a step-like density profile. The ESWs
can be excited either by the electric or by the magnetic part of the Lorentz
force exerted by the laser and, correspondingly, have frequencies around
or , where is the laser frequency.Comment: 4 EPS figures, Revte
The and decays in perturbative QCD approach
In perturbative QCD approach, we investigate the and decays,
which include the Cabibbo-Kobayashi-Maskawa (CKM) favored decays and the
Cabibbo-Kobayashi-Maskawa-suppressed decays, where T denotes a light tensor
meson. From our calculation, we find that the nonfactorizable emission diagrams
and the annihilation type diagrams are important, especially for those color
suppressed channels. For those decays with a tensor meson emitted, the
factorizable emission diagrams vanish owing to the fact that a tensor meson can
not be produced through the local (V-A) or tensor current. The numerical
results show that the predictions for the branching ratios of considered
charmed B decays are in the range of to for those
CKM-favored decays (governed by ) and in the range of to
for those CKM-suppressed decays (governed by ). We also
predict large transverse polarization contributions in many of the decay channels.Comment: 30 pages, 3 figures, typos correctet in Figs. 1 and
Strong and weak coupling limits in optics of quantum well excitons
A transition between the strong (coherent) and weak (incoherent) coupling
limits of resonant interaction between quantum well (QW) excitons and bulk
photons is analyzed and quantified as a function of the incoherent damping rate
caused by exciton-phonon and exciton-exciton scattering. For confined QW
polaritons, a second, anomalous, damping-induced dispersion branch arises and
develops with increasing damping. In this case, the strong-weak coupling
transition is attributed to a critical damping rate, when the intersection of
the normal and damping-induced dispersion branches occurs. For the radiative
states of QW excitons, i.e., for radiative QW polaritons, the transition is
described as a qualitative change of the photoluminescence spectrum at grazing
angles along the QW structure. Furthermore, we show that the radiative
corrections to the QW exciton states with in-plane wavevector approaching the
photon cone are universally scaled by an energy parameter rather than diverge.
The strong-weak coupling transition rates are also proportional to the same
energy parameter. The numerical evaluations are given for a GaAs single quantum
well with realistic parameters.Comment: Published in Physical Review B. 29 pages, 12 figure
Critical sound attenuation in a diluted Ising system
The field-theoretic description of dynamical critical effects of the
influence of disorder on acoustic anomalies near the temperature of the
second-order phase transition is considered for three-dimensional Ising-like
systems. Calculations of the sound attenuation in pure and dilute Ising-like
systems near the critical point are presented. The dynamical scaling function
for the critical attenuation coefficient is calculated. The influence of
quenched disorder on the asymptotic behaviour of the critical ultrasonic
anomalies is discussed.Comment: 12 RevTeX pages, 4 figure
Magnetic state in URu2Si2, UPd2Al3 and UNi2Al3 probed by point contacts
The antiferromagnetic (AFM) state has been investigated in the three
heavy-fermion compounds URu2Si2, UPd2Al3, and UNi2Al3 by measuring dV/dI(V)
curves of point contacts at different temperatures (1.5-20 K) and magnetic
fields (0-28 T). The zero-bias maximum in dV/dI(V) for URu2Si2 points to a
partially gapped Fermi-surface related to the itinerant nature of the AFM state
contrary to UPd2Al3 where analogous features have not been found. The AFM state
in UNi2Al3 has more similarities with URu2Si2. For URu2Si2, the same critical
field of about 40 T along the easy c axis is found for all features in dV/dI(V)
corresponding to the Neel temperature, the gap in the electronic density of
states, and presumably the ordered moments.Comment: 10 pages incl. 5 figures, LaTex 2
Point-contact spectroscopy on URuSi
Tunnel and point contact experiments have been made in a URuSi single
crystal along the c-axis. The experiments were performed changing temperature
and contact size in a low temperature scanning tunneling microscope. A
resonance develops at the Fermi level at K. This resonance splits
and becomes asymmetric when the 17.5 K phase transition is crossed. These
results are consistent with the existence of Kondo like bound states of the
U ionic configurations and the conduction electrons. Below the
transition, these configurations are split by the development of quadrupolar
ordering. The peak separation can be interpreted as a direct measurement of the
order parameter. Measurements on a policrystalline UAu_2Si_2$ sample are also
reported, with a comparative study of the behavior of both materials.Comment: 4 pages (Latex) + 2 postscript figure
Properties of Light Flavour Baryons in Hypercentral quark model
The light flavour baryons are studied within the quark model using the hyper
central description of the three-body system. The confinement potential is
assumed as hypercentral coulomb plus power potential () with power
index . The masses and magnetic moments of light flavour baryons are
computed for different power index, starting from 0.5 to 1.5. The
predicted masses and magnetic moments are found to attain a saturated value
with respect to variation in beyond the power index 1.0. Further
we computed transition magnetic moments and radiative decay width of light
flavour baryons. The results are in good agreement with known experimental as
well as other theoretical models.Comment: Accepted in Pramana J. of Physic
Rare Decays of \Lambda_b->\Lambda + \gamma and \Lambda_b ->\Lambda + l^{+} l^{-} in the Light-cone Sum Rules
Within the Standard Model, we investigate the weak decays of and with the light-cone
sum rules approach. The higher twist distribution amplitudes of
baryon to the leading conformal spin are included in the sum rules for
transition form factors. Our results indicate that the higher twist
distribution amplitudes almost have no influences on the transition form
factors retaining the heavy quark spin symmetry, while such corrections can
result in significant impacts on the form factors breaking the heavy quark spin
symmetry. Two phenomenological models (COZ and FZOZ) for the wave function of
baryon are also employed in the sum rules for a comparison, which can
give rise to the form factors approximately 5 times larger than that in terms
of conformal expansion. Utilizing the form factors calculated in LCSR, we then
perform a careful study on the decay rate, polarization asymmetry and
forward-backward asymmetry, with respect to the decays of , .Comment: 38 pages, 15 figures, some typos are corrected and more references
are adde
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