5,811 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
General analysis of the rare Bc->D^*_s l^+ l^- decay beyond the standard model
The general analysis of the rare Bc->D^*_s l^+ l^- decay is presented by
using the most general, model independent effective Hamiltonian. The
dependencies of the branching ratios, longitudinal, normal and transversal
polarization asymmetries for l^- and the combined asymmetries for l^- and l^+
on the new Wilson coefficients are investigated. Our analysis shows that the
lepton polarization asymmetries are very sensitive to the scalar and tensor
type interactions, which will be very useful in looking for new physics beyond
the standard model.Comment: 27 Pages, 14 Figure
New physics effects to the lepton polarizations in the B -> K l^+ l^- decay
Using the general, model independent form of the effective Hamiltonian, the
general expressions of the longitudinal, normal and transversal polarization
asymmetries for (l^-) and (l^+) and combinations of them for the exclusive (B
-> K l^+ l^-) decay are found. The sensitivity of lepton polarizations and
their combinations on new Wilson coefficients are studied. It is found that
there exist regions of Wilson coefficients for which the branching ratio
coincides with the Standard Model result while the lepton polarizations differ
substantially from the standard model prediction. Hence, studying lepton
polarization in these regions of new Wilson coefficients can serve as a
promising tool for establishing new physics beyond the Standard Model.Comment: 18 pp, 14 figures (postscript formatted), LaTex formatte
Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers
Ferromagnetic resonance and static magnetic properties of CoFe/Al2O3/CoFe/Py
and CoFe/Al2O3/CoFeB/Py magnetic tunnel junctions and of 25nm thick
single-layer Permalloy(Py) films have been studied as a function of temperature
down to 2K. The temperature dependence of the ferromagnetic resonance excited
in the Py layers in magnetic tunnel junctions shows knee-like enhancement of
the resonance frequency accompanied by an anomaly in the magnetization near
60K. We attribute the anomalous static and dynamic magnetic response at low
temperatures to interface stress induced magnetic reorientation transition at
the Py interface which could be influenced by dipolar soft-hard layer coupling
through the Al2O3 barrier
Low frequency noise due to magnetic inhomogeneities in submicron FeCoB/MgO/FeCoB magnetic tunnel junctions
We report on room temperature low frequency noise due to magnetic
inhomogeneities/domain walls (MI/DWs) in elliptic submicron FeCoB/MgO/FeCoB
magnetic tunnel junctions with an area between 0.0245 and 0.0675{\mu}m2. In the
smaller area junctions we found an unexpected random telegraph noise (RTN1),
deeply in the parallel state, possibly due to stray field induced MI/DWs in the
hard layer. The second noise source (RTN2) is observed in the antiparallel
state for the largest junctions. Strong asymmetry of RTN2 and of related
resistance steps with current indicate spin torque acting on the MI/DWs in the
soft layer at current densities below 5x10^5 A/cm2.Comment: 12 pages, 4 figure
Lepton polarization correlations in
In this work we will study the polarizations of both leptons () in the
decay channel . In the case of the dileptonic inclusive
decay , where apart from the polarization asymmetries
of single lepton , one can also observe the polarization asymmetries of
both leptons simultaneously. If this sort of measurement is possible then we
can have, apart from decay rate, FB asymmetry and the six single lepton
polarization asymmetries (three each for and ), nine more
double polarization asymmetries. This will give us a very useful tool in more
strict testing of SM and the physics beyond. We discuss the double polarization
asymmetries of leptons in the decay mode within
the SM and the Minimal Supersymmetric extensions of it.Comment: 21 pages, 21 figures; version to match paper to appear in PR
Sparsity of integer solutions in the average case
We examine how sparse feasible solutions of integer programs are, on average. Average case here means that we fix the constraint matrix and vary the right-hand side vectors. For a problem in standard form with m equations, there exist LP feasible solutions with at most m many nonzero entries. We show that under relatively mild assumptions, integer programs in standard form have feasible solutions with O(m) many nonzero entries, on average. Our proof uses ideas from the theory of groups, lattices, and Ehrhart polynomials. From our main theorem we obtain the best known upper bounds on the integer Carathéodory number provided that the determinants in the data are small
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
Investigation of heavy-heavy pseudoscalar mesons in thermal QCD Sum Rules
We investigate the mass and decay constant of the heavy-heavy pseudoscalar,
, and mesons in the framework of finite temperature QCD
sum rules. The annihilation and scattering parts of spectral density are
calculated in the lowest order of perturbation theory. Taking into account the
additional operators arising at finite temperature, the nonperturbative
corrections are also evaluated. The masses and decay constants remain unchanged
under , but after this point, they start to diminish with
increasing the temperature. At critical or deconfinement temperature, the decay
constants reach approximately to 35% of their values in the vacuum, while the
masses are decreased about 7%, 12% and 2% for , and
states, respectively. The results at zero temperature are in a good consistency
with the existing experimental values as well as predictions of the other
nonperturbative approaches.Comment: 11 Pages, 2 Tables and 6 Figure
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