8,981 research outputs found
Wakefield generation in magnetized plasmas
We consider wakefield generation in plasmas by electromagnetic pulses
propagating perpendicular to a strong magnetic field, in the regime where the
electron cyclotron frequency is equal to or larger than the plasma frequency.
PIC-simulations reveal that for moderate magnetic field strengths previous
results are re-produced, and the wakefield wavenumber spectrum has a clear peak
at the inverse skin depth. However, when the cyclotron frequency is
significantly larger than the plasma frequency, the wakefield spectrum becomes
broad-band, and simultaneously the loss rate of the driving pulse is much
enhanced. A set of equations for the scalar and vector potentials reproducing
these results are derived, using only the assumption of a weakly nonlinear
interaction.Comment: 6 pages, 8 figure
Spin contribution to the ponderomotive force in a plasma
The concept of a ponderomotive force due to the intrinsic spin of electrons
is developed. An expression containing both the classical as well as the
spin-induced ponderomotive force is derived. The results are used to
demonstrate that an electromagnetic pulse can induce a spin-polarized plasma.
Furthermore, it is shown that for certain parameters, the nonlinear
back-reaction on the electromagnetic pulse from the spin magnetization current
can be larger than that from the classical free current. Suitable parameter
values for a direct test of this effect are presented.Comment: 4 pages, 2 figures, version accepted for publication in Physical
Review Letter
Gravity waves from inflating brane or Mirrors moving in adS
We study tensor perturbations in a model with inflating Randall--Sundrum-type
brane embedded in five-dimensional anti-de Sitter (adS) bulk. In this
model, a natural {\it in}-vacuum of gravitons is the vacuum defined in static
adS frame. We show that this vacuum is, in fact, the same as the {\it
in}-vacuum defined in the frame with de Sitter (dS) slicing, in which the
brane is at rest. Thus, during inflation, gravitons on and off the brane remain
in their vacuum state. We then study graviton creation by the brane on which
inflation terminates at some moment of time. We mostly consider gravitons whose
wavelengths at the end of inflation exceed both the horizon size and the
adS radius. Creation of these gravitons is dominated by (zero mode)--(zero
mode) Bogoliubov coefficients and, apart from an overall factor, the spectrum
of produced gravitons is the same as in four-dimensional theory.
``Kaluza--Klein'' gravitons are also produced, but this effect is subdominant.
Graviton spectra at somewhat higher momenta are also presented for
completeness.Comment: 33 pages, 1 figur
On initial conditions for the Hot Big Bang
We analyse the process of reheating the Universe in the electroweak theory
where the Higgs field plays a role of the inflaton. We estimate the maximal
temperature of the Universe and fix the initial conditions for
radiation-dominated phase of the Universe expansion in the framework of the
Standard Model (SM) and of the nuMSM -- the minimal extension of the SM by
three right-handed singlet fermions. We show that the inflationary epoch is
followed by a matter dominated stage related to the Higgs field oscillations.
We investigate the energy transfer from Higgs-inflaton to the SM particles and
show that the radiation dominated phase of the Universe expansion starts at
temperature T_r~(3-15)*10^{13} GeV, where the upper bound depends on the Higgs
boson mass. We estimate the production rate of singlet fermions at preheating
and find that their concentrations at T_r are negligibly small. This suggests
that the sterile neutrino Dark Matter (DM) production and baryogenesis in the
nuMSM with Higgs-driven inflation are low energy phenomena, having nothing to
do with inflation. We study then a modification of the nuMSM, adding to its
Lagrangian higher dimensional operators suppressed by the Planck scale. The
role of these operators in Higgs-driven inflation is clarified. We find that
these operators do not contribute to the production of Warm Dark Matter (WDM)
and to baryogenesis. We also demonstrate that the sterile neutrino with mass
exceeding 100 keV (a Cold Dark Matter (CDM) candidate) can be created during
the reheating stage of the Universe in necessary amounts. We argue that the
mass of DM sterile neutrino should not exceed few MeV in order not to overclose
the Universe.Comment: 41 pages, 5 figures. Journal version accepted in JCA
Two-body Photodisintegration of He with Full Final State Interaction
The cross sections of the processes He()H and
He()He are calculated taking into account the full final
state interaction via the Lorentz integral transform (LIT) method. This is the
first consistent microscopic calculation beyond the three--body breakup
threshold. The results are obtained with a semirealistic central NN potential
including also the Coulomb force. The cross sections show a pronounced dipole
peak at 27 MeV which lies within the rather broad experimental band. At higher
energies, where experimental uncertainties are considerably smaller, one finds
a good agreement between theory and experiment. The calculated sum of three--
and four--body photodisintegration cross sections is also listed and is in fair
agreement with the data.Comment: 18 pages, 6 figure
Ultra cold neutron trap as a tool to search for dark matter with long-range radius of forces
The problem of possible application of an ultracold neutron (UCN) trap as a
detector of dark matter particles with long-range radius of forces has been
considered. Transmission of small recoil energy in scattering is a
characteristic of long-range forces. The main advantage of the ultracold
neutron technique lies in possibility of detecting recoil energy as small as
eV. Here are presented constraints on the interaction potential
parameters: for dark matter particles and
neutrons, as well as on the density value of long-range dark matter on the
Earth. The possible mechanism of accumulation of long-range dark matter on the
Earth surface is considered and the factor of density increase on the Earth
surface is evaluated. The results of the first experiment on search of
astronomical day variation of ultracold neutron storage time are under
discussion.Comment: 17 pages, 19 figures. arXiv admin note: substantial text overlap with
arXiv:1109.339
TeV gamma-UHECR anisotropy by decaying nuclei in flight: first neutrino traces?
Ultra High Cosmic Rays) made by He-like lightest nuclei might solve the AUGER
extragalactic clustering along Cen A. Moreover He like UHECR nuclei cannot
arrive from Virgo because the light nuclei fragility and opacity above a few
Mpc, explaining the Virgo UHECR absence. UHECR signals are spreading along
Cen-A as observed because horizontal galactic arms magnetic fields, bending
them on vertical angles. Cen A events by He-like nuclei are deflected as much
as the observed clustered ones; proton will be more collimated while heavy
(iron) nuclei are too much dispersed. Such a light nuclei UHECR component
coexist with the other Auger heavy nuclei and with the Hires nucleon
composition. Remaining UHECR spread group may hint for correlations with other
gamma (MeV-Al^{26} radioactive) maps, mainly due to galactic SNR sources as
Vela pulsar, the brightest, nearest GeV source. Other nearest galactic gamma
sources show links with UHECR via TeV correlated maps. We suggest that UHECR
are also heavy radioactive galactic nuclei as Ni^{56}, Ni^{57} and Co^{60}
widely bent by galactic fields. UHECR radioactivity (in and
channels) and decay in flight at hundreds keV is boosted (by huge Lorentz
factor (nearly a billion) leading to PeVs electrons and consequent synchrotron
TeVs gamma offering UHECR-TeV correlated sky anisotropy. Moreover also rarest
and non-atmospheric electron and tau neutrinos secondaries at PeVs, as the
first two rarest shower just discovered in ICECUBE, maybe the first signature
of such expected radioactive secondary tail.Comment: 7 pages,3 figures. arXiv admin note: substantial text overlap with
arXiv:1201.015
Development of Speed-power Qualities of Young Sportsmen-Swimmers.
У статті обґрунтовано розвиток швидкісно-силових якостей залежно від віку, статі та рівня підготовленості юних спортсменів, узагальнено досвід теорії й практики з проблем підвищення ефективності швидкісно-силової підготовки юних плавців, дано науково-методичні рекомендації щодо використання вправ різного характеру з метою покращення спортивних результатів. In the article
obgruntovano development of speed-power qualities in dependence on age, floor and level of preparedness of young
sportsmen, the experience of theory and practice from the problems of increase of efficiency of speed-power
preparation of young swimmers is generalized,scientific-methodical recommendations are given after the use of
exercises of a different character with the purpose of improvement of sporting results
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