497 research outputs found
Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks
We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass
Immittance Matching for Multi-dimensional Open-system Photonic Crystals
An electromagnetic (EM) Bloch wave propagating in a photonic crystal (PC) is
characterized by the immittance (impedance and admittance) of the wave. The
immittance is used to investigate transmission and reflection at a surface or
an interface of the PC. In particular, the general properties of immittance are
useful for clarifying the wave propagation characteristics. We give a general
proof that the immittance of EM Bloch waves on a plane in infinite one- and
two-dimensional (2D) PCs is real when the plane is a reflection plane of the PC
and the Bloch wavevector is perpendicular to the plane. We also show that the
pure-real feature of immittance on a reflection plane for an infinite
three-dimensional PC is good approximation based on the numerical calculations.
The analytical proof indicates that the method used for immittance matching is
extremely simplified since only the real part of the immittance function is
needed for analysis without numerical verification. As an application of the
proof, we describe a method based on immittance matching for qualitatively
evaluating the reflection at the surface of a semi-infinite 2D PC, at the
interface between a semi-infinite slab waveguide (WG) and a semi-infinite 2D PC
line-defect WG, and at the interface between a semi-infinite channel WG and a
semi-infinite 2D PC slab line-defect WG.Comment: 8 pages, 6 figure
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
The femtosecond optical pump-probe technique was used to study dynamics of
photoexcited electrons and coherent optical phonons in transition metals Zn and
Cd as a function of temperature and excitation level. The optical response in
time domain is well fitted by linear combination of a damped harmonic
oscillation because of excitation of coherent phonon and a
subpicosecond transient response due to electron-phonon thermalization. The
electron-phonon thermalization time monotonically increases with temperature,
consistent with the thermomodulation scenario, where at high temperatures the
system can be well explained by the two-temperature model, while below
50 K the nonthermal electron model needs to be applied. As the
lattice temperature increases, the damping of the coherent phonon
increases, while the amplitudes of both fast electronic response and the
coherent phonon decrease. The temperature dependence of the damping of
the phonon indicates that population decay of the coherent optical
phonon due to anharmonic phonon-phonon coupling dominates the decay process. We
present a model that accounts for the observed temperature dependence of the
amplitude assuming the photoinduced absorption mechanism, where the signal
amplitude is proportional to the photoinduced change in the quasiparticle
density. The result that the amplitude of the phonon follows the
temperature dependence of the amplitude of the fast electronic transient
indicates that under the resonant condition both electronic and phononic
responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review
Experimental Constraints on the Neutrino Oscillations and a Simple Model of Three Flavour Mixing
A simple model of the neutrino mixing is considered, which contains only one
right-handed neutrino field, coupled via the mass term to the three usual
left-handed fields. This is a simplest model that allows for three-flavour
neutrino oscillations. The existing experimental limits on the neutrino
oscillations are used to obtain constraints on the two free mixing parameters
of the model. A specific sum rule relating the oscillation probabilities of
different flavours is derived.Comment: 10 pages, 3 figures in post script, Latex, IFT 2/9
Generalized "Quasi-classical" Ground State for an Interacting Two Level System
We treat a system (a molecule or a solid) in which electrons are coupled
linearly to any number and type of harmonic oscillators and which is further
subject to external forces of arbitrary symmetry. With the treatment restricted
to the lowest pair of electronic states, approximate "vibronic"
(vibration-electronic) ground state wave functions are constructed having the
form of simple, closed expressions. The basis of the method is to regard
electronic density operators as classical variables. It extends an earlier
"guessed solution", devised for the dynamical Jahn-Teller effect in cubic
symmetry, to situations having lower (e.g., dihedral) symmetry or without any
symmetry at all. While the proposed solution is expected to be quite close to
the exact one, its formal simplicity allows straightforward calculations of
several interesting quantities, like energies and vibronic reduction (or Ham)
factors. We calculate for dihedral symmetry two different -factors (""
and "") and a -factor. In simplified situations we obtain . The formalism enables quantitative estimates to be made for the dynamical
narrowing of hyperfine lines in the observed ESR spectrum of the dihedral
cyclobutane radical cation.Comment: 28 pages, 4 figure
Braille text entry on smartwatches : an evaluation of methods for composing the Braille cell
Smartwatches are gaining popularity on market with a set of features comparable to smartphones in a wearable device. This novice technology brings new interaction paradigms and challenges for blind users, who have difficulties dealing with touchscreens. Among a variety of tasks that must be studied, text entry is analyzed, considering that current existing solutions may be unsatisfactory (as voice input) or even unfeasible (as working with tiny QWERTY keyboards) for a blind user. More specifically, this paper presents a study on possible solutions for composing a Braille cell on smart-watches. Five prototypes were developed and different feedback features were proposed. These are confronted with seven specialists on an evaluation study that results in a qualitative analysis of which strategies can be more useful for blind users in a Braille text entry.Postprin
Detecting the (Quasi-)Two-Body Decays of Leptons in Short-Baseline Neutrino Oscillation Experiments
Novel detector schemes are proposed for the short-baseline neutrino
experiments of next generation, aimed at exploring the large-
domain of \omutau oscillations in the appearance mode. These schemes emphasize
good spectrometry for charged particles and for electromagnetic showers and
efficient reconstruction of \ypi_gg decays. The basic elements are a sequence
of relatively thin emulsion targets, immersed in magnetic field and
interspersed with electronic trackers, and a fine-grained electromagnetic
calorimeter built of lead glass. These elements act as an integral whole in
reconstructing the electromagnetic showers. This conceptual scheme shows good
performance in identifying the (quasi-)two-body decays by their
characteristic kinematics and in selecting the electronic decays of the .Comment: 34 pages, 8 figure
Is CP Violation Observable in Long Baseline Neutrino Oscillation Experiments ?
We have studied CP violation originated by the phase of the neutrino mixing
matrix in the long baseline neutrino oscillation experiments. The direct
measurements of CP violation is the difference of the transition probabilities
between CP-conjugate channels. In those experiments, the CP violating effect is
not suppressed if the highest neutrino mass scale is taken to be 1\sim 5 \eV,
which is appropriate for the cosmological hot dark matter. Assuming the
hierarchy for the neutrino masses, the upper bounds of CP violation have been
caluculated for three cases, in which mixings are constrained by the recent
short baseline ones. The calculated upper bounds are larger than ,
which will be observable in the long baseline accelerator experiments. The
matter effect, which is not CP invariant, has been also estimated in those
experiments.Comment: 28 pages, LaTex file, 6 figures included using epsfig Matter effect
is estimated(Figs.3(a) (b)). Physical parameters are change
A first measurement of the interaction cross section of the tau neutrino
The DONuT experiment collected data in 1997 and published first results in
2000 based on four observed charged-current (CC) interactions. The
final analysis of the data collected in the experiment is presented in this
paper, based on protons on target using the 800 GeV
Tevatron beam at Fermilab. The number of observed CC interactions is
9, from a total of 578 observed neutrino interactions. We calculated the
energy-independent part of the tau-neutrino CC cross section (), relative to the well-known and cross sections. The
ratio / was found to be
. The CC cross section was found to be cm. Both results are in
agreement the Standard Model.Comment: 37 pages, 15 figure
A High Statistics Search for Electron-Neutrino --> Tau-Neutrino Oscillations
We present new limits on nu_e to nu_tau and nu_e to nu_sterile oscillations
by searching for electron neutrino dissappearance in the high-energy wide-band
CCFR neutrino beam. Sensitivity to nu_tau appearance comes from tau decay modes
in which a large fraction of the energy deposited is electromagnetic. The beam
is composed primarily of muon neutrinos but this analysis uses the 2.3%
electron neutrino component of the beam. Electron neutrino energies range from
30 to 600 GeV and flight lengths vary from 0.9 to 1.4 km. This limit improves
the sensitivity of existing limits and obtains a lowest 90% confidence upper
limit in sin**2(2*alpha) of 9.9 x 10**(-2) at delta-m**2 of 125 eV**2.Comment: submitted to Phys. Rev. D. Rapid Com
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