1,717 research outputs found
Response to. comment on optic nerve sheath diameter ultrasound evaluation in intensive care unit: possible role and clinical aspects in neurological critical patients' daily monitoring
Comment on "Optic Nerve Sheath Diameter Ultrasound Evaluation in Intensive Care Unit: Possible Role and Clinical Aspects in Neurological Critical Patients' Daily Monitoring"
Noncommutative 6D Gauge Higgs Unification Models
The influence of higher dimensions in noncommutative field theories is
considered. For this purpose, we analyze the bosonic sector of a recently
proposed 6 dimensional SU(3) orbifold model for the electroweak interactions.
The corresponding noncommutative theory is constructed by means of the
Seiberg-Witten map in 6D. We find in the reduced bosonic interactions in 4D
theory, couplings which are new with respect to other known 4D noncommutative
formulations of the Standard Model using the Seiberg-Witten map.
Phenomenological implications due to the noncommutativity of extra dimensions
are explored. In particular, assuming that the commutative model leads to the
standard model values, a bound -5.63 10^{-8} GeV^{-2}< theta <1.06
10^{-7}GeV^{-2} on the corresponding noncommutativity scale is derived from
current experimental constraints on the S and T oblique parameters. This bound
is used to predict a possibly significant impact of noncommutativity effects of
extra dimensions on the rare Higgs boson decay H-> gamma gamma.Comment: Latex file, 11 pages, 1 figur
How do wave packets spread? Time evolution on Ehrenfest time scales
We derive an extension of the standard time dependent WKB theory which can be
applied to propagate coherent states and other strongly localised states for
long times. It allows in particular to give a uniform description of the
transformation from a localised coherent state to a delocalised Lagrangian
state which takes place at the Ehrenfest time. The main new ingredient is a
metaplectic operator which is used to modify the initial state in a way that
standard time dependent WKB can then be applied for the propagation.
We give a detailed analysis of the phase space geometry underlying this
construction and use this to determine the range of validity of the new method.
Several examples are used to illustrate and test the scheme and two
applications are discussed: (i) For scattering of a wave packet on a barrier
near the critical energy we can derive uniform approximations for the
transition from reflection to transmission. (ii) A wave packet propagated along
a hyperbolic trajectory becomes a Lagrangian state associated with the unstable
manifold at the Ehrenfest time, this is illustrated with the kicked harmonic
oscillator.Comment: 30 pages, 3 figure
WKB Propagation of Gaussian Wavepackets
We analyze the semiclassical evolution of Gaussian wavepackets in chaotic
systems. We prove that after some short time a Gaussian wavepacket becomes a
primitive WKB state. From then on, the state can be propagated using the
standard TDWKB scheme. Complex trajectories are not necessary to account for
the long-time propagation. The Wigner function of the evolving state develops
the structure of a classical filament plus quantum oscillations, with phase and
amplitude being determined by geometric properties of a classical manifold.Comment: 4 pages, 4 figures; significant improvement
Semiclassical Description of Wavepacket Revival
We test the ability of semiclassical theory to describe quantitatively the
revival of quantum wavepackets --a long time phenomena-- in the one dimensional
quartic oscillator (a Kerr type Hamiltonian). Two semiclassical theories are
considered: time-dependent WKB and Van Vleck propagation. We show that both
approaches describe with impressive accuracy the autocorrelation function and
wavefunction up to times longer than the revival time. Moreover, in the Van
Vleck approach, we can show analytically that the range of agreement extends to
arbitrary long times.Comment: 10 pages, 6 figure
Parallax of PSR J1744-1134 and the Local Interstellar Medium
We present the annual trigonometric parallax of PSR J1744-1134 derived from
an analysis of pulse times of arrival. The measured parallax, pi = 2.8+/-0.3
mas ranks among the most precisely determined distances to any pulsar. The
parallax distance of 357+/-39 pc is over twice that derived from the dispersion
measure using the Taylor & Cordes model for the Galactic electron distribution.
The mean electron density in the path to the pulsar, n_e = (0.0088 +/- 0.0009)
cm^{-3}, is the lowest for any disk pulsar. We have compared the n_e for PSR
J1744-1134 with those for another 11 nearby pulsars with independent distance
estimates. We conclude that there is a striking asymmetry in the distribution
of electrons in the local interstellar medium. The electron column densities
for pulsars in the third Galactic quadrant are found to be systematically
higher than for those in the first. The former correlate with the position of
the well known local HI cavity in quadrant three. The excess electrons within
the cavity may be in the form of HII clouds marking a region of interaction
between the local hot bubble and a nearby superbubble.Comment: revised version accepted for publication in ApJ Letters; reanalysis
of uncertainty in parallax measure and changes to fig
Ex-nihilo II: Examination Syllabi and the Sequencing of Cosmology Education
Cosmology education has become an integral part of modern physics courses.
Directed by National Curricula, major UK examination boards have developed
syllabi that contain explicit statements about the model of the Big Bang and
the strong observational evidence that supports it. This work examines the
similarities and differences in these specifications, addresses when cosmology
could be taught within a physics course, what should be included in this
teaching and in what sequence it should be taught at different levels.Comment: 9 pages. Accepted for publication in a special issue of Physics
Educatio
Semiclassical spatial correlations in chaotic wave functions
We study the spatial autocorrelation of energy eigenfunctions corresponding to classically chaotic systems in the semiclassical regime.
Our analysis is based on the Weyl-Wigner formalism for the spectral average
of , defined as the average over eigenstates within an energy window
centered at . In this framework is the Fourier
transform in momentum space of the spectral Wigner function . Our study reveals the chord structure that
inherits from the spectral Wigner function showing the interplay between the
size of the spectral average window, and the spatial separation scale. We
discuss under which conditions is it possible to define a local system
independent regime for . In doing so, we derive an expression
that bridges the existing formulae in the literature and find expressions for
valid for any separation size .Comment: 24 pages, 3 figures, submitted to PR
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