767 research outputs found
Poincare Invariant Three-Body Scattering
Relativistic Faddeev equations for three-body scattering are solved at
arbitrary energies in terms of momentum vectors without employing a partial
wave decomposition. Relativistic invariance is incorporated withing the
framework of Poincar\'e invariant quantum mechanics. Based on a Malfliet-Tjon
interaction, observables for elastic and breakup scattering are calculated and
compared to non-relativistic ones.Comment: 4 pages, 2 figures. Proceedings of the workshop "Critical Stability
of Few-Body Quantum Systems" 200
RHESSI Results -- Time For a Rethink?
Hard X-rays and gamma-rays are the most direct signatures of energetic
electrons and ions in the sun's atmosphere which is optically thin at these
energies and their radiation involves no coherent processes. Being collisional
they are complementary to gyro-radiation in probing atmospheric density as
opposed to magnetic field and the electrons are primarily 10--100 keV in
energy, complementing the (>100 keV) electrons likely responsible for microwave
bursts.
The pioneering results of the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) are raising the first new major questions concerning solar energetic
particles in many years. Some highlights of these results are discussed --
primarily around RHESSI topics on which the authors have had direct research
involvement -- particularly when they are raising the need for re-thinking of
entrenched ideas. Results and issues are broadly divided into discoveries in
the spatial, temporal and spectral domains, with the main emphasis on flare
hard X-rays/fast electrons but touching also on gamma-rays/ions, non-flare
emissions, and the relationship to radio bursts.Comment: Proceedings CESRA Workshop 2004: "The High Energy Solar Corona:
Waves, Eruptions, Particles", Lecture Notes in Physics, 2006 (accepted
Pair excitations and the mean field approximation of interacting Bosons, I
In our previous work \cite{GMM1},\cite{GMM2} we introduced a correction to
the mean field approximation of interacting Bosons. This correction describes
the evolution of pairs of particles that leave the condensate and subsequently
evolve on a background formed by the condensate. In \cite{GMM2} we carried out
the analysis assuming that the interactions are independent of the number of
particles . Here we consider the case of stronger interactions. We offer a
new transparent derivation for the evolution of pair excitations. Indeed, we
obtain a pair of linear equations describing their evolution. Furthermore, we
obtain apriory estimates independent of the number of particles and use these
to compare the exact with the approximate dynamics
Supersymmteric Null-like Holographic Cosmologies
We construct a new class of 1/4-BPS time dependent domain-wall solutions with
null-like metric and dilaton in type II supergravities, which admit a null-like
big bang singularity. Based on the domain-wall/QFT correspondence, these
solutions are dual to 1/4-supersymmetric quantum field theories living on a
boundary cosmological background with time dependent coupling constant and UV
cutoff. In particular we evaluate the holographic function for the
2-dimensional dual field theory living on the corresponding null-like
cosmology. We find that this function runs in accordance with the
-theorem as the boundary universe evolves, this means that the number of
degrees of freedom is divergent at big bang and suggests the possible
resolution of big bang singularity.Comment: 26 pages;v2 references adde
Spin in relativistic quantum theory
We discuss the role of spin in Poincar\'e invariant formulations of quantum
mechanics.Comment: 54 page
Low-energy cutoffs in electron spectra of solar flares: statistical survey
The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base
(February 2002 -- May 2006) has been searched to find solar flares with weak
thermal components and flat photon spectra. Using a regularised inversion
technique, we determine the mean electron flux distribution from count spectra
of a selection of events with flat photon spectra in the 15--20 keV energy
range. Such spectral behaviour is expected for photon spectra either affected
by photospheric albedo or produced by electron spectra with an absence of
electrons in a given energy range, e.g. a low-energy cutoff in the mean
electron spectra of non-themal particles. We have found 18 cases which exhibit
a statistically significant local minimum (a dip) in the range of 10--20 keV.
The positions and spectral indices of events with low-energy cutoff indicate
that such features are likely to be the result of photospheric albedo. It is
shown that if the isotropic albedo correction was applied, all low-energy
cutoffs in the mean electron spectrum were removed and hence the low energy
cutoffs in the mean electron spectrum of solar flares above 12 keV cannot
be viewed as real features in the electron spectrum. If low-energy cutoffs
exist in the mean electron spectra, the energy of low energy cutoffs should be
less than 12 keV.Comment: 9 pages, 5 figures, submitted to Solar Physic
Semiparametric estimation of the proportional rates model for recurrent events data with missing event category
Proportional rates models are frequently used for the analysis of recurrent event data with multiple event categories. When some of the event categories are missing, a conventional approach is to either exclude the missing data for a complete-case analysis or employ a parametric model for the missing event type. It is well known that the complete-case analysis is inconsistent when the missingness depends on covariates, and the parametric approach may incur bias when the model is misspecified. In this paper, we aim to provide a more robust approach using a rate proportion method for the imputation of missing event types. We show that the log-odds of the event type can be written as a semiparametric generalized linear model, facilitating a theoretically justified estimation framework. Comprehensive simulation studies were conducted demonstrating the improved performance of the semiparametric method over parametric procedures. Multiple types of Pseudomonas aeruginosa infections of young cystic fibrosis patients were analyzed to demonstrate the feasibility of our proposed approach
Vortices and dynamics in trapped Bose-Einstein condensates
I review the basic physics of ultracold dilute trapped atomic gases, with
emphasis on Bose-Einstein condensation and quantized vortices. The hydrodynamic
form of the Gross-Pitaevskii equation (a nonlinear Schr{\"o}dinger equation)
illuminates the role of the density and the quantum-mechanical phase. One
unique feature of these experimental systems is the opportunity to study the
dynamics of vortices in real time, in contrast to typical experiments on
superfluid He. I discuss three specific examples (precession of single
vortices, motion of vortex dipoles, and Tkachenko oscillations of a vortex
array). Other unusual features include the study of quantum turbulence and the
behavior for rapid rotation, when the vortices form dense regular arrays.
Ultimately, the system is predicted to make a quantum phase transition to
various highly correlated many-body states (analogous to bosonic quantum Hall
states) that are not superfluid and do not have condensate wave functions. At
present, this transition remains elusive. Conceivably, laser-induced synthetic
vector potentials can serve to reach this intriguing phase transition.Comment: Accepted for publication in Journal of Low Temperature Physics,
conference proceedings: Symposia on Superfluids under Rotation (Lammi,
Finland, April 2010
Independent Eigenstates of Angular Momentum in a Quantum N-body System
The global rotational degrees of freedom in the Schr\"{o}dinger equation for
an -body system are completely separated from the internal ones. After
removing the motion of center of mass, we find a complete set of
independent base functions with the angular momentum . These are
homogeneous polynomials in the components of the coordinate vectors and the
solutions of the Laplace equation, where the Euler angles do not appear
explicitly. Any function with given angular momentum and given parity in the
system can be expanded with respect to the base functions, where the
coefficients are the functions of the internal variables. With the right choice
of the base functions and the internal variables, we explicitly establish the
equations for those functions. Only (3N-6) internal variables are involved both
in the functions and in the equations. The permutation symmetry of the wave
functions for identical particles is discussed.Comment: 24 pages, no figure, one Table, RevTex, Will be published in Phys.
Rev. A 64, 0421xx (Oct. 2001
Quantum nucleation in ferromagnets with tetragonal and hexagonal symmetries
The phenomenon of quantum nucleation is studied in a ferromagnet in the
presence of a magnetic field at an arbitrary angle. We consider the
magnetocrystalline anisotropy with tetragonal symmetry and that with hexagonal
symmetry, respectively. By applying the instanton method in the
spin-coherent-state path-integral representation, we calculate the dependence
of the rate of quantum nucleation and the crossover temperature on the
orientation and strength of the field for a thin film and for a bulk solid. Our
results show that the rate of quantum nucleation and the crossover temperature
depend on the orientation of the external magnetic field distinctly, which
provides a possible experimental test for quantum nucleation in nanometer-scale
ferromagnets.Comment: 19 pages and 3 figures, Final version and accepted by Phys. Rev. B
(Feb. B1 2001
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