7,732 research outputs found
Absence of a consistent classical equation of motion for a mass-renormalized point charge
The restrictions of analyticity, relativistic (Born) rigidity, and negligible
O(a) terms involved in the evaluation of the self electromagnetic force on an
extended charged sphere of radius "a" are explicitly revealed and taken into
account in order to obtain a classical equation of motion of the extended
charge that is both causal and conserves momentum-energy. Because the
power-series expansion used in the evaluation of the self force becomes invalid
during transition time intervals immediately following the application and
termination of an otherwise analytic externally applied force, transition
forces must be included during these transition time intervals to remove the
noncausal pre-acceleration and pre-deceleration from the solutions to the
equation of motion without the transition forces. For the extended charged
sphere, the transition forces can be chosen to maintain conservation of
momentum-energy in the causal solutions to the equation of motion within the
restrictions of relativistic rigidity and negligible O(a) terms under which the
equation of motion is derived. However, it is shown that renormalization of the
electrostatic mass to a finite value as the radius of the charge approaches
zero introduces a violation of momentum-energy conservation into the causal
solutions to the equation of motion of the point charge if the magnitude of the
external force becomes too large. That is, the causal classical equation of
motion of a point charge with renormalized mass experiences a high acceleration
catastrophe.Comment: 13 pages, No figure
New Exclusion Limits for the Search of Scalar and Pseudoscalar Axion-Like Particles from "Light Shining Through a Wall"
Physics beyond the Standard Model predicts the possible existence of new
particles that can be searched at the low energy frontier in the sub-eV range.
The OSQAR photon regeneration experiment looks for "Light Shining through a
Wall" from the quantum oscillation of optical photons into "Weakly Interacting
Sub-eV Particles", such as axion or Axion-Like Particles (ALPs), in a 9 T
transverse magnetic field over the unprecedented length of m.
In 2014, this experiment has been run with an outstanding sensitivity, using an
18.5 W continuous wave laser emitting in the green at the single wavelength of
532 nm. No regenerated photons have been detected after the wall, pushing the
limits for the existence of axions and ALPs down to an unprecedented level for
such a type of laboratory experiment. The di-photon couplings of possible
pseudo-scalar and scalar ALPs can be constrained in the nearly massless limit
to be less than GeV and
GeV, respectively, at 95% Confidence Level.Comment: 6 pages, 6 figure
High energy neutrino yields from astrophysical sources II: Magnetized sources
We calculate the yield of high energy neutrinos produced in astrophysical
sources for arbitrary interaction depths and magnetic field strengths
. We take into account energy loss processes like synchrotron radiation and
diffusion of charged particles in turbulent magnetic fields as well as the
scattering of secondaries on background photons and the direct production of
charm neutrinos. Meson-photon interactions are simulated with an extended
version of the SOPHIA model. Diffusion leads to an increased path-length before
protons leave the source of size R_s and therefore magnetized sources lose
their transparency below the energy , with and 1 for Kolmogorov and
Bohm diffusion, respectively. Moreover, the neutrino flux is suppressed above
the energy where synchrotron energy losses become important for charged
particles. As a consequence, the energy spectrum and the flavor composition of
neutrinos are strongly modified both at low and high energies even for sources
with \tau_0\lsim 1.Comment: 15 pages, 16 figure
The Trans-Pacific Partnership Agreement: Looking Ahead to the Next Steps
Pressure has been building for the conclusion of the 12-country Trans-Pacific Partnership (TPP) negotiations. Getting the deal done is important, but the TPP is not just another free trade agreement (FTA). It represents the chance to set a trade agenda for the future across a wide range of topics for countries throughout the Asia-Pacific region. This means that the agreement should not be settled in haste. More importantly, it also means that key decisions need to be reached about broader issues related to the institutional structure of the TPP. These decisions must be made now, before the deal is closed, on issues such as how to create the TPP as a living agreement, the formation of a TPP Secretariat, and the clarification of entry conditions for future members such as the People’s Republic of China (PRC). These choices must be made deliberately and carefully even while officials are struggling with reaching closure on the most highly sensitive issues still remaining in the agreement. It will not be easy, but wise decisions are necessary now to ensure the long-term success of the TPP
Direct parametric reconstruction with joint motion estimation/correction for dynamic brain PET data
Direct reconstruction of parametric images from raw photon counts has been shown to improve the quantitative analysis of dynamic positron emission tomography (PET) data. However it suffers from subject motion which is inevitable during the typical acquisition time of 1-2 hours. In this work we propose a framework to jointly estimate subject head motion and reconstruct the motion-corrected parametric images directly from raw PET data, so that the effects of distorted tissue-to-voxel mapping due to subject motion can be reduced in reconstructing the parametric images with motion-compensated attenuation correction and spatially aligned temporal PET data. The proposed approach is formulated within the maximum likelihood framework, and efficient solutions are derived for estimating subject motion and kinetic parameters from raw PET photon count data. Results from evaluations on simulated [11C]raclopride data using the Zubal brain phantom and real clinical [18F]florbetapir data of a patient with Alzheimer’s disease show that the proposed joint direct parametric reconstruction motion correction approach can improve the accuracy of quantifying dynamic PET data with large subject motion
Search for weakly interacting sub-eV particles with the OSQAR laser-based experiment: results and perspectives
Recent theoretical and experimental studies highlight the possibility of new
fundamental particle physics beyond the Standard Model that can be probed by
sub-eV energy experiments. The OSQAR photon regeneration experiment looks for
"Light Shining through a Wall" (LSW) from the quantum oscillation of optical
photons into "Weakly Interacting Sub-eV Particles" (WISPs), like axion or
axion-like particles (ALPs), in a 9 T transverse magnetic field over the
unprecedented length of m. No excess of events has been
detected over the background. The di-photon couplings of possible new light
scalar and pseudo-scalar particles can be constrained in the massless limit to
be less than GeV. These results are very close to the
most stringent laboratory constraints obtained for the coupling of ALPs to two
photons. Plans for further improving the sensitivity of the OSQAR experiment
are presented.Comment: 7 pages, 7 figure
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
Annealing-Dependent Magnetic Depth Profile in Ga[1-x]Mn[x]As
We have studied the depth-dependent magnetic and structural properties of
as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron
reflectometry. In addition to increasing total magnetization, the annealing
process was observed to produce a significantly more homogeneous distribution
of the magnetization. This difference in the films is attributed to the
redistribution of Mn at interstitial sites during the annealing process. Also,
we have seen evidence of significant magnetization depletion at the surface of
both as-grown and annealed films.Comment: 5 pages, 3 figure
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