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

Self-forces on extended bodies in electrodynamics

In this paper, we study the bulk motion of a classical extended charge in flat spacetime. A formalism developed by W. G. Dixon is used to determine how the details of such a particle's internal structure influence its equations of motion. We place essentially no restrictions (other than boundedness) on the shape of the charge, and allow for inhomogeneity, internal currents, elasticity, and spin. Even if the angular momentum remains small, many such systems are found to be affected by large self-interaction effects beyond the standard Lorentz-Dirac force. These are particularly significant if the particle's charge density fails to be much greater than its 3-current density (or vice versa) in the center-of-mass frame. Additional terms also arise in the equations of motion if the dipole moment is too large, and when the `center-of-electromagnetic mass' is far from the `center-of-bare mass' (roughly speaking). These conditions are often quite restrictive. General equations of motion were also derived under the assumption that the particle can only interact with the radiative component of its self-field. These are much simpler than the equations derived using the full retarded self-field; as are the conditions required to recover the Lorentz-Dirac equation.Comment: 30 pages; significantly improved presentation; accepted for publication in Phys. Rev.

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 $\tau_0$ and magnetic field strengths $B$. 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 $E\sim 10^{18}{\rm eV} (R_s/{\rm pc}) (B/{\rm mG}) \tau_0^{1/\alpha}$, with $\alpha=1/3$ 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

Cell cycle-dependent cytotoxicity and induction of apoptosis by liposomal N4-hexadecyl-1-beta-D-arabinofuranosylcytosine.

The clonogenic growth inhibition, the cell cycle dependence of N4-hexadecyl-1-beta-D-arabinofuranosylcytosine (NHAC) cytotoxicity and the capability to induce apoptosis in ara-C-sensitive and -resistant HL-60 cells were investigated and compared with arabinofuranosylcytosine (ara-C). In the clonogenic assay with sensitive HL-60 cells, ara-C was slightly more effective than a liposomal preparation of NHAC, whereas in the resistant cells, NHAC revealed its potency to overcome ara-C resistance, resulting in a 23-fold lower 50% inhibitory concentration compared with ara-C. Cell cycle dependent cytotoxicity and induction of apoptosis were studied by flow cytometry, using the bromodeoxyuridine-propidium iodide and terminal transferase method respectively. In contrast to ara-C, NHAC exerted no phase-specific toxicity at low concentrations (< 40 microM). At higher concentrations the S-phase-specific toxicity increased, probably resulting from ara-C formed from NHAC. NHAC induced apoptosis at higher drug concentrations than ara-C, however apoptosis appeared not to be limited to the S-phase cells. Apoptosis occurred in both cell lines within 2-4 h after drug exposure. These results give further evidence that NHAC exerts its cytotoxicity by different mechanisms of action than ara-C and might therefore be active in ara-C-resistant tumours

Combining clinical factors and quantitative ultrasound improves the detection of women both at low and high risk for hip fracture

Summary: We hypothesized that combining clinical risk factors (CRF) with the heel stiffness index (SI) measured via quantitative ultrasound (QUS) would improve the detection of women both at low and high risk for hip fracture. Categorizing women by risk score improved the specificity of detection to 42.4%, versus 33.8% using CRF alone and 38.4% using the SI alone. This combined CRF-SI score could be used wherever and whenever DXA is not readily accessible. Introduction and hypothesis: Several strategies have been proposed to identify women at high risk for osteoporosis-related fractures; we wanted to investigate whether combining clinical risk factors (CRF) and heel QUS parameters could provide a more accurate tool to identify women at both low and high risk for hip fracture than either CRF or QUS alone. Methods: We pooled two Caucasian cohorts, EPIDOS and SEMOF, into a large database named "EPISEM”, in which 12,064 women, 70 to 100years old, were analyzed. Amongst all the CRF available in EPISEM, we used only the ones which were statistically significant in a Cox multivariate model. Then, we constructed a risk score, by combining the QUS-derived heel stiffness index (SI) and the following seven CRF: patient age, body mass index (BMI), fracture history, fall history, diabetes history, chair-test results, and past estrogen treatment. Results: Using the composite SI-CRF score, 42% of the women who did not report a hip fracture were found to be at low risk at baseline, and 57% of those who subsequently sustained a fracture were at high risk. Using the SI alone, corresponding percentages were 38% and 52%; using CRF alone, 34% and 53%. The number of subjects in the intermediate group was reduced from 5,400 (including 112 hip fractures) and 5,032 (including 111 hip fractures) to 4549 (including 100 including fractures) for the CRF and QUS alone versus the combination score. Conclusions: Combining clinical risk factors to heel bone ultrasound appears to correctly identify more women at low risk for hip fracture than either the stiffness index or the CRF alone; it improves the detection of women both at low and high ris

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

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 $2 \times 14.3$ 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 $3.5\cdot 10^{-8}$ GeV$^{-1}$ and $3.2\cdot 10^{-8}$ GeV$^{-1}$, respectively, at 95% Confidence Level.Comment: 6 pages, 6 figure

Suspended liminality: Vacillating affects in cyberbullying/research

This paper develops a concept of liminal hotspots in the context of i) a secondary analysis of a cyberbullying case involving a group of school children from a Danish school, and ii) an altered auto-ethnography in which the authors ‘entangle’ their own experiences with the case analysis. These two sources are used to build an account of a liminal hotspot conceived as an occasion of troubled and suspended transformative transition in which a liminal phase is extended and remains unresolved. The altered auto-ethnography is used to explore the affectivity at play in liminal hotspots, and this liminal affectivity is characterised in terms of volatility, vacillation, suggestibility and paradox