A New Basis for QED Bound State Computations

A simple method to compute QED bound state properties is presented, in which binding energy effects are treated non-perturbatively. It is shown that to take the effects of all ladder Coulomb photon exchanges into account, one can simply perform the derivative of standard QED amplitudes with respect to the external momentum. For example, the derivative of the light-by-light scattering amplitude gives an amplitude for orthopositronium decay to three photons where any number of Coulomb photon exchanges between the e-e+ is included. Various applications are presented. From them, it is shown that binding energy must be treated non-perturbatively in order to preserve the analyticity of positronium decay amplitudes. Interesting perspectives for quarkonium physics are briefly sketched.Comment: LaTeX, 23 pages, 16 figures. Minor corrections. Some comments adde

Sifting the Network: Performing Packet Triage with NFR

We describe a set of Network Flight Recorder scripts designed to detect network intrusions. After developing scripts that detect some known attacks, we focus on sifting scripts that attempt to remove "normal" traffic from a packet stream, leaving behind fewer packets requiring manual inspection. We conclude with a description of our experiences with the NFR product.http://deepblue.lib.umich.edu/bitstream/2027.42/107922/1/citi-tr-98-6.pd

Evaluating Delayed Write in a Multilevel Caching File System

Delayed write in a multilevel file system cache hierarchy offers a way to improve performance in diverse client/server scenarios, such as integrating mass store into a distributed file system or providing distributed file system access over low-speed links. Using file system traces and cache simulations, we explore extensions and modifications to the traditional client caching model employed in such file systems as AFS, Sprite, and DFS. High cache hit rates at an intermediate cache server—a machine logically interposed between clients and servers that provides cached file service to the clients—combined with high client cache hit rates lend practicality to an integrated mass storage file system. In such a system, magnetic tape or optical-based mass storage devices may be used as a first-class data repository, fronted by disk and RAM caches to offer acceptable access times to the large, but slow, mass storage system. Similarly, a high cache hit rate is necessary for users accessing file systems via low-speed links, where a delayed write intermediate caching server can mediate traffic to make better use of available bandwidth. In an example taken from mobile computing, an intermediate server might be used as a docking station at a user's home. This arrangement would be convenient for users of mobile computers who upload large amounts of data generated while operating in disconnected mode. Simulations of delayed write caching strategies are applicable to both the mass storage and low-speed link scenarios.http://deepblue.lib.umich.edu/bitstream/2027.42/107940/1/citi-tr-95-9.pd

Weak solutions to problems involving inviscid fluids

We consider an abstract functional-differential equation derived from the pressure-less Euler system with variable coefficients that includes several systems of partial differential equations arising in the fluid mechanics. Using the method of convex integration we show the existence of infinitely many weak solutions for prescribed initial data and kinetic energy

Positronium Decay : Gauge Invariance and Analyticity

The construction of positronium decay amplitudes is handled through the use of dispersion relations. In this way, emphasis is put on basic QED principles: gauge invariance and soft-photon limits (analyticity). A firm grounding is given to the factorization approaches, and some ambiguities in the spin and energy structures of the positronium wavefunction are removed. Non-factorizable amplitudes are naturally introduced. Their dynamics is described, especially regarding the enforcement of gauge invariance and analyticity through delicate interferences. The important question of the completeness of the present theoretical predictions for the decay rates is then addressed. Indeed, some of those non-factorizable contributions are unaccounted for by NRQED analyses. However, it is shown that such new contributions are highly suppressed, being of order alpha^3. Finally, a particular effective form factor formalism is constructed for parapositronium, allowing a thorough analysis of binding energy effects and analyticity implementation.Comment: 34 pages, 13 figure

Enhancing Collaboration Between Primary and Subspeciality Care Providers for Children and Youth with Special Health Care Needs

Summary: The goals of this guide are to discuss the complementary roles of generalist and subspecialist physicians in providing coordinated and effective care for children and youth with special health care needs. We will emphasize the centrality of family-professional partnerships. We also will describe various models for collaboration among generalist and subspecialist physicians and families. Ultimately, the value of this guide will be to serve as a framework for discussion about how primary and subspecialty care physicians can work collaboratively to enhance the quality of care that children and youth with special health care needs and their families receive

Access Control in a Workstation-Based Distributed Computing Environment

This paper describes the mechanisms employed to control access to system services on the IFS project. We base our distributed computing environment on systems that we trust, and run those systems in physically secure rooms. From that base, we add services, modifying them to interoperate with existing access control mechanisms. Some weaknesses remain in our environment; we conclude with a description of present vulnerabilities and future plans.http://deepblue.lib.umich.edu/bitstream/2027.42/107869/1/citi-tr-90-2.pd

Gravitational Spin-Orbit Coupling through Third-Subleading Post-Newtonian Order: From First-Order Self-Force to Arbitrary Mass Ratios

Exploiting simple yet remarkable properties of relativistic gravitational scattering, we use first-order self-force (linear-in-mass-ratio) results to obtain arbitrary-mass-ratio results for the complete third-subleading post-Newtonian (4.5PN) corrections to the spin-orbit sector of spinning-binary conservative dynamics, for generic (bound or unbound) orbits and spin orientations. We thereby improve important ingredients of models of gravitational waves from spinning binaries, and we demonstrate the improvement in accuracy by comparing against aligned-spin numerical simulations of binary black holes

Nonlinear propagation equations in fibers with multiple modes—Transitions between representation bases

The transverse pattern of the field that propagates in a fiber supporting multiple modes can always be described as a superposition of the patterns of the individual fiber modes. Yet, the use of other bases is often found to be more convenient, with the most famous example being that of linearly polarized modes in weakly guiding fibers. The nonlinear propagation equations contain coefficients that involve overlap integrals between the lateral profiles of multiple propagation modes. A fundamental question that has been raised in this context is whether it is legitimate to compute these coefficients from the overlap integrals between elements of alternative bases for the field representation. In this paper, we show that the answer to this question is positive in the most general sense. This result is significant in the context of space-division multiplexed transmission in multi-mode and multi-core fibers.The transverse pattern of the field that propagates in a fiber supporting multiple modes can always be described as a superposition of the patterns of the individual fiber modes. Yet, the use of other bases is often found to be more convenient, with the most famous example being that of linearly polarized modes in weakly guiding fibers. The nonlinear propagation equations contain coefficients that involve overlap integrals between the lateral profiles of multiple propagation modes. A fundamental question that has been raised in this context is whether it is legitimate to compute these coefficients from the overlap integrals between elements of alternative bases for the field representation. In this paper, we show that the answer to this question is positive in the most general sense. This result is significant in the context of space-division multiplexed transmission in multi-mode and multi-core fibers

How Abundant is Iron in the Core of the Perseus Cluster?

The analysis of Perseus data collected with the Medium Energy Concentrator Spectrometer (MECS) on board Beppo-SAX shows that the ratio of the flux of the 8 keV line complex (dominated by Fe K$_{\beta}$ emission) over the 6.8 keV line complex (dominated by Fe K$_{\alpha}$ emission) is significantly larger than predicted by standard thermal emission codes. Moreover the analysis of spatially resolved spectra shows that the above ratio decreases with increasing cluster radius. We find that, amongst the various explanations we consider, the most likely requires the plasma to be optically thick for resonant scattering at the energy of the Fe K$_{\alpha}$ line. We argue that if this is the case, then measures of the iron abundance made using standard thermal emission codes, that assume optically thin emission, can significantly underestimate the true iron abundance. In the case of the core of Perseus we estimate the true abundance to be $\sim$ 0.9 solar in a circular region with radius of $\sim 60$ kpc and centered on NGC 1275. Finally we speculate that similar results may hold for the core of other rich clusters.Comment: 19 pages, 3 Postscript figure