Infrared divergences and non-lightlike eikonal lines in Sudakov processes

We propose a method to analyze infrared contributions to non-inclusive processes in QCD. We use the one-loop Sudakov form factor as a working example. Borrowing techniques from renormalization theory, we construct counterterms for the contributions from the soft and collinear regions, and we relate them to eikonal Wilson lines taken along non-lightlike directions.Comment: Latex, 1 postscript figure. Version 2: presentation improved and typos corrected, results unchange

A Unified Theory for the Effects of Stellar Perturbations and Galactic Tides on Oort Cloud Comets

We examine the effects of passing field stars on the angular momentum of a nearly radial orbit of an Oort cloud comet bound to the Sun. We derive the probability density function (PDF) of the change in angular momentum from one stellar encounter, assuming a uniform and isotropic field of perturbers. We show that the total angular momentum follows a Levy flight, and determine its distribution function. If there is an asymmetry in the directional distribution of perturber velocities, the marginal probability distribution of each component of the angular momentum vector can be different. The constant torque attributed to Galactic tides arises from a non-cancellation of perturbations with an impact parameter of order the semimajor axis of the comet. When the close encounters are rare, the angular momentum is best modeled by the stochastic growth of stellar encounters. If trajectories passing between the comet and sun occur frequently, the angular momentum exhibits the coherent growth attributed to the Galactic tides.Comment: 8 pages, 2 figures; accepted to A

How do adults with cystic fibrosis cope following a diagnosis of diabetes?

The official published version of the article can be obtained from the link below.Aim. This paper is a report of a study examining the experience of adults with cystic fibrosis in adapting to the diagnosis of diabetes, a second chronic illness. Background. Diabetes is a common complication of cystic fibrosis; the onset signifies the development of a second chronic illness. Both cystic fibrosis and diabetes are complex conditions, which require daily treatment schedules as part of their management. However, it is unclear how people already living with cystic fibrosis respond to the diagnosis of diabetes. Method. A qualitative method was chosen to obtain an ‘insider’ experience of adjusting to a second chronic illness. Semi-structured interviews were conducted in 2004 with 22 adults with cystic fibrosis-related diabetes. The data were analysed using interpretative phenomenological analysis. Findings. Four recurring themes were identified: emotional response to diagnosis of diabetes, looking for an understanding, learning to live with diabetes, and limiting the impact of diagnosis. Having cystic fibrosis appeared helpful in limiting the impact of the diagnosis of diabetes. Juggling conflicting dietary demands of cystic fibrosis and diabetes coupled with the lack of practical professional advice available was seen as one of the biggest challenges in adapting to diabetes. Conclusion. Healthcare professionals need increased awareness of diabetes amongst adults with cystic fibrosis and provide adequate support and structured evidence-based education throughout the course of the illness, particularly in relation to diet. Nevertheless, patients’ familiarity with regular daily routines and problem-solving attitudes, already developed in the context of cystic fibrosis, may be drawn on to limit the impact of diabetes

The tectonic evolution of central and Northern Madagascar and its place in the final assembly of Gondwana

Copyright © 2002 by The University of Chicago. All rights reserved.Recent work in central and northern Madagascar has identified five tectonic units of the East African Orogen (EAO), a large collisional zone fundamental to the amalgamation of Gondwana. These five units are the Antongil block, the Antananarivo block, the Tsaratanana sheet, the Itremo sheet, and the Bemarivo belt. Geochronological, lithological, metamorphic, and geochemical characteristics of these units and their relationships to each other are used as a type area to compare and contrast with surrounding regions of Gondwana. The Antananarivo block of central Madagascar, part of a broad band of pre-1000-Ma continental crust that stretches from Yemen through Somalia and eastern Ethiopia into Madagascar, is sandwiched between two suture zones we interpret as marking strands of the Neoproterozoic Mozambique Ocean. The eastern suture connects the Al-Mukalla terrane (Yemen), the Maydh greenstone belt (northern Somalia), the Betsimisaraka suture (east Madagascar), and the Palghat-Cauvery shear zone system (south India). The western suture projects the Al-Bayda terrane (Yemen) through a change in crustal age in Ethiopia to the region west of Madagascar. Our new framework for the central EAO links the Mozambique belt with the Arabian/Nubian Shield and highlights the power of tectonic analysis in unraveling the complex tectonic collage of the EAO.Alan S. Collins and Brian F. Windle

Inclusive Diffraction at HERA

New precision measurements of inclusive diffractive deep-inelastic ep scattering interactions, performed by the H1 and ZEUS collaborations at the HERA collider, are discussed. A new set of diffractive parton distributions, determined from recent high precision H1 data, is presented.Comment: 5 pages, to appear in the proceedings of the 31st Intl. Conference on High Energy Physics ICHEP 2002, Amsterdam, July 200

Low density gas dynamic wall boundary conditions

Low density nozzles or large expansion ratio nozzles used in space experience rarefaction effects near their exit in the form of velocity slip and temperature jump at the walls. In addition, the boundary layers become very thick and there is a very strong viscous/inviscid interaction. For these reasons no existing design technique has been found to accurately predict the nozzle flow properties up to the nozzle exit. The objective of this investigation was to examine the slip boundary conditions and formulate them in a form appropriate for use with a full Navier-Stokes numerical code. The viscous/inviscid interaction would automatically be accounted for by using a compressible Navier-Stokes code. Through examination of the interaction of molecules with solid surfaces, a model for the distribution function of the reflected molecules has been determined and this distribution function has been used to develop a new slip boundary condition that can be shown to yield more realistic surface boundary conditions

Protoplanet Dynamics in a Shear-Dominated Disk

The velocity dispersion, or eccentricity distribution, of protoplanets interacting with planetesimals is set by a balance between dynamical friction and viscous stirring. We calculate analytically the eccentricity distribution function of protoplanets embedded in a cold, shear-dominated planetesimal swarm. We find a distinctly non-Rayleigh distribution with a simple analytical form. The peak of the distribution lies much lower than the root-mean-squared value, indicating that while most of the bodies have similarly small eccentricities, a small subset of the population contains most of the thermal energy. We also measure the shear-dominated eccentricity distribution using numerical simulations. The numerical code treats each protoplanet explicitly and adds an additional force term to each body to represent the dynamical friction of the planetesimals. Without fitting any parameters, the eccentricity distribution of protoplanets in the N-body simulation agrees with the analytical results. This distribution function provides a useful tool for testing hybrid numerical simulations of late-stage planet formation.Comment: 8 pages, 2 figure

Soft gluons and gauge-invariant subtractions in NLO parton-shower Monte Carlo event generators

We address the problem of decomposing graphs in perturbative QCD into terms associated with particular regions. Motivated by asking how to incorporate next-to-leading order (NLO) QCD corrections in parton-shower algorithms, we require that: (a) The integrand for the hard part is to be integrable even if the corrections are applied to a process that is not infrared and collinear safe. (b) The splitting between the terms should be defined gauge-invariantly. (c) The dependence on cut-offs should obey homogeneous evolution equations. In the context of one-gluon-emission graphs for deep inelastic scattering, we explain a subtractive technique that is based on gauge-invariant Wilson-line operators. Appropriate organization of subtractions involving the soft region allows a connection to previous work where evolution equations with respect to the directions of the Wilson lines have been derived.Comment: 11 pages, 2 figures; v2: comments and references added, results unchange

Co-orbital Oligarchy

We present a systematic examination of the changes in semi-major axis caused by the mutual interactions of a group of massive bodies orbiting a central star in the presence of eccentricity dissipation. For parameters relevant to the oligarchic stage of planet formation, dynamical friction keeps the typical eccentricities small and prevents orbit crossing. Interactions at impact parameters greater than several Hill radii cause the protoplanets to repel each other; if the impact parameter is instead much less than the Hill radius, the protoplanets shift slightly in semi-major axis but remain otherwise unperturbed. If the orbits of two or more protoplanets are separated by less than a Hill radius, they are each pushed towards an equilibrium spacing between their neighbors and can exist as a stable co-orbital system. In the shear-dominated oligarchic phase of planet formation we show that the feeding zones contain several oligarchs instead of only one. Growth of the protoplanets in the oligarchic phase drives the disk to an equilibrium configuration that depends on the mass ratio of protoplanets to planetesimals, $\Sigma/\sigma$. Early in the oligarchic phase, when $\Sigma/\sigma$ is low, the spacing between rows of co-orbital oligarchs are about 5 Hill radii wide, rather than the 10 Hill radii cited in the literature. It is likely that at the end of oligarchy the average number of co-orbital oligarchs is greater than unity. In the outer solar system this raises the disk mass required to form the ice giants. In the inner solar system this lowers the mass of the final oligarchs and requires more giant impacts than previously estimated. This result provides additional evidence that Mars is not an untouched leftover from the oligarchic phase, but must be composed of several oligarchs assembled through giant impacts.Comment: 10 pages, 8 figures. v2 includes major revisions including additional results motivated by the referee's comment

Structure Functions are not Parton Probabilities

We explain why contrary to common belief, the deep inelastic scattering structure functions are not related to parton probabilities in the target.Comment: 4 pages. Invited talk presented during the `International Light-Cone Workshop', Trento, ECT, September 3-11, 2001. Updated Report-Number