Diffusion-limited reactions and mortal random walkers in confined geometries

Motivated by the diffusion-reaction kinetics on interstellar dust grains, we study a first-passage problem of mortal random walkers in a confined two-dimensional geometry. We provide an exact expression for the encounter probability of two walkers, which is evaluated in limiting cases and checked against extensive kinetic Monte Carlo simulations. We analyze the continuum limit which is approached very slowly, with corrections that vanish logarithmically with the lattice size. We then examine the influence of the shape of the lattice on the first-passage probability, where we focus on the aspect ratio dependence: Distorting the lattice always reduces the encounter probability of two walkers and can exhibit a crossover to the behavior of a genuinely one-dimensional random walk. The nature of this transition is also explained qualitatively.Comment: 18 pages, 16 figure

A First Search for coincident Gravitational Waves and High Energy Neutrinos using LIGO, Virgo and ANTARES data from 2007

We present the results of the first search for gravitational wave bursts associated with high energy neutrinos. Together, these messengers could reveal new, hidden sources that are not observed by conventional photon astronomy, particularly at high energy. Our search uses neutrinos detected by the underwater neutrino telescope ANTARES in its 5 line configuration during the period January - September 2007, which coincided with the fifth and first science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed for candidate gravitational-wave signals coincident in time and direction with the neutrino events. No significant coincident events were observed. We place limits on the density of joint high energy neutrino - gravitational wave emission events in the local universe, and compare them with densities of merger and core-collapse events.Comment: 19 pages, 8 figures, science summary page at http://www.ligo.org/science/Publication-S5LV_ANTARES/index.php. Public access area to figures, tables at https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=p120000

Protocolized weaning from mechanical ventilation: ICU physicians' views

Background. The use of protocols during weaning from mechanical ventilation is uncommon in the UK, despite research pointing to their potential benefits. This may be because the research evidence is considered not to apply in different settings. Intensive care unit consultant physicians are the major decision-makers in weaning in the UK and any attempt to introduce protocolized weaning will require consideration of their views.Aim. The aim of this paper is to report a study exploring intensive care physicians’ views on (i) weaning from mechanical ventilation, (ii) the utility of weaning protocols and (iii) nurses’ roles in the weaning process. A specific goal was to identify potential aids and barriers to developing weaning protocols and their introduction into clinical practice.Methods. Qualitative interviews were conducted with a purposive sample of 10 consultant physicians in two intensive care units in Northern Ireland and subjected to content analysis.Findings. The primary themes identified were (i) information required for weaning decisions and clinical judgement, (ii) professional boundaries, (iii) protocol issues and (iv) timing of weaning. Three types of information were deemed to be required for weaning decisions – empirical objective, empirical subjective and abstract – and interviewees considered that it would be challenging to incorporate all into a protocol. They were divided on whether protocols were useful when nursing experience was limited. Some groups of patients were thought more suitable than others for protocolized weaning.Conclusions. Although local physicians were supportive in theory, introduction of protocolized weaning is likely to be difficult because of the breadth of information required for successful decision-making. Consultant views in this study were not consistent with American findings that physicians’ caution may unnecessarily prolong weaning