962 research outputs found
Ice nucleation from aqueous NaCl droplets with and without marine diatoms
Ice formation in the atmosphere by homogeneous and heterogeneous nucleation is one of the least understood processes in cloud microphysics and climate. Here we describe our investigation of the marine environment as a potential source of atmospheric IN by experimentally observing homogeneous ice nucleation from aqueous NaCl droplets and comparing against heterogeneous ice nucleation from aqueous NaCl droplets containing intact and fragmented diatoms. Homogeneous and heterogeneous ice nucleation are studied as a function of temperature and water activity, <i>a</i><sub>w</sub>. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, &omega;<sub>het</sub>, ice nucleation rate coefficients, <i>J</i><sub>het</sub>, and differential and cumulative ice nuclei spectra, <i>k(T)</i> and <i>K(T)</i>, respectively. Homogeneous freezing temperatures and corresponding nucleation rate coefficients are in agreement with the water activity based homogeneous ice nucleation theory within experimental and predictive uncertainties. Our results confirm, as predicted by classical nucleation theory, that a stochastic interpretation can be used to describe the homogeneous ice nucleation process. Heterogeneous ice nucleation initiated by intact and fragmented diatoms can be adequately represented by a modified water activity based ice nucleation theory. A horizontal shift in water activity, &Delta;<i>a</i><sub>w, het</sub> = 0.2303, of the ice melting curve can describe median heterogeneous freezing temperatures. Individual freezing temperatures showed no dependence on available diatom surface area and aqueous volume. Determined at median diatom freezing temperatures for <i>a</i><sub>w</sub> from 0.8 to 0.99, &omega;<sub>het</sub><u>~</u>0.11<sup>+0.06</sup><sub>&minus;0.05</sub> s<sup>−1</sup>, <i>J</i><sub>het</sub><u>~</u>1.0<sup>+1.16</sup><sub>&minus;0.61</sub>&times;10<sup>4</sup> cm<sup>−2</sup> s<sup>−1</sup>, and <i>K</i><u>~</u>6.2<sup>+3.5</sup><sub>&minus;4.1</sub> &times;10<sup>4</sup> cm<sup>−2</sup>. The experimentally derived ice nucleation rates and nuclei spectra allow us to estimate ice particle production which we subsequently use for a comparison with observed ice crystal concentrations typically found in cirrus and polar marine mixed-phase clouds. Differences in application of time-dependent and time-independent analyses to predict ice particle production are discussed
Stability of complex hyperbolic space under curvature-normalized Ricci flow
Using the maximal regularity theory for quasilinear parabolic systems, we
prove two stability results of complex hyperbolic space under the
curvature-normalized Ricci flow in complex dimensions two and higher. The first
result is on a closed manifold. The second result is on a complete noncompact
manifold. To prove both results, we fully analyze the structure of the
Lichnerowicz Laplacian on complex hyperbolic space. To prove the second result,
we also define suitably weighted little H\"{o}lder spaces on a complete
noncompact manifold and establish their interpolation properties.Comment: Some typos in version 2 are correcte
New Dimensions for Wound Strings: The Modular Transformation of Geometry to Topology
We show, using a theorem of Milnor and Margulis, that string theory on
compact negatively curved spaces grows new effective dimensions as the space
shrinks, generalizing and contextualizing the results in hep-th/0510044.
Milnor's theorem relates negative sectional curvature on a compact Riemannian
manifold to exponential growth of its fundamental group, which translates in
string theory to a higher effective central charge arising from winding
strings. This exponential density of winding modes is related by modular
invariance to the infrared small perturbation spectrum. Using self-consistent
approximations valid at large radius, we analyze this correspondence explicitly
in a broad set of time-dependent solutions, finding precise agreement between
the effective central charge and the corresponding infrared small perturbation
spectrum. This indicates a basic relation between geometry, topology, and
dimensionality in string theory.Comment: 28 pages, harvmac big. v2: references and KITP preprint number added,
minor change
Ricci flow for homogeneous compact models of the universe
Using quaternions, we give a concise derivation of the Ricci tensor for
homogeneous spaces with topology of the 3-dimensional sphere. We derive
explicit and numerical solutions for the Ricci flow PDE and discuss their
properties. In the collapse (or expansion) of these models, the interplay of
the various components of the Ricci tensor are studied. We dedicate this paper
to honor the work of Josh Goldberg.Comment: 18 pages, 2 figure
Adolescent Self-Consent for Biomedical HIV Prevention Research: Implications for Institutional Review Board Approval and Implementation
Purpose
The Adolescent Medicine Trials Network Protocol 113 (ATN113) is an open-label, multisite demonstration project and Phase II safety study of human immunodeficiency virus (HIV) preexposure prophylaxis with 15- to 17-year-old young men who have sex with men that requires adolescent consent for participation. The purpose of this study was to examine factors related to the process by which Institutional Review Boards (IRBs) and researchers made decisions regarding whether to approve and implement ATN113 so as to inform future biomedical HIV prevention research with high-risk adolescent populations.
Methods
Participants included 17 researchers at 13 sites in 12 states considering ATN113 implementation. Qualitative descriptive methods were used. Data sources included interviews and documents generated during the initiation process.
Results
A common process for initiating ATN113 emerged, and informants described how they identified and addressed practical, ethical, and legal challenges that arose. Informants described the process as responding to the protocol, preparing for IRB submission, abstaining from or proceeding with submission, responding to IRB concerns, and reacting to the outcomes. A complex array of factors impacting approval and implementation were identified, and ATN113 was ultimately implemented in seven of 13 sites. Informants also reflected on lessons learned that may help inform future biomedical HIV prevention research with high-risk adolescent populations.
Conclusions
The results illustrate factors for consideration in determining whether to implement such trials, demonstrate that such protocols have the potential to be approved, and highlight a need for clearer standards regarding biomedical HIV prevention research with high-risk adolescent populations
Minors' and Young Adults' Experiences of the Research Consent Process in a Phase II Safety Study of Pre-exposure Prophylaxis for HIV
PURPOSE:
There is a persistent HIV epidemic among sexual and gender minority adolescents in the U.S. Oral pre-exposure prophylaxis (PrEP) is an efficacious prevention strategy, but not yet approved for minors. Minors' access to biomedical HIV prevention technologies is impeded by the ethical and legal complexities of consent to research participation. We explore autonomous consent and study experiences among minor and adult participants in Project PrEPare, a Phase II safety study of PrEP for HIV prevention.
METHODS:
Data for this mixed-methods descriptive study were collected via self-administered web-survey and in-depth telephone interviews in early 2016. Eligible participants were previously enrolled in Project PrEPare. We attempted to contact 191 participants; 74 were reached and expressed interest in participating and 58 enrolled.
RESULTS:
Participants nearly universally felt well informed, understood the study, and freely volunteered with the clear understanding they could withdraw any time. All felt supported by study staff, but a small minority wished for more support during enrollment. Minors were more likely than adults to indicate a wish for more support in decision-making, and adults expressed higher satisfaction with their decision compared to minors. There was no association between elements of consent and Project PrEPare study outcomes.
CONCLUSIONS:
Participants had an overwhelmingly positive experience in a Phase II safety study of PrEP for HIV prevention. Some minors wished for more support during the decision-making process, but none consulted their parents about the decision. Our results support the inclusion of decisional supports in consent processes for adolescents, while also protecting their privacy
Formal matched asymptotics for degenerate Ricci flow neckpinches
Gu and Zhu have shown that Type-II Ricci flow singularities develop from
nongeneric rotationally symmetric Riemannian metrics on , for all . In this paper, we describe and provide plausibility arguments for a
detailed asymptotic profile and rate of curvature blow-up that we predict such
solutions exhibit
The LHCb Outer Tracker Front End Electronics
This note provides an overview of the front-end electronics used to readout the drift-times of the LHCb Outer Tracker straw tube chambers. The main functional components of the readout are the ASDBLR ASIC for amplification and signal digitization, the OTIS ASIC for the time measurement and for the L0 buffering, and the GOL ASIC to serialize the digital data for the optical data transmission. The L1 buffer board used to receive the data which is sent via the optical link is a common LHCb development and is not described here. This note supersedes an earlier document [1]
Using Advanced Mass Spectrometry Techniques to Fully Characterize Atmospheric Organic Carbon: Current Capabilities and Remaining Gaps
Organic compounds in the atmosphere vary widely in their molecular composition and chemical properties, so no single instrument can reasonably measure the entire range of ambient compounds. Over the past decade, a new generation of in-situ, field-deployable mass spectrometers has dramatically improved our ability to detect, identify, and quantify these organic compounds, but no systematic approach has been developed to assess the extent to which currently available tools capture the entire space of chemical identity and properties that is expected in the atmosphere. Reduced-parameter frameworks that have been developed to describe atmospheric mixtures are exploited here to characterize the range of chemical properties accessed by a suite of instruments. Multiple chemical spaces (e.g. oxidation state of carbon vs. volatility, and oxygen number vs. carbon number) were populated with ions measured by several mass spectrometers, with gas- and particle-phase -pinene oxidation products serving as the test mixture of organic compounds. Few gaps are observed in the coverage of the parameter spaces by the instruments employed in this work, though the full extent to which comprehensive measurement was achieved is difficult to assess due to uncertainty in the composition of the mixture. Overlaps between individual ions and regions in parameter space were identified, both between gas- and particle-phase measurements, and within each phase. These overlaps were conservatively found to account for little (<10%) of the measured mass. However, challenges in identifying overlaps and in accurately converting molecular formulas into chemical properties (such as volatility or reactivity) highlight a continued need to incorporate structural information into atmospheric measurements
Investigating Off-shell Stability of Anti-de Sitter Space in String Theory
We propose an investigation of stability of vacua in string theory by
studying their stability with respect to a (suitable) world-sheet
renormalization group (RG) flow. We prove geometric stability of (Euclidean)
anti-de Sitter (AdS) space (i.e., ) with respect to the simplest
RG flow in closed string theory, the Ricci flow. AdS space is not a fixed point
of Ricci flow. We therefore choose an appropriate flow for which it is a fixed
point, prove a linear stability result for AdS space with respect to this flow,
and then show this implies its geometric stability with respect to Ricci flow.
The techniques used can be generalized to RG flows involving other fields. We
also discuss tools from the mathematics of geometric flows that can be used to
study stability of string vacua.Comment: 29 pages, references added in this version to appear in Classical and
Quantum Gravit
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