Collision damping in the pi 3He -> d'N reaction near the threshold

We present a simple quantum mechanical model exploiting the optical potential approach for the description of collision damping in the reaction pi 3He -> d'N near the threshold, which recently has been measured at TRIUMF. The influence of the open d'N -> NNN channel is taken into account. It leads to a suppression factor of about ten in the d' survival probability. Applications of the method to other reactions are outlined.Comment: RevTeX4, 14 pages, 3 Postscript figures, uses epsfig.sty, to appear in Phys.Rev.

Pion double charge exchange on 4He

The doubly differential cross sections for the $^4$He$(\pi^+,\pi^-) 4p$ reaction were calculated using both a two-nucleon sequential single charge exchange model and an intranuclear cascade code. Final state interactions between the two final protons which were the initial neutrons were included in both methods. At incident pion energies of 240 and 270 MeV the low-energy peak observed experimentally in the energy spectrum of the final pions can be understood only if the contribution of pion production is included. The calculated cross sections are compared with data.Comment: 25 pages, 9 figure

Optimization of the derivative expansion in the nonperturbative renormalization group

We study the optimization of nonperturbative renormalization group equations truncated both in fields and derivatives. On the example of the Ising model in three dimensions, we show that the Principle of Minimal Sensitivity can be unambiguously implemented at order $\partial^2$ of the derivative expansion. This approach allows us to select optimized cut-off functions and to improve the accuracy of the critical exponents $\nu$ and $\eta$. The convergence of the field expansion is also analyzed. We show in particular that its optimization does not coincide with optimization of the accuracy of the critical exponents.Comment: 13 pages, 9 PS figures, published versio

Heisenberg frustrated magnets: a nonperturbative approach

Frustrated magnets are a notorious example where the usual perturbative methods are in conflict. Using a nonperturbative Wilson-like approach, we get a coherent picture of the physics of Heisenberg frustrated magnets everywhere between $d=2$ and $d=4$. We recover all known perturbative results in a single framework and find the transition to be weakly first order in $d=3$. We compute effective exponents in good agreement with numerical and experimental data.Comment: 5 pages, Revtex, technical details available at http://www.lpthe.jussieu.fr/~tissie

A non perturbative approach of the principal chiral model between two and four dimensions

We investigate the principal chiral model between two and four dimensions by means of a non perturbative Wilson-like renormalization group equation. We are thus able to follow the evolution of the effective coupling constants within this whole range of dimensions without having recourse to any kind of small parameter expansion. This allows us to identify its three dimensional critical physics and to solve the long-standing discrepancy between the different perturbative approaches that characterizes the class of models to which the principal chiral model belongs.Comment: 5 pages, 1 figure, Revte

Glacier outflow dissolved organic matter as a window into seasonally changing carbon sources: Leverett Glacier, Greenland

The Greenland Ice Sheet is losing mass at a remarkable rate as a result of climatic warming. This mass loss coincides with the export of dissolved organic matter (DOM) in glacial meltwaters. However, little is known about how the source and composition of exported DOM changes over the melt season, which is key for understanding its fate in downstream ecosystems. Over the 2015 ablation season, we sampled the outflow of Leverett Glacier, a large land‐terminating glacier of the Greenland Ice Sheet. Dissolved organic carbon (DOC) concentrations and DOM fluorescence were analyzed to assess the evolution of DOM sources over the course of the melt season. DOC concentrations and red‐shifted fluorescence were highly associated (R2 > 0.95) and suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein‐like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, and these sources were differentiated using fluorescence characteristics. Red‐shifted fluorescence components were robust proxies for DOC concentration. Finally, the majority of DOM flux, which occurs during the outburst and postoutburst periods, was characterized by protein‐like fluorescence from supraglacial and potentially subglacial microbial sources. As protein‐like fluorescence is linked to the bioavailability of DOM, the observed changes likely reflect seasonal variations in the impact of glacial inputs on secondary production in downstream ecosystems due to shifting hydrologic regimes

Few-body resonances in light nuclei

We have localized several few-body resonances in light nuclei, using methods which can properly handle two- or three-body resonant states. Among other results, we predict the existence of a three-neutron resonance, small spin-orbit splittings between the low-lying states in He-5 and Li-5, the nonexistence of the soft dipole resonance in He-6, new 1+ states in Li-8 and B-8, and the presence of a nonlinear amplification phenomenon in the 0+_2 state of C-12

The strategic economic governance of Greater Manchester's local labour market by the local state: implications for young workers

This article explores how work and employment conditions for young workers are affected by the actions of the state at the spatial scale of the locality. The article argues that young workers have experienced deteriorating labour market conditions following shifts in the form which capitalist accumulation takes in the UK. This shift has altered the composition of the national state which has in turn led to changes in how it regulates both local labour markets and the economic strategies of the local state. One result of these changes is the diffusion of neoliberal labour market reforms which have led to negative material consequences for young workers; these are manifest in the expansion of low-waged work concentrated in a small number of sectors, and characterized by an intensified labour process

Single-Molecule Force Spectroscopy: Experiments, Analysis, and Simulations

International audienceThe mechanical properties of cells and of subcellular components are important to obtain a mechanistic molecular understanding of biological processes. The quantification of mechanical resistance of cells and biomolecules using biophysical methods matured thanks to the development of nanotechnologies such as optical and magnetic tweezers, the biomembrane force probe and atomic force microscopy (AFM). The quantitative nature of force spectroscopy measurements has converted AFM into a valuable tool in biophysics. Force spectroscopy allows the determination of the forces required to unfold protein domains and to disrupt individual receptor/ligand bonds. Molecular simulation as a computational microscope allows investigation of similar biological processes with an atomistic detail. In this chapter, we first provide a step-by-step protocol of force spectroscopy including sample preparation, measurement and analysis of force spectroscopy using AFM and its interpretation in terms of available theories. Next, we present the background for molecular dynamics (MD) simulations focusing on steered molecular dynamics (SMD) and the importance of bridging of computational tools with experimental technique

Cytoskeletal protein kinases: titin and its relations in mechanosensing

Titin, the giant elastic ruler protein of striated muscle sarcomeres, contains a catalytic kinase domain related to a family of intrasterically regulated protein kinases. The most extensively studied member of this branch of the human kinome is the Ca2+–calmodulin (CaM)-regulated myosin light-chain kinases (MLCK). However, not all kinases of the MLCK branch are functional MLCKs, and about half lack a CaM binding site in their C-terminal autoinhibitory tail (AI). A unifying feature is their association with the cytoskeleton, mostly via actin and myosin filaments. Titin kinase, similar to its invertebrate analogue twitchin kinase and likely other “MLCKs”, is not Ca2+–calmodulin-activated. Recently, local protein unfolding of the C-terminal AI has emerged as a common mechanism in the activation of CaM kinases. Single-molecule data suggested that opening of the TK active site could also be achieved by mechanical unfolding of the AI. Mechanical modulation of catalytic activity might thus allow cytoskeletal signalling proteins to act as mechanosensors, creating feedback mechanisms between cytoskeletal tension and tension generation or cellular remodelling. Similar to other MLCK-like kinases like DRAK2 and DAPK1, TK is linked to protein turnover regulation via the autophagy/lysosomal system, suggesting the MLCK-like kinases have common functions beyond contraction regulation