Paul Scherrer Institute

About: Paul Scherrer Institute is a facility organization based out in Villigen, Switzerland. It is known for research contribution in the topics: Neutron & Large Hadron Collider. The organization has 9248 authors who have published 23984 publications receiving 890129 citations. The organization is also known as: PSI.

Scaling form of viscosity at all length-scales in poly(ethylene glycol) solutions studied by fluorescence correlation spectroscopy and capillary electrophoresis

Abstract: We measured the viscosity of poly(ethylene glycol) (PEG 6000, 12 000, 20 000) in water using capillary electrophoresis and fluorescence correlation spectroscopy with nanoscopic probes of different diameters (from 1.7 to 114 nm). For a probe of diameter smaller than the radius of gyration of PEG (e.g.rhodamine B or lyzozyme) the measured nanoviscosity was orders of magnitude smaller than the macroviscosity. For sizes equal to (or larger than) the polymer radius of gyration, macroscopic value of viscosity was measured. A mathematical relation for macro and nanoviscosity was found as a function of PEG radius of gyration, Rg, correlation length in semi-dilute solution, ξ, and probe size, R. For R Rg, both nano and macroviscosity follow the same curve, exp(b(R/ξ)a), where a and b are two constants close to unity. This mathematical relation was shown to equally well describe rhodamine (of size 1.7 nm) in PEG 20 000 and the macroviscosity of PEG 8 000 000, whose radius of gyration exceeds 200 nm. Additionally, for the smallest probes (rhodamine B and lysozyme) we have verified, using capillary electrophoresis and fluorescence correlation spectroscopy, that the Stokes–Einstein (SE) relation holds, providing that we use a size-dependent viscosity in the formula. The SE relation is correct even in PEG solutions of very high viscosity (three orders of magnitude larger than that of water).

Phase-contrast X-ray microtomography links Cretaceous seeds with Gnetales and Bennettitales

Abstract: The study of the emergence of flowering plants has been revolutionized over the past 25 years by the discovery of many exquisitely preserved fossil flowers. But fossil gymnosperms (conifers) have received less attention. Friis et al. have used a recently developed synchrotron radiation X-ray tomographic technique to redress the balance. Their images reveal internal structures at very high resolution. The seed structures of some Cretaceous gymnosperms suggest evolutionary links with Gnetales (an obscure and evolutionarily hard-to-place gymnosperm group with three living genera) and the Bennettitales (an important extinct group of cycad-like plants). A relatively new imaging technique is used to reveal that some Cretaceous gymnosperm seeds have evolutionary links with Gnetales (an evolutionarily hard-to-place gymnosperm group with three living genera) and the Bennetitales (an extinct group of cycad-like plants). The link between Gnetales and Bennetitales may have important consequences for our understanding of the evolution of seed plants, including flowering plants. Over the past 25 years the discovery and study of Cretaceous plant mesofossils has yielded diverse and exquisitely preserved fossil flowers that have revolutionized our knowledge of early angiosperms1, but remains of other seed plants in the same mesofossil assemblages2,3 have so far received little attention. These fossils, typically only a few millimetres long, have often been charred in natural fires and preserve both three-dimensional morphology and cellular detail. Here we use phase-contrast-enhanced synchrotron-radiation X-ray tomographic microscopy to clarify the structure of small charcoalified gymnosperm seeds from the Early Cretaceous of Portugal and North America. The new information links these seeds to Gnetales (including Erdtmanithecales, a putatively closely related fossil group2), and to Bennettitales—important extinct Mesozoic seed plants with cycad-like leaves and flower-like reproductive structures. The results suggest that the distinctive seed architecture of Gnetales, Erdtmanithecales and Bennettitales defines a clade containing these taxa. This has significant consequences for hypotheses of seed plant phylogeny by providing support for key elements of the controversial anthophyte hypothesis, which links angiosperms, Bennettitales and Gnetales.

Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WT e 2

Abstract: The authors present here several advances towards a comprehensive understanding of WTe${}_{2}$. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe${}_{2}$. Moreover, they demonstrate for the first time the presence of large surface state Fermi arcs on both surfaces. Using surface electronic structure calculations, they further demonstrate that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. Contrary to common belief, the observation of surface state Fermi arcs is thus not suitable to robustly identify a type-II Weyl semimetal. Finally, the authors use the observation of Fermi arcs and distinct top and bottom surfaces to clarify the controversial bulk electronic structure of WTe${}_{2}$. They show that the bulk Fermi surface is formed by three-dimensional electron and hole pockets with areas that are found to be in good agreement with transport experiments, with the exception of small hole pockets that have not been observed in quantum oscillation experiments.

Electroweak corrections to Higgs-strahlung off W/Z bosons at the Tevatron and the LHC with Hawk

Abstract: The associate production of Higgs bosons with W or Z bosons, known as Higgs-strahlung, is an important search channel for Higgs bosons at the hadron colliders Tevatron and LHC for low Higgs-boson masses. We refine a previous calculation of next-to-leading-order electroweak corrections (and recalculate the QCD corrections) upon including the leptonic decay of the W/Z bosons, thereby keeping the fully differential information of the 2-lepton + Higgs final state. The gauge invariance of the W/Z-resonance treatment is ensured by the use of the complex-mass scheme. The electroweak corrections, which are at the level of − (5−10)% for total cross sections, further increase in size with increasing transverse momenta p T in differential cross sections. For instance, for p T,H $ {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{ > }} $ 200 GeV, which is the interesting range at the LHC, the electroweak corrections to WH production reach about −14% for M H = 120 GeV. The described corrections are implemented in the Hawk Monte Carlo program, which was initially designed for the vector-boson-fusion channel, and are discussed for various distributions in the production channels $ {\text{pp}}/{\text{p}}\overline {\text{p}} \to {\text{H}} + l{{v}_{l}}{{l}^{ - }}{{l}^{ + }}/ + X $ .