318 research outputs found
Passive States for Essential Observers
The aim of this note is to present a unified approach to the results given in
\cite{bb99} and \cite{bs04} which also covers examples of models not presented
in these two papers (e.g. -dimensional Minkowski space-time for ).
Assuming that a state is passive for an observer travelling along certain
(essential) worldlines, we show that this state is invariant under the isometry
group, is a KMS-state for the observer at a temperature uniquely determined by
the structure constants of the Lie algebra involved and fulfills (a variant of)
the Reeh-Schlieder property. Also the modular objects associated to such a
state and the observable algebra of an observer are computed and a version of
weak locality is examined.Comment: 27 page
Structural Insight into Eukaryotic Sterol Transport through Niemann-Pick Type C Proteins
Niemann-Pick type C (NPC) proteins are essential for sterol homeostasis, believed to drive sterol integration into the lysosomal membrane before redistribution to other cellular membranes. Here, using a combination of crystallography, cryo-electron microscopy, and biochemical and in vivo studies on the Saccharomyces cerevisiae NPC system (NCR1 and NPC2), we present a framework for sterol membrane integration. Sterols are transferred between hydrophobic pockets of vacuolar NPC2 and membrane-protein NCR1. NCR1 has its N-terminal domain (NTD) positioned to deliver a sterol to a tunnel connecting NTD to the luminal membrane leaflet 50 Å away. A sterol is caught inside this tunnel during transport, and a proton-relay network of charged residues in the transmembrane region is linked to this tunnel supporting a proton-driven transport mechanism. We propose a model for sterol integration that clarifies the role of NPC proteins in this essential eukaryotic pathway and that rationalizes mutations in patients with Niemann-Pick disease type C
Selective Visualization of Fluorescent Sterols in Caenorhabditis elegans by Bleach-Rate-Based Image Segmentation
The nematode Caenorhabditis elegans is a genetically tractable model organism to investigate sterol transport. In vivo imaging of the fluorescent sterol, dehydroergosterol (DHE), is challenged by C. elegans' high autofluorescence in the same spectral region as emission of DHE. We present a method to detect DHE selectively, based on its rapid bleaching kinetics compared to cellular autofluorescence. Worms were repeatedly imaged on an ultraviolet-sensitive wide field (UV-WF) microscope, and bleaching kinetics of DHE were fitted on a pixel-basis to mathematical models describing the intensity decay. Bleach-rate constants were determined for DHE in vivo and confirmed in model membranes. Using this method, we could detect enrichment of DHE in specific tissues like the nerve ring, the spermateca and oocytes. We confirm these results in C. elegans gut-granule-loss (glo) mutants with reduced autofluorescence and compare our method with three-photon excitation microscopy of sterol in selected tissues. Bleach-rate-based UV-WF imaging is a useful tool for genetic screening experiments on sterol transport, as exemplified by RNA interference against the rme-2 gene coding for the yolk receptor and for worm homologues of Niemann-Pick C disease proteins. Our approach is generally useful for identifying fluorescent probes in the presence of high cellular autofluorescence
Determination of the total width of the eta' meson
Taking advantage of both the low-emittance proton-beam of the Cooler
Synchrotron COSY and the high momentum precision of the COSY-11 detector
system, the mass distribution of the eta' meson was measured with a resolution
of 0.33 MeV/c^2 (FWHM), improving the experimental mass resolution by almost an
order of magnitude with respect to previous results. Based on the sample of
more than 2300 reconstructed pp --> pp eta' events the total width of the eta'
meson was determined to be 0.226 +- 0.017(stat.) +- 0.014(syst.) MeV/c^2.Comment: 4 pages, 4 figure
Quantum-to-Classical Correspondence and Hubbard-Stratonovich Dynamical Systems, a Lie-Algebraic Approach
We propose a Lie-algebraic duality approach to analyze non-equilibrium
evolution of closed dynamical systems and thermodynamics of interacting quantum
lattice models (formulated in terms of Hubbard-Stratonovich dynamical systems).
The first part of the paper utilizes a geometric Hilbert-space-invariant
formulation of unitary time-evolution, where a quantum Hamiltonian is viewed as
a trajectory in an abstract Lie algebra, while the sought-after evolution
operator is a trajectory in a dynamic group, generated by the algebra via
exponentiation. The evolution operator is uniquely determined by the
time-dependent dual generators that satisfy a system of differential equations,
dubbed here dual Schrodinger-Bloch equations, which represent a viable
alternative to the conventional Schrodinger formulation. These dual
Schrodinger-Bloch equations are derived and analyzed on a number of specific
examples. It is shown that deterministic dynamics of a closed classical
dynamical system occurs as action of a symmetry group on a classical manifold
and is driven by the same dual generators as in the corresponding quantum
problem. This represents quantum-to-classical correspondence. In the second
part of the paper, we further extend the Lie algebraic approach to a wide class
of interacting many-particle lattice models. A generalized Hubbard-Stratonovich
transform is proposed and it is used to show that the thermodynamic partition
function of a generic many-body quantum lattice model can be expressed in terms
of traces of single-particle evolution operators governed by the dynamic
Hubbard-Stratonovich fields. Finally, we derive Hubbard-Stratonovich dynamical
systems for the Bose-Hubbard model and a quantum spin model and use the
Lie-algebraic approach to obtain new non-perturbative dual descriptions of
these theories.Comment: 25 pages, 1 figure; v2: citations adde
Isospin dependence of the eta prime meson production in nucleon-nucleon collisions
A comparison of the close-to-threshold total cross section for the eta prime
production in pp --> pp eta prime and pn --> pn eta prime reactions constitutes
a tool to investigate the eta prime meson structure and the reaction mechanism
in the channels of isospin I=0 and I=1 and may provide insight into the
flavour-singlet (perhaps also into gluonium) content of the eta prime meson. In
this contribution we present preliminary results of measurement of the
quasi-free production of the eta prime meson in the proton-neutron collisions
conducted using the COSY-11 facility.Comment: Presented at 10th International Workshop on Meson Production,
Properties and Interaction (MESON 2008), Cracow, Poland, 6 - 10 June 200
Low-Energy \Lambda-\p Scattering Parameters from the Reaction
Constraints on the spin-averaged scattering length and effective
range have been obtained from measurements of the reaction
close to the production threshold by comparing model phase-space Dalitz plot
occupations with experimental ones. The data fix well the position of the
virtual bound state in the system. Combining this with information
from elastic scattering measurements at slightly higher energies,
together with the fact that the hyperdeuteron is not bound, leads to a new
determination of the low energy scattering parameters.Comment: 18 pages, 7 figure
Near threshold eta meson production in dp collisions
Preliminary results of recent measurements of the near threshold eta meson
production in the dp-->dp eta reaction are presented. The experiment was
performed at the COSY-Juelich accelerator with the use of the COSY-11 detection
system. Data were taken for three values of deuteron beam momenta corresponding
to excess energies of 3.2, 6.1 and 9.2 MeV. The energy dependence of the total
cross section confirms a strong effect of the final state interaction.Comment: Talk given at 9th International Workshop on Meson Production,
Properties and Interaction, Cracow, Poland, 9-13 June 2006. 5 pages, 3
figure
Measuring the Polarization of a Rapidly Precessing Deuteron Beam
This paper describes a time-marking system that enables a measurement of the
in-plane (horizontal) polarization of a 0.97-GeV/c deuteron beam circulating in
the Cooler Synchrotron (COSY) at the Forschungszentrum J\"ulich. The clock time
of each polarimeter event is used to unfold the 120-kHz spin precession and
assign events to bins according to the direction of the horizontal
polarization. After accumulation for one or more seconds, the down-up
scattering asymmetry can be calculated for each direction and matched to a
sinusoidal function whose magnitude is proportional to the horizontal
polarization. This requires prior knowledge of the spin tune or polarization
precession rate. An initial estimate is refined by re-sorting the events as the
spin tune is adjusted across a narrow range and searching for the maximum
polarization magnitude. The result is biased toward polarization values that
are too large, in part because of statistical fluctuations but also because
sinusoidal fits to even random data will produce sizeable magnitudes when the
phase is left free to vary. An analysis procedure is described that matches the
time dependence of the horizontal polarization to templates based on
emittance-driven polarization loss while correcting for the positive bias. This
information will be used to study ways to extend the horizontal polarization
lifetime by correcting spin tune spread using ring sextupole fields and thereby
to support the feasibility of searching for an intrinsic electric dipole moment
using polarized beams in a storage ring. This paper is a combined effort of the
Storage Ring EDM Collaboration and the JEDI Collaboration.Comment: 28 pages, 15 figures, prepared for Physical Review ST - Accelerators
and Beam
Measurement of the absolute differential cross section of proton-proton elastic scattering at small angles
The differential cross section for proton-proton elastic scattering has been
measured at a beam energy of 1.0 GeV and in 200 MeV steps from 1.6 to 2.8 GeV
for centre-of-mass angles in the range from 12-16 degrees to 25-30 degrees,
depending on the energy. Absolute normalisations of typically 3% were achieved
by studying the energy losses of the circulating beam of the COSY storage ring
as it passed repeatedly through the windowless hydrogen target of the ANKE
magnetic spectrometer. It is shown that the data have a significant impact upon
a partial wave analysis. After extrapolating the differential cross sections to
the forward direction, the results are broadly compatible with the predictions
of forward dispersion relations
- …