194 research outputs found
Electroproduction of Charmonia off Protons and Nuclei
Elastic virtual photoproduction of charmonia on nucleons is calculated in a
parameter free way with the light-cone dipole formalism and the same input:
factorization in impact parameters, light-cone wave functions for the photons
and the charmonia, and the universal phenomenological dipole cross section
which is fitted to other data. The charmonium wave functions are calculated
with four known realistic potentials, and two models for the dipole cross
section are tested. Very good agreement with data for the cross section of
charmonium electroproduction is found in a wide range of and . Using
the ingredients from those calculations we calculate also exclusive
electroproduction of charmonia off nuclei. Here new effects become important,
(i) color filtering of the pair on its trajectory through nuclear
matter, (ii) dependence on the finite lifetime of the fluctuation
(coherence length) and (iii) gluon shadowing in a nucleus compared to the one
in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as
functions of are presented. The results can be tested with future
electron-nucleus colliders or in the peripheral collisions of relativistic
heavy ions.Comment: Talk at 2-nd International Workshop on Hadron Physics, 25-29
September 2002, Coimbra, Portugal. To appear in the Workshop Proceedings
(will be published by the American Institute of Physics
From Registration to Recounts Revisited: Developments in the Election Ecosystems of Five Midwestern States
As a follow-up to a study of problems during the 2006 elections, examines the states' continuing adjustments to institutional arrangements, voter registration databases, convenience voting, and post-election processes in the 2008 elections
Genomic stability in response to high versus low linear energy transfer radiation in Arabidopsis thaliana.
Low linear energy transfer (LET) gamma rays and high LET HZE (high atomic weight, high energy) particles act as powerful mutagens in both plants and animals. DNA damage generated by HZE particles is more densely clustered than that generated by gamma rays. To understand the genetic requirements for resistance to high versus low LET radiation, a series of Arabidopsis thaliana mutants were exposed to either 1GeV Fe nuclei or gamma radiation. A comparison of effects on the germination and subsequent growth of seedlings led us to conclude that the relative biological effectiveness (RBE) of the two types of radiation (HZE versus gamma) are roughly 3:1. Similarly, in wild-type lines, loss of somatic heterozygosity was induced at an RBE of about a 2:1 (HZE versus gamma). Checkpoint and repair defects, as expected, enhanced sensitivity to both agents. The "replication fork" checkpoint, governed by ATR, played a slightly more important role in resistance to HZE-induced mutagenesis than in resistance to gamma induced mutagenesis
High atomic weight, high-energy radiation (HZE) induces transcriptional responses shared with conventional stresses in addition to a core "DSB" response specific to clastogenic treatments.
Plants exhibit a robust transcriptional response to gamma radiation which includes the induction of transcripts required for homologous recombination and the suppression of transcripts that promote cell cycle progression. Various DNA damaging agents induce different spectra of DNA damage as well as "collateral" damage to other cellular components and therefore are not expected to provoke identical responses by the cell. Here we study the effects of two different types of ionizing radiation (IR) treatment, HZE (1 GeV Fe(26+) high mass, high charge, and high energy relativistic particles) and gamma photons, on the transcriptome of Arabidopsis thaliana seedlings. Both types of IR induce small clusters of radicals that can result in the formation of double strand breaks (DSBs), but HZE also produces linear arrays of extremely clustered damage. We performed these experiments across a range of time points (1.5-24 h after irradiation) in both wild-type plants and in mutants defective in the DSB-sensing protein kinase ATM. The two types of IR exhibit a shared double strand break-repair-related damage response, although they differ slightly in the timing, degree, and ATM-dependence of the response. The ATM-dependent, DNA metabolism-related transcripts of the "DSB response" were also induced by other DNA damaging agents, but were not induced by conventional stresses. Both Gamma and HZE irradiation induced, at 24 h post-irradiation, ATM-dependent transcripts associated with a variety of conventional stresses; these were overrepresented for pathogen response, rather than DNA metabolism. In contrast, only HZE-irradiated plants, at 1.5 h after irradiation, exhibited an additional and very extensive transcriptional response, shared with plants experiencing "extended night." This response was not apparent in gamma-irradiated plants
Allocating the Burden of Proof in Administrative and Judicial Proceedings under the Individuals with Disabilities Education Act
Investigating biomechanical noise in neuroblastoma cells using the quartz crystal microbalance.
Quantifying cellular behaviour by motility and morphology changes is increasingly important in formulating an understanding of fundamental physiological phenomena and cellular mechanisms of disease. However, cells are complex biological units, which often respond to external environmental factors by manifesting subtle responses that may be difficult to interpret using conventional biophysical measurements. This paper describes the adaptation of the quartz crystal microbalance (QCM) to monitor neuroblastoma cells undergoing environmental stress wherein the frequency stability of the device can be correlated to changes in cellular state. By employing time domain analysis of the resulting frequency fluctuations, it is possible to study the variations in cellular motility and distinguish between different cell states induced by applied external heat stress. The changes in the frequency fluctuation data are correlated to phenotypical physical response recorded using optical microscopy under identical conditions of environmental stress. This technique, by probing the associated biomechanical noise, paves the way for its use in monitoring cell activity, and intrinsic motility and morphology changes, as well as the modulation resulting from the action of drugs, toxins and environmental stress.This is the author accepted manuscript. The final version is available from Royal Society Publishing via http://dx.doi.org/10.1098/rsif.2014.138
Graphene Rings in Magnetic Fields: Aharonov-Bohm Effect and Valley Splitting
We study the conductance of mesoscopic graphene rings in the presence of a
perpendicular magnetic field by means of numerical calculations based on a
tight-binding model. First, we consider the magnetoconductance of such rings
and observe the Aharonov-Bohm effect. We investigate different regimes of the
magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm
oscillations are suppressed. Results for both clean (ballistic) and disordered
(diffusive) rings are presented. Second, we study rings with smooth mass
boundary that are weakly coupled to leads. We show that the valley degeneracy
of the eigenstates in closed graphene rings can be lifted by a small magnetic
flux, and that this lifting can be observed in the transport properties of the
system.Comment: 12 pages, 9 figure
suppression in Pb+Pb collisions and broadening
We have analysed the NA50 data, on the centrality dependence of
broadening of 's, in Pb+Pb collisions, at the CERN-SPS. The data were
analysed in a QCD based model, where 's are suppressed in 'nuclear'
medium. Without any free parameter, the model could explain the NA50
broadening data. The data were also analysed in a QGP based threshold model,
where suppression is 100% above a critical density. The QGP based
model could not explain the NA50 broadening data. We have also predicted
the centrality dependence of suppression and broadening at RHIC
energy. Both the models, the QGP based threshold model and the QCD based
nuclear absorption model, predict broadening very close to each other.Comment: The paper was completely revised. The conclusion is also changed. 5
pages, 4 figure
Spatial mapping and manipulation of two tunnel-coupled quantum dots
The metallic tip of a scanning force microscope operated at 300 mK is used to
locally induce a potential in a fully controllable double quantum dot defined
via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning
gate techniques we record spatial images of the current through the sample for
different numbers of electrons on the quantum dots (i.e., for different quantum
states). Owing to the spatial resolution of current maps, we are able to
determine the spatial position of the individual quantum dots, and investigate
their apparent relative shifts due to the voltage applied to a single gate
Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy
We report high resolution angle-resolved photoemission spectroscopy (ARPES)
studies of the electronic structure of BaFeAs, which is one of the
parent compounds of the Fe-pnictide superconductors. ARPES measurements have
been performed at 20 K and 300 K, corresponding to the orthorhombic
antiferromagnetic phase and the tetragonal paramagnetic phase, respectively.
Photon energies between 30 and 175 eV and polarizations parallel and
perpendicular to the scattering plane have been used. Measurements of the Fermi
surface yield two hole pockets at the -point and an electron pocket at
each of the X-points. The topology of the pockets has been concluded from the
dispersion of the spectral weight as a function of binding energy. Changes in
the spectral weight at the Fermi level upon variation of the polarization of
the incident photons yield important information on the orbital character of
the states near the Fermi level. No differences in the electronic structure
between 20 and 300 K could be resolved. The results are compared with density
functional theory band structure calculations for the tetragonal paramagnetic
phase.Comment: 11 pages, 5 figure
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