51,093 research outputs found
Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere
The photospheric magnetic field outside of active regions and the network has
a ubiquitous and dynamic line-of-sight component that strengthens from disk
center to limb as expected for a nearly horizontal orientation. This component
shows a striking time variation with an average temporal rms near the limb of
1.7 G at ~3" resolution. In our moderate resolution observations the nearly
horizontal component has a frequency variation power law exponent of -1.4 below
1.5 mHz and is spatially patchy on scales up to ~15 arcsec. The field may be a
manifestation of changing magnetic connections between eruptions and evolution
of small magnetic flux elements in response to convective motions. It shows no
detectable latitude or longitude variations.Comment: 7 pages, 6 figures, submitted to ApJ letters, quality of figures
significantly degraded here by compression requirement
Renormalizability of the open string sigma model and emergence of D-branes
Rederiving the one-loop divergences for the most general coupling of the open
string sigma model by the heat kernel technique, we distinguish the classical
background field from the mean field of the effective action. The latter is
arbitrary, i.e. does not fulfil the boundary conditions. As a consequence a new
divergent counter term strongly suggests the introduction of another external
one-form field (beside the usual gauge field), coupled to the normal derivative
at the boundary. Actually such a field has been proposed in the literature for
different reasons, but its full impact never seems to have thoroughly
investigated before. The beta function for the resulting renormalizable model
is calculated and the consequences are discussed, including the ones for the
Born-Infeld action. The most exciting property of the new coupling is that it
enters the coefficient in front of the normal derivative in Neumann boundary
conditions. For certain values of the background fields this coefficient
vanishes, leading to Dirichlet boundary conditions. This provides a natural
mechanism for the emergence of D-branes.Comment: 24 pages, a reference and discussion (about 1 page, sec. 3.3 and 4.1)
added, typos correcte
RooStatsCms: a tool for analyses modelling, combination and statistical studies
The RooStatsCms (RSC) software framework allows analysis modelling and
combination, statistical studies together with the access to sophisticated
graphics routines for results visualisation. The goal of the project is to
complement the existing analyses by means of their combination and accurate
statistical studies.Comment: Proceedings of the 11th Topical Seminar on Innovative Particle and
Radiation Detectors. 4 pages and 5 figure
Vanishing geodesic distance on spaces of submanifolds and diffeomorphisms
The -metric or Fubini-Study metric on the non-linear Grassmannian of all
submanifolds of type in a Riemannian manifold induces geodesic
distance 0. We discuss another metric which involves the mean curvature and
shows that its geodesic distance is a good topological metric. The vanishing
phenomenon for the geodesic distance holds also for all diffeomorphism groups
for the -metric.Comment: 26 pages, LATEX, final versio
Elliptic Flow and Initial Eccentricity in Cu+Cu and Au+Au Collisions at RHIC
We present a systematic study of elliptic flow as a function of centrality,
pseudorapidity, transverse momentum and energy for Cu+Cu and Au+Au collisions
from the PHOBOS experiment. New data on elliptic flow in Cu+Cu collisions at
22.4 GeV are shown. Elliptic flow scaled by participant eccentricity is found
to be similar for both systems when collisions with the same number of
participants or the same average area density are compared. This similarity is
observed over a wide range in pseudorapidity and transverse momentum,
indicating that participant eccentricity is the relevant quantity for
generating the azimuthal asymmetry leading to the observed elliptic flow.Comment: 5 pages, 4 figures, the 19th International Conference On Ultra
relativistic Nucleus-Nucleus Collisions (Quark Matter 2006), Shanghai China,
Nov. 14-20, 200
Multimodal MRI-based Imputation of the Aβ+ in Early Mild Cognitive Impairment.
ObjectiveTo identify brain atrophy from structural-MRI and cerebral blood flow(CBF) patterns from arterial spin labeling perfusion-MRI that are best predictors of the Aβ-burden, measured as composite 18F-AV45-PET uptake, in individuals with early mild cognitive impairment(MCI). Furthermore, to assess the relative importance of imaging modalities in classification of Aβ+/Aβ- early mild cognitive impairment.MethodsSixty-seven ADNI-GO/2 participants with early-MCI were included. Voxel-wise anatomical shape variation measures were computed by estimating the initial diffeomorphic mapping momenta from an unbiased control template. CBF measures normalized to average motor cortex CBF were mapped onto the template space. Using partial least squares regression, we identified the structural and CBF signatures of Aβ after accounting for normal cofounding effects of age, sex, and education.Results18F-AV45-positive early-MCIs could be identified with 83% classification accuracy, 87% positive predictive value, and 84% negative predictive value by multidisciplinary classifiers combining demographics data, ApoE ε4-genotype, and a multimodal MRI-based Aβ score.InterpretationMultimodal-MRI can be used to predict the amyloid status of early-MCI individuals. MRI is a very attractive candidate for the identification of inexpensive and non-invasive surrogate biomarkers of Aβ deposition. Our approach is expected to have value for the identification of individuals likely to be Aβ+ in circumstances where cost or logistical problems prevent Aβ detection using cerebrospinal fluid analysis or Aβ-PET. This can also be used in clinical settings and clinical trials, aiding subject recruitment and evaluation of treatment efficacy. Imputation of the Aβ-positivity status could also complement Aβ-PET by identifying individuals who would benefit the most from this assessment
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