3,569 research outputs found
Resolution and enhancement in nanoantenna-based fluorescence microscopy
Single gold nanoparticles can act as nanoantennas for enhancing the
fluorescence of emitters in their near-fields. Here we present experimental and
theoretical studies of scanning antenna-based fluorescence microscopy as a
function of the diameter of the gold nanoparticle. We examine the interplay
between fluorescence enhancement and spatial resolution and discuss the
requirements for deciphering single molecules in a dense sample. Resolutions
better than 20 nm and fluorescence enhancement up to 30 times are demonstrated
experimentally. By accounting for the tip shaft and the sample interface in
finite-difference time-domain calculations, we explain why the measured
fluorescence enhancements are higher in the presence of an interface than the
values predicted for a homogeneous environment.Comment: 10 pages, 3 figures. accepted for publication in Nano Letter
Statistical Mechanics of Support Vector Networks
Using methods of Statistical Physics, we investigate the generalization
performance of support vector machines (SVMs), which have been recently
introduced as a general alternative to neural networks. For nonlinear
classification rules, the generalization error saturates on a plateau, when the
number of examples is too small to properly estimate the coefficients of the
nonlinear part. When trained on simple rules, we find that SVMs overfit only
weakly. The performance of SVMs is strongly enhanced, when the distribution of
the inputs has a gap in feature space.Comment: REVTeX, 4 pages, 2 figures, accepted by Phys. Rev. Lett (typos
corrected
Precise Charm- and Bottom-Quark Masses: Theoretical and Experimental Uncertainties
Recent theoretical and experimental improvements in the determination of
charm and bottom quark masses are discussed. A new and improved evaluation of
the contribution from the gluon condensate to the
charm mass determination and a detailed study of potential uncertainties in the
continuum cross section for production is presented, together with a
study of the parametric uncertainty from the -dependence of our
results. The final results, MeV and
MeV, represent, together with a closely related lattice
determination MeV, the presently most precise
determinations of these two fundamental Standard Model parameters. A critical
analysis of the theoretical and experimental uncertainties is presented.Comment: 12 pages, presented at Quarks~2010, 16th International Seminar of
High Energy Physics, Kolomna, Russia, June 6-12, 2010; v2: references adde
Phase Diagram and Storage Capacity of Sequence Processing Neural Networks
We solve the dynamics of Hopfield-type neural networks which store sequences
of patterns, close to saturation. The asymmetry of the interaction matrix in
such models leads to violation of detailed balance, ruling out an equilibrium
statistical mechanical analysis. Using generating functional methods we derive
exact closed equations for dynamical order parameters, viz. the sequence
overlap and correlation- and response functions, in the thermodynamic limit. We
calculate the time translation invariant solutions of these equations,
describing stationary limit-cycles, which leads to a phase diagram. The
effective retarded self-interaction usually appearing in symmetric models is
here found to vanish, which causes a significantly enlarged storage capacity of
, compared to \alpha_\c\sim 0.139 for Hopfield networks
storing static patterns. Our results are tested against extensive computer
simulations and excellent agreement is found.Comment: 17 pages Latex2e, 2 postscript figure
Differentiation of cerebral tumors using multi-section echo planar MR perfusion imaging
Objective: We have investigated the performance of magnetic resonance (MR) perfusion imaging to differentiate between astrocytomas grade II, grade III and glioblastomas in a prospective study. Materials and methods: In 33 patients with suspected supratentorial primary cerebral tumors we performed multi-section Echo Planar MR perfusion imaging. Regional cerebral blood volume (rCBV) maps were calculated and the maximum rCBV was determined from the entire lesion. This value was divided by the mean rCBV value from the contralateral side, which provided the rCBV index used in this study. The rCBV index was correlated with the histological tumor classification after stereotactic biopsy (n=7) or open resection (n=26). Results: The maximum rCBV index was 1.2±0.8 for grade II astrocytomas (n=3), 4.0±1.2 for grade III astrocytomas (n=13), and 10.3±3.3 for glioblastomas (n=17). The difference between grade III astrocytomas and glioblastomas was highly significant (P<0.001). Discussion and conclusion: The rCBV index measured with multi-section Echo Planar MR perfusion is capable of differentiating grade III astrocytomas from glioblastomas. It serves as an additional parameter to establish a diagnosis in cases where it is not possible to clearly differentiate between these types of tumors on the basis of conventional MR imaging. MR perfusion imaging also provides information about spatial heterogeneities within a tumor which might improve diagnostic performance. This technology may also be of interest for follow-up examinations after histological diagnosis and further treatment
Analysis of Bidirectional Associative Memory using SCSNA and Statistical Neurodynamics
Bidirectional associative memory (BAM) is a kind of an artificial neural
network used to memorize and retrieve heterogeneous pattern pairs. Many efforts
have been made to improve BAM from the the viewpoint of computer application,
and few theoretical studies have been done. We investigated the theoretical
characteristics of BAM using a framework of statistical-mechanical analysis. To
investigate the equilibrium state of BAM, we applied self-consistent signal to
noise analysis (SCSNA) and obtained a macroscopic parameter equations and
relative capacity. Moreover, to investigate not only the equilibrium state but
also the retrieval process of reaching the equilibrium state, we applied
statistical neurodynamics to the update rule of BAM and obtained evolution
equations for the macroscopic parameters. These evolution equations are
consistent with the results of SCSNA in the equilibrium state.Comment: 13 pages, 4 figure
Electroweak corrections to W-boson pair production at the LHC
Vector-boson pair production ranks among the most important Standard-Model
benchmark processes at the LHC, not only in view of on-going Higgs analyses.
These processes may also help to gain a deeper understanding of the electroweak
interaction in general, and to test the validity of the Standard Model at
highest energies. In this work, the first calculation of the full one-loop
electroweak corrections to on-shell W-boson pair production at hadron colliders
is presented. We discuss the impact of the corrections on the total cross
section as well as on relevant differential distributions. We observe that
corrections due to photon-induced channels can be amazingly large at energies
accessible at the LHC, while radiation of additional massive vector bosons does
not influence the results significantly.Comment: 29 pages, 15 figures, 4 tables; some references and comments on
\gamma\gamma -> WW added; matches version published in JHE
Simple model for decay of superdeformed nuclei
Recent theoretical investigations of the decay mechanism out of a
superdeformed nuclear band have yielded qualitatively different results,
depending on the relative values of the relevant decay widths. We present a
simple two-level model for the dynamics of the tunneling between the
superdeformed and normal-deformed bands, which treats decay and tunneling
processes on an equal footing. The previous theoretical results are shown to
correspond to coherent and incoherent limits of the full tunneling dynamics.
Our model accounts for experimental data in both the A~150 mass region, where
the tunneling dynamics is coherent, and in the A~190 mass region, where the
tunneling dynamics is incoherent.Comment: 4 page
QCD corrections to decay-lepton polar and azimuthal angular distributions in e+e- -> t tbar in the soft-gluon approximation
QCD corrections to order alpha_s in the soft-gluon approximation to angular
distributions of decay charged leptons in the process e+e- -> t tbar followed
by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass
frame. As compared to distributions in the top rest frame, these have the
advantage that they would allow direct comparison with experiment without the
need to reconstruct the top rest frame. The results also do not depend on the
choice of a spin quantization axis for t or tbar. Analytic expression for the
triple distribution in the polar angle of t and polar and azimuthal angles of
the lepton is obtained. Analytic expression is also derived for the
distribution in the charged-lepton polar angle. Numerical values are discussed
for total c.m. energies of 400 GeV, 800 GeV and 1500 GeV.Comment: 21 pages, Latex, 6 figures included in the submission. To appear in
Pramana - Journal of Physics; expanded version of hep-ph/0011321, v
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