763 research outputs found
Theoretical Update on Two Non-Resonant Three-Body Channels in Charmed Meson Decays
Predictions of two channels in the three-bod decays of the charmed mesons are
made within the heavy hadron chiral perturbation theory. There still exists the
problem that the theoretical expectation is too small compared to the
experimental data.Comment: 13 pages with 2 figures included. To be publishe
Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals
The relaxation of strongly-confined electrons and holes between 1P and 1S
levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond
transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor
nanocrystals, both electrons and holes are strongly confined in PbSe
nanocrystals. Despite the large electron and hole energy level spacings (at
least 12 times the optical phonon energy), we consistently observe picosecond
time-scale relaxation. Existing theories of carrier relaxation cannot account
for these experimental results. Mechanisms that could possibly circumvent the
phonon bottleneck in IV-VI quantum dots are discussed
Development of a Three Dimensional Neural Sensing Device by a Stacking Method
This study reports a new stacking method for assembling a 3-D microprobe array. To date, 3-D array structures have usually been assembled with vertical spacers, snap fasteners and a supporting platform. Such methods have achieved 3-D structures but suffer from complex assembly steps, vertical interconnection for 3-D signal transmission, low structure strength and large implantable opening. By applying the proposed stacking method, the previous techniques could be replaced by 2-D wire bonding. In this way, supporting platforms with slots and vertical spacers were no longer needed. Furthermore, ASIC chips can be substituted for the spacers in the stacked arrays to achieve system integration, design flexibility and volume usage efficiency. To avoid overflow of the adhesive fluid during assembly, an anti-overflow design which made use of capillary action force was applied in the stacking method as well. Moreover, presented stacking procedure consumes only 35 minutes in average for a 4 × 4 3-D microprobe array without requiring other specially made assembly tools. To summarize, the advantages of the proposed stacking method for 3-D array assembly include simplified assembly process, high structure strength, smaller opening area and integration ability with active circuits. This stacking assembly technique allows an alternative method to create 3-D structures from planar components
On the Short Distance Part of the QCD Anomaly Contribution to the b --> s eta' Amplitude
In addressing the B --> eta' K puzzle, there has been a considerable hope in
the literature to resolve it by the QCD anomaly contribution to the b --> s
eta' amplitude. This contribution corresponds to the electroweak b --> s g* g*
transition followed by the off-shell gluon fusion g* g* --> eta'. In the
present paper we perform a critical reassessment of this issue. We show that
for the hard virtual gluons in a loop there is a well defined short distance
amplitude corresponding to a remnant of the QCD anomaly. However, we find that
it cannot account for the measured amplitude.
In addition, we point out that the reduction of the gluon fusion vertex for
the off-shell gluons is compensated by an absence of the claimed suppression in
the electroweak vertex, and that some nonperturbative contributions related to
the QCD anomaly may still be viable in explaining the physical B --> eta' K
amplitude.Comment: 11 pages, 4 figures, LaTeX + elsart.cls, to appear in Phys. Lett.
Multiplexed, High Density Electrophysiology with Nanofabricated Neural Probes
Extracellular electrode arrays can reveal the neuronal network correlates of behavior with single-cell, single-spike, and sub-millisecond resolution. However, implantable electrodes are inherently invasive, and efforts to scale up the number and density of recording sites must compromise on device size in order to connect the electrodes. Here, we report on silicon-based neural probes employing nanofabricated, high-density electrical leads. Furthermore, we address the challenge of reading out multichannel data with an application-specific integrated circuit (ASIC) performing signal amplification, band-pass filtering, and multiplexing functions. We demonstrate high spatial resolution extracellular measurements with a fully integrated, low noise 64-channel system weighing just 330 mg. The on-chip multiplexers make possible recordings with substantially fewer external wires than the number of input channels. By combining nanofabricated probes with ASICs we have implemented a system for performing large-scale, high-density electrophysiology in small, freely behaving animals that is both minimally invasive and highly scalable
Susceptibility to glaucoma: differential comparison of the astrocyte transcriptome from glaucomatous African American and Caucasian American donors
Comparison of gene expression in normal and glaucomatous eyes from Caucasian American and African American donors reveals differences that might reflect different susceptibility to glaucoma
Charming penguin contributions to charmless B decays into two pseudoscalar mesons
We present estimates of the charming penguin contribution to B => K pi, pi
pi,K eta, K eta' decays due to intermediate charmed meson states. We find that
this contribution is indeed significant for B => K pi decays, and its
inclusion, together with the tree and penguin terms, produces large branching
ratios in agreement with data, though the analysis is affected by large
theoretical uncertainties. On the other hand, for B => pi pi, K eta, K eta'
decays, the effect of the charming penguin contribution is more modest. We also
compute CP asymmetries for B => K pi, pi pi decays and we obtain rather large
results.Comment: 12 pages, 4 figures, LaTeX2e with epsfig. Minor changes in the text,
this version will appear in Phys. Rev.
Nonresonant Three-body Decays of D and B Mesons
Nonresonant three-body decays of D and B mesons are studied. It is pointed
out that if heavy meson chiral perturbation theory (HMChPT) is applied to the
heavy-light strong and weak vertices and assumed to be valid over the whole
kinematic region, then the predicted decay rates for nonresonant charmless
3-body B decays will be too large and especially B^- --> pi^- K^+ K^- greatly
exceeds the current experimental limit. This can be understood as chiral
symmetry has been applied there twice beyond its region of validity. If HMChPT
is applied only to the strong vertex and the weak transition is accounted for
by the form factors, the dominant B^* pole contribution to the tree-dominated
direct three-body B decays will become small and the branching ratio will be of
order 10^{-6}. The decay modes B^- --> (K^- h^+ h^-)_{NR} and bar{B}^0 -->
(bar{K}^0 h^+h^-)_{NR} for h = pi, K are penguin dominated. We apply HMChPT in
two different cases to study the direct 3-body D decays and compare the results
with experiment. Theoretical uncertainties are discussed.Comment: 24 pages, 2 figures. New experimental results of direct 3-body D
decays as Reported at ICHEP2002 are included. To appear in Phys. Re
BACE1 activity impairs neuronal glucose oxidation:rescue by beta-hydroxybutyrate and lipoic acid
Glucose hypometabolism and impaired mitochondrial function in neurons have been suggested to play early and perhaps causative roles in Alzheimer's disease (AD) pathogenesis. Activity of the aspartic acid protease, beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), responsible for beta amyloid peptide generation, has recently been demonstrated to modify glucose metabolism. We therefore examined, using a human neuroblastoma (SH-SY5Y) cell line, whether increased BACE1 activity is responsible for a reduction in cellular glucose metabolism. Overexpression of active BACE1, but not a protease-dead mutant BACE1, protein in SH-SY5Y cells reduced glucose oxidation and the basal oxygen consumption rate, which was associated with a compensatory increase in glycolysis. Increased BACE1 activity had no effect on the mitochondrial electron transfer process but was found to diminish substrate delivery to the mitochondria by inhibition of key mitochondrial decarboxylation reaction enzymes. This BACE1 activity-dependent deficit in glucose oxidation was alleviated by the presence of beta hydroxybutyrate or α-lipoic acid. Consequently our data indicate that raised cellular BACE1 activity drives reduced glucose oxidation in a human neuronal cell line through impairments in the activity of specific tricarboxylic acid cycle enzymes. Because this bioenergetic deficit is recoverable by neutraceutical compounds we suggest that such agents, perhaps in conjunction with BACE1 inhibitors, may be an effective therapeutic strategy in the early-stage management or treatment of AD
Angular redistribution of near-infrared emission from quantum dots in 3D photonic crystals
We study the angle-resolved spontaneous emission of near-infrared light
sources in 3D photonic crystals over a wavelength range from 1200 to 1550 nm.
To this end PbSe quantum dots are used as light sources inside titania inverse
opal photonic crystals. Strong deviations from the Lambertian emission profile
are observed. An attenuation of 60 % is observed in the angle dependent radiant
flux emitted from the samples due to photonic stop bands. At angles that
correspond to the edges of the stop band the emitted flux is increased by up to
34 %. This increase is explained by the redistribution of Bragg-diffracted
light over the available escape angles. The results are quantitatively
explained by an expanded escape-function model. This model is based on
diffusion theory and adapted to photonic crystals using band structure
calculations. Our results are the first angular redistributions and escape
functions measured at near-infrared, including telecom, wavelengths. In
addition, this is the first time for this model to be applied to describe
emission from samples that are optically thick for the excitation light and
relatively thin for the photoluminesence light.Comment: 24 pages, 8 figures (current format = single column, double spaced
- …