2,666 research outputs found
String Theory in the Penrose Limit of AdS_2 x S^2
The string theory in the Penrose limit of AdS_2 x S^2 is investigated. The
specific Penrose limit is the background known as the Nappi-Witten spacetime,
which is a plane-wave background with an axion field. The string theory on it
is given as the Wess-Zumino-Novikov-Witten (WZNW) model on non-semi-simple
group H_4. It is found that, in the past literature, an important type of
irreducible representations of the corresponding algebra, h_4, were missed. We
present this "new" representations, which have the type of continuous series
representations. All the three types of representations of the previous
literature can be obtained from the "new" representations by setting the
momenta in the theory to special values. Then we realized the affine currents
of the WZNW model in terms of four bosonic free fields and constructed the
spectrum of the theory by acting the negative frequency modes of free fields on
the ground level states in the h_4 continuous series representation. The
spectrum is shown to be free of ghosts, after the Virasoro constraints are
satisfied. In particular we argued that there is no need for constraining one
of the longitudinal momenta to have unitarity. The tachyon vertex operator,
that correspond to a particular state in the ground level of the string
spectrum, is constructed. The operator products of the vertex operator with the
currents and the energy-momentum tensor are shown to have the correct forms,
with the correct conformal weight of the vertex operator.Comment: 30 pages, Latex, no figure
Test beam measurement of the first prototype of the fast silicon pixel monolithic detector for the TT-PET project
The TT-PET collaboration is developing a PET scanner for small animals with
30 ps time-of-flight resolution and sub-millimetre 3D detection granularity.
The sensitive element of the scanner is a monolithic silicon pixel detector
based on state-of-the-art SiGe BiCMOS technology. The first ASIC prototype for
the TT-PET was produced and tested in the laboratory and with minimum ionizing
particles. The electronics exhibit an equivalent noise charge below 600 e- RMS
and a pulse rise time of less than 2 ns, in accordance with the simulations.
The pixels with a capacitance of 0.8 pF were measured to have a detection
efficiency greater than 99% and, although in the absence of the
post-processing, a time resolution of approximately 200 ps
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A monolithic ASIC demonstrator for the Thin Time-of-Flight PET scanner
Time-of-flight measurement is an important advancement in PET scanners to improve image reconstruction with a lower delivered radiation dose. This article describes the monolithic ASIC for the TT-PET project, a novel idea for a high-precision PET scanner for small animals. The chip uses a SiGe Bi-CMOS process for timing measurements, integrating a fully-depleted pixel matrix with a low-power BJT-based front-end per channel, integrated on the same 100 µm thick die. The target timing resolution of the scanner is 30 ps RMS for electrons from the conversion of 511 keV photons. The system will include 1.6 million channels across almost 2000 different chips. A full-featured demonstrator chip with a 3×10 matrix of 500×500 µm2 pixels was fabricated to validate each block. Its design and experimental results are presented here. © 2019 CERN
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A 50 ps resolution monolithic active pixel sensor without internal gain in SiGe BiCMOS technology
A monolithic pixelated silicon detector designed for high time resolution has been produced in the SG13G2 130 nm SiGe BiCMOS technology of IHP. This proof-of-concept chip contains hexagonal pixels of 65 µm and 130 µm side. The SiGe front-end electronics implemented provides an equivalent noise charge of 90 and 160 e- for a pixel capacitance of 70 and 220 fF, respectively, and a total time walk of less than 1 ns. Lab measurements with a 90Sr source show a time resolution of the order of 50 ps. This result is competitive with silicon technologies that integrate an avalanche gain mechanism. © 2019 CERN
Point Interaction in two and three dimensional Riemannian Manifolds
We present a non-perturbative renormalization of the bound state problem of n
bosons interacting with finitely many Dirac delta interactions on two and three
dimensional Riemannian manifolds using the heat kernel. We formulate the
problem in terms of a new operator called the principal or characteristic
operator. In order to investigate the problem in more detail, we then restrict
the problem to one particle sector. The lower bound of the ground state energy
is found for general class of manifolds, e.g., for compact and Cartan-Hadamard
manifolds. The estimate of the bound state energies in the tunneling regime is
calculated by perturbation theory. Non-degeneracy and uniqueness of the ground
state is proven by Perron-Frobenius theorem. Moreover, the pointwise bounds on
the wave function is given and all these results are consistent with the one
given in standard quantum mechanics. Renormalization procedure does not lead to
any radical change in these cases. Finally, renormalization group equations are
derived and the beta-function is exactly calculated. This work is a natural
continuation of our previous work based on a novel approach to the
renormalization of point interactions, developed by S. G. Rajeev.Comment: 43 page
A Novel Alternating Cell Directions Implicit Method for the Solution of Incompressible Navier Stokes Equations on Unstructured Grids
In this paper, A Novel Alternating Cell Direction Implicit Method (ACDI) is researched which allows implementation of fast line implicit methods on quadrilateral unstructured meshes. In ACDI method, designated alternating cell directions are taken along a series of contiguous cells within the unstructured grid domain and used as implicit lines similar to Line Gauss Seidel Method (LGS). ACDI method applied earlier for the solution of potential flows is extended for the solution of the incompressible Navier-Stokes equations on unstructured grids. The system of equations is solved by using the Symmetric Line Gauss-Seidel (SGS) method along the alternating cell directions. Laminar flow fields over a single element NACA-0008 airfoil are computed by using structured and unstructured quadrilateral grids, and inviscid Euler flow solutions are given for the NACA-23012b multielement airfoil. The predictive capability of the method is validated against the data taken from the experimental or the other numerical studies and the efficiency of the ACDI method is compared with the implicit Point Gauss Seidel (PGS) method. In the selected validation cases, the results show that a reduction in total computation between 18% and 23% is achieved by the ACDI method over the PGS. In general, the results show that the ACDI method is a fast, efficient, robust and versatile method that can handle complicated unstructured grid cases with equal ease as with the structured grids
GATA-targeted compounds modulate cardiac subtype cell differentiation in dual reporter stem cell line
BackgroundPharmacological modulation of cell fate decisions and developmental gene regulatory networks holds promise for the treatment of heart failure. Compounds that target tissue-specific transcription factors could overcome non-specific effects of small molecules and lead to the regeneration of heart muscle following myocardial infarction. Due to cellular heterogeneity in the heart, the activation of gene programs representing specific atrial and ventricular cardiomyocyte subtypes would be highly desirable. Chemical compounds that modulate atrial and ventricular cell fate could be used to improve subtype-specific differentiation of endogenous or exogenously delivered progenitor cells in order to promote cardiac regeneration.MethodsTranscription factor GATA4-targeted compounds that have previously shown in vivo efficacy in cardiac injury models were tested for stage-specific activation of atrial and ventricular reporter genes in differentiating pluripotent stem cells using a dual reporter assay. Chemically induced gene expression changes were characterized by qRT-PCR, global run-on sequencing (GRO-seq) and immunoblotting, and the network of cooperative proteins of GATA4 and NKX2-5 were further explored by the examination of the GATA4 and NKX2-5 interactome by BioID. Reporter gene assays were conducted to examine combinatorial effects of GATA-targeted compounds and bromodomain and extraterminal domain (BET) inhibition on chamber-specific gene expression.ResultsGATA4-targeted compounds 3i-1000 and 3i-1103 were identified as differential modulators of atrial and ventricular gene expression. More detailed structure-function analysis revealed a distinct subclass of GATA4/NKX2-5 inhibitory compounds with an acetyl lysine-like domain that contributed to ventricular cells (%Myl2-eGFP+). Additionally, BioID analysis indicated broad interaction between GATA4 and BET family of proteins, such as BRD4. This indicated the involvement of epigenetic modulators in the regulation of GATA-dependent transcription. In this line, reporter gene assays with combinatorial treatment of 3i-1000 and the BET bromodomain inhibitor (+)-JQ1 demonstrated the cooperative role of GATA4 and BRD4 in the modulation of chamber-specific cardiac gene expression.ConclusionsCollectively, these results indicate the potential for therapeutic alteration of cell fate decisions and pathological gene regulatory networks by GATA4-targeted compounds modulating chamber-specific transcriptional programs in multipotent cardiac progenitor cells and cardiomyocytes. The compound scaffolds described within this study could be used to develop regenerative strategies for myocardial regeneration.Peer reviewe
Isometric Embeddings and Noncommutative Branes in Homogeneous Gravitational Waves
We characterize the worldvolume theories on symmetric D-branes in a
six-dimensional Cahen-Wallach pp-wave supported by a constant Neveu-Schwarz
three-form flux. We find a class of flat noncommutative euclidean D3-branes
analogous to branes in a constant magnetic field, as well as curved
noncommutative lorentzian D3-branes analogous to branes in an electric
background. In the former case the noncommutative field theory on the branes is
constructed from first principles, related to dynamics of fuzzy spheres in the
worldvolumes, and used to analyse the flat space limits of the string theory.
The worldvolume theories on all other symmetric branes in the background are
local field theories. The physical origins of all these theories are described
through the interplay between isometric embeddings of branes in the spacetime
and the Penrose-Gueven limit of AdS3 x S3 with Neveu-Schwarz three-form flux.
The noncommutative field theory of a non-symmetric spacetime-filling D-brane is
also constructed, giving a spatially varying but time-independent
noncommutativity analogous to that of the Dolan-Nappi model.Comment: 52 pages; v2: References adde
Time resolution and power consumption of a monolithic silicon pixel prototype in SiGe BiCMOS technology
SiGe BiCMOS technology can be used to produce ultra-fast, low-power silicon
pixel sensors that provide state-of-the-art time resolution even without an
internal gain mechanism. The development of such sensors requires the
identification of the main factors that may degrade the timing performance and
the characterisation of the dependance of the sensor time resolution on the
amplifier power consumption. Measurements with a source of
a prototype sensor produced in SG13G2 technology from IHP Microelectronics,
shows a time resolution of 140 ps at an amplifier current of 7 A and 45 ps at higher power consumption. A full simulation shows that the
resolution on the measurement of the signal time-over-threshold, used to
correct for time walk, is the main factor affecting the timing performance
Atmospheric Pressure Mass Spectrometry of Single Viruses and Nanoparticles by Nanoelectromechanical Systems
Mass spectrometry of intact nanoparticles and viruses can serve as a potent
characterization tool for material science and biophysics. Inaccessible by
widespread commercial techniques, the mass of single nanoparticles and viruses
(>10MDa) can be readily measured by NEMS (Nanoelectromechanical Systems) based
Mass Spectrometry, where charged and isolated analyte particles are generated
by Electrospray Ionization (ESI) in air and transported onto the NEMS resonator
for capture and detection. However, the applicability of NEMS as a practical
solution is hindered by their miniscule surface area, which results in poor
limit-of-detection and low capture efficiency values. Another hindrance is the
necessity to house the NEMS inside complex vacuum systems, which is required in
part to focus analytes towards the miniscule detection surface of the NEMS.
Here, we overcome both limitations by integrating an ion lens onto the NEMS
chip. The ion lens is composed of a polymer layer, which charges up by
receiving part of the ions incoming from the ESI tip and consequently starts to
focus the analytes towards an open window aligned with the active area of the
NEMS electrostatically. With this integrated system, we have detected the mass
of gold and polystyrene nanoparticles under ambient conditions and with two
orders-of-magnitude improvement in capture efficiency compared to the
state-of-the-art. We then applied this technology to obtain the mass spectrum
of SARS-CoV-2 and BoHV-1 virions. With the increase in analytical throughput,
the simplicity of the overall setup and the operation capability under ambient
conditions, the technique demonstrates that NEMS Mass Spectrometry can be
deployed for mass detection of engineered nanoparticles and biological samples
efficiently.Comment: 38 pages, 6 figure
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