3,091 research outputs found
A new formulation of the effective theory for heavy particles
We derive the effective theories for heavy particles with a functional
integral approach by integrating away the states with high velocity and with
high virtuality. This formulation is non-perturbative and has a close
connection with the Wilson renormalization group transformation. The fixed
point hamiltonian of our transformation coincides with the static hamiltonian
and irrelevant operators can be identified with the usual corrections to
the static theory. No matching condition has to be imposed between the full and
the static theory operators with our approach. The values of the matching
constants come out as a dynamical effect of the renormalization group flow.Comment: 26 pages, plain Latex + 4 postscript figures (appended at the end),
Preprint Roma1 993-94 (some missing lines in a few formulas have been
restored; minor changes
In-plane and Out-of-plane Plasma Resonances in Optimally Doped La1.84Sr0.16CuO4
We addressed the inconsistency between the electron mass anisotropy ratios
determined by the far-infrared experiments and DC conductivity measurements. By
eliminating possible sources of error and increasing the sensitivity and
resolution in the far-infrared reflectivity measurement on the single
crystalline and on the polycrystalline La1.84Sr0.16CuO4, we have unambiguously
identified that the source of the mass anisotropy problem is in the estimation
of the free electron density involved in the charge transport and
superconductivity. In this study we found that only 2.8 % of the total
doping-induced charge density is itinerant at optimal doping. Our result not
only resolves the mass anisotropy puzzle but also points to a novel electronic
structure formed by the rest of the electrons that sets the stage for the high
temperature superconductivity
An Elusive Z' Coupled to Beauty
By extending the standard gauge group to SU(3)_c \times SU(2)_L \times U(1)_Y
\times U(1)_X with X charges carried only by the third family we accommodate
the LEP measurement of R_b and predict a potentially measurable discrepancy in
A_{FB}^{b} in e^+e^- scattering and that D^0\bar{D}^0 mixing may be near its
experimental limit. The Z', which explicitly violates the GIM mechanism, can
nevertheless be naturally consistent with FCNC constraints. Direct detection of
the Z' is possible but challenging.Comment: 12 pages, plus 1 Postscript figure, uses revtex, Discussion of FCNC
extende
Strong and radiative decays of X(3872) as a hadronic molecule with a negative parity
Properties of X(3872) are studied by regarding it as a hadronic
molecule with in the phenomenological Lagrangian approach. We
find that our model with about 97.6% isospin zero component explains the
existing data nicely, for example, the ratio . We predict
the partial widths of the radiative decays of ,
and the strong decays of ,
as well as . Our analysis
shows that the measurement of the ratio may signal the nature
of X(3872)
Homeostatic competition drives tumor growth and metastasis nucleation
We propose a mechanism for tumor growth emphasizing the role of homeostatic
regulation and tissue stability. We show that competition between surface and
bulk effects leads to the existence of a critical size that must be overcome by
metastases to reach macroscopic sizes. This property can qualitatively explain
the observed size distributions of metastases, while size-independent growth
rates cannot account for clinical and experimental data. In addition, it
potentially explains the observed preferential growth of metastases on tissue
surfaces and membranes such as the pleural and peritoneal layers, suggests a
mechanism underlying the seed and soil hypothesis introduced by Stephen Paget
in 1889 and yields realistic values for metastatic inefficiency. We propose a
number of key experiments to test these concepts. The homeostatic pressure as
introduced in this work could constitute a quantitative, experimentally
accessible measure for the metastatic potential of early malignant growths.Comment: 13 pages, 11 figures, to be published in the HFSP Journa
Making Neural Networks FAIR
Research on neural networks has gained significant momentum over the past few
years. Because training is a resource-intensive process and training data
cannot always be made available to everyone, there has been a trend to reuse
pre-trained neural networks. As such, neural networks themselves have become
research data. In this paper, we first present the neural network ontology
FAIRnets Ontology, an ontology to make existing neural network models findable,
accessible, interoperable, and reusable according to the FAIR principles. Our
ontology allows us to model neural networks on a meta-level in a structured
way, including the representation of all network layers and their
characteristics. Secondly, we have modeled over 18,400 neural networks from
GitHub based on this ontology, which we provide to the public as a knowledge
graph called FAIRnets, ready to be used for recommending suitable neural
networks to data scientists
Average Heating Rate of Hot Atmospheres in Distant Clusters by Radio AGN: Evidence for Continuous AGN Heating
We examine atmospheric heating by radio active galactic nuclei (AGN) in
distant X-ray clusters by cross correlating clusters selected from the 400
Square Degree (400SD) X-ray Cluster survey with radio sources in the NRAO VLA
Sky Survey. Roughly 30% of the clusters show radio emission above a flux
threshold of 3 mJy within a projected radius of 250 kpc. The radio emission is
presumably associated with the brightest cluster galaxy. The mechanical jet
power for each radio source was determined using scaling relations between
radio power and cavity (mechanical) power determined for nearby clusters,
groups, and galaxies with hot atmospheres containing X-ray cavities. The
average jet power of the central radio AGN is approximately \ergs. We find no significant correlation between radio power, hence
mechanical jet power, and the X-ray luminosities of clusters in the redshift
range 0.1 -- 0.6. This implies that the mechanical heating rate per particle is
higher in lower mass, lower X-ray luminosity clusters. The jet power averaged
over the sample corresponds to an atmospheric heating of approximately 0.2 keV
per particle within R. Assuming the current AGN heating rate does not
evolve but remains constant to redshifts of 2, the heating rate per particle
would rise by a factor of two. We find that the energy injected from radio AGN
contribute substantially to the excess entropy in hot atmospheres needed to
break self-similarity in cluster scaling relations. The detection frequency of
radio AGN is inconsistent with the presence of strong cooling flows in 400SD
clusters, but does not exclude weak cooling flows. It is unclear whether
central AGN in 400SD clusters are maintained by feedback at the base of a
cooling flow. Atmospheric heating by radio AGN may retard the development of
strong cooling flows at early epochs.Comment: ApJ in pres
Direct Formation of Supermassive Black Holes via Multi-Scale Gas Inflows in Galaxy Mergers
Observations of distant bright quasars suggest that billion solar mass
supermassive black holes (SMBHs) were already in place less than a billion
years after the Big Bang. Models in which light black hole seeds form by the
collapse of primordial metal-free stars cannot explain their rapid appearance
due to inefficient gas accretion. Alternatively, these black holes may form by
direct collapse of gas at the center of protogalaxies. However, this requires
metal-free gas that does not cool efficiently and thus is not turned into
stars, in contrast with the rapid metal enrichment of protogalaxies. Here we
use a numerical simulation to show that mergers between massive protogalaxies
naturally produce the required central gas accumulation with no need to
suppress star formation. Merger-driven gas inflows produce an unstable, massive
nuclear gas disk. Within the disk a second gas inflow accumulates more than 100
million solar masses of gas in a sub-parsec scale cloud in one hundred thousand
years. The cloud undergoes gravitational collapse, which eventually leads to
the formation of a massive black hole. The black hole can grow to a billion
solar masses in less than a billion years by accreting gas from the surrounding
disk.Comment: 26 pages, 4 Figures, submitted to Nature (includes Supplementary
Information
Multi-scale initial conditions for cosmological simulations
We discuss a new algorithm to generate multi-scale initial conditions with
multiple levels of refinements for cosmological "zoom-in" simulations. The
method uses an adaptive convolution of Gaussian white noise with a real space
transfer function kernel together with an adaptive multi-grid Poisson solver to
generate displacements and velocities following first (1LPT) or second order
Lagrangian perturbation theory (2LPT). The new algorithm achieves RMS relative
errors of order 10^(-4) for displacements and velocities in the refinement
region and thus improves in terms of errors by about two orders of magnitude
over previous approaches. In addition, errors are localized at coarse-fine
boundaries and do not suffer from Fourier-space induced interference ringing.
An optional hybrid multi-grid and Fast Fourier Transform (FFT) based scheme is
introduced which has identical Fourier space behaviour as traditional
approaches. Using a suite of re-simulations of a galaxy cluster halo our real
space based approach is found to reproduce correlation functions, density
profiles, key halo properties and subhalo abundances with per cent level
accuracy. Finally, we generalize our approach for two-component baryon and
dark-matter simulations and demonstrate that the power spectrum evolution is in
excellent agreement with linear perturbation theory. For initial baryon density
fields, it is suggested to use the local Lagrangian approximation in order to
generate a density field for mesh based codes that is consistent with
Lagrangian perturbation theory instead of the current practice of using the
Eulerian linearly scaled densities.Comment: 22 pages, 24 figures. MNRAS in press. Updated affiliation
All Optical Implementation of Multi-Spin Entanglement in a Semiconductor Quantum Well
We use ultrafast optical pulses and coherent techniques to create spin
entangled states of non-interacting electrons bound to donors (at least three)
and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the
exchange interaction between localized excitons and paramagnetic impurities,
can in principle be applied to entangle a large number of spins.Comment: 17 pages, 3 figure
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