10,026 research outputs found
Generalised group field theories and quantum gravity transition amplitudes
We construct a generalised formalism for group field theories, in which the
domain of the field is extended to include additional proper time variables, as
well as their conjugate mass variables. This formalism allows for different
types of quantum gravity transition amplitudes in perturbative expansion, and
we show how both causal spin foam models and the usual a-causal ones can be
derived from it, within a sum over triangulations of all topologies. We also
highlight the relation of the so-derived causal transition amplitudes with
simplicial gravity actions.Comment: RevTeX; 6 pages, 2 figure
Radial and angular rotons in trapped dipolar gases
We study Bose-Einstein condensates with purely dipolar interactions in oblate
(pancake) traps. We find that the condensate always becomes unstable to
collapse when the number of particles is sufficiently large. We analyze the
instability, and find that it is the trapped-gas analogue of the
``roton-maxon'' instability previously reported for a gas that is unconfined in
two dimensions. In addition, we find that under certain circumstances, the
condensate wave function attains a biconcave shape, with its maximum density
away from the center of the gas. These biconcave condensates become unstable
due to azimuthl excitation - an angular roton.Comment: 4 pages, 3 figure
A revisitation of the 1888 H.Hertz experiment
We propose a revisitation of the original experiment performed by H. Hertz in
1888. With a simple setup it is possible to produce electromagnetic waves with
a frequency in the range of 3 MHz. By performing Fourier analysis of the signal
captured by a resonant antenna it is possible to study the behaviour of the RLC
series circuit, frequency splitting of coupled resonances and finally the
characteristics of the near-field emitted by the loop antenna
Solid phase chemistry to covalently and reversibly capture thiolated RNA.
Here, we describe an approach to enrich newly transcribed RNAs from primary mouse neurons using 4-thiouridine (s4U) metabolic labeling and solid phase chemistry. This one-step enrichment procedure captures s4U-RNA by using highly efficient methane thiosulfonate (MTS) chemistry in an immobilized format. Like solution-based methods, this solid-phase enrichment can distinguish mature RNAs (mRNA) with differential stability, and can be used to reveal transient RNAs such as enhancer RNAs (eRNAs) and primary microRNAs (pri-miRNAs) from short metabolic labeling. Most importantly, the efficiency of this solid-phase chemistry made possible the first large scale measurements of RNA polymerase II (RNAPII) elongation rates in mouse cortical neurons. Thus, our approach provides the means to study regulation of RNA metabolism in specific tissue contexts as a means to better understand gene expression in vivo
The pharmacology and function of receptors for short-chain fatty acids
Despite some blockbuster G protein–coupled receptor (GPCR) drugs, only a small fraction (∼15%) of the more than 390 nonodorant GPCRs have been successfully targeted by the pharmaceutical industry. One way that this issue might be addressed is via translation of recent deorphanization programs that have opened the prospect of extending the reach of new medicine design to novel receptor types with potential therapeutic value. Prominent among these receptors are those that respond to short-chain free fatty acids of carbon chain length 2–6. These receptors, FFA2 (GPR43) and FFA3 (GPR41), are each predominantly activated by the short-chain fatty acids acetate, propionate, and butyrate, ligands that originate largely as fermentation by-products of anaerobic bacteria in the gut. However, the presence of FFA2 and FFA3 on pancreatic β-cells, FFA3 on neurons, and FFA2 on leukocytes and adipocytes means that the biologic role of these receptors likely extends beyond the widely accepted role of regulating peptide hormone release from enteroendocrine cells in the gut. Here, we review the physiologic roles of FFA2 and FFA3, the recent development and use of receptor-selective pharmacological tool compounds and genetic models available to study these receptors, and present evidence of the potential therapeutic value of targeting this emerging receptor pair
Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses
We consider the fundamental problem of charged particles moving along and
around a curved magnetic field line, revising the synchro-curvature radiation
formulae introduced by Cheng and Zhang (1996). We provide more compact
expressions to evaluate the spectrum emitted by a single particle, identifying
the key parameter that controls the transition between the curvature-dominated
and the synchrotron-dominated regime. This parameter depends on the local
radius of curvature of the magnetic field line, the gyration radius, and the
pitch angle. We numerically solve the equations of motion for the emitting
particle by considering self-consistently the radiative losses, and provide the
radiated spectrum produced by a particle when an electric acceleration is
balanced by its radiative losses, as it is assumed to happen in the outer gaps
of pulsar's magnetospheres. We compute the average spectrum radiated throughout
the particle trajectory finding that the slope of the spectrum before the peak
depends on the location and size of the emission region. We show how this
effect could then lead to a variety of synchro-curvature spectra. Our results
reinforce the idea that the purely synchrotron or curvature losses are, in
general, inadequate to describe the radiative reaction on the particle motion,
and the spectrum of emitted photons. Finally, we discuss the applicability of
these calculations to different astrophysical scenarios.Comment: 9 pages, 5 figures, 2 tables. Accepted for publication in MNRAS main
journal. References update
Post-T Tauri stars: a false problem
We consider the problem of the apparent lack of old T Tauri stars in low-mass
star forming regions in the framework of the standard model of low-mass star
formation. We argue that the similarity between molecular cloud lifetime and
ambipolar diffusion timescale implies that star formation does not take place
instantaneously, nor at a constant rate. We conclude that the probability of
finding a large population of old stars in a star forming region is
intrinsically very small and that the post-T Tauri problem is by and large not
existent.Comment: 6 pages (LaTeX), no Figures to be published in The Astrophysical
Journal Letter
Factor Price Equalization in Heckscher-Ohlin Model
This paper investigates the likelihood of factor-price equalization under the simple assumptions of Heckscher-Ohlin Theory. Factor-price equalization is also directly related to whether countries specialize or not in the global market. A full-equilibrium in the world requires not only the equilibrium in the production side of the economy, but also the supply-demand equality in the world. However, once we obtain an equilibrium in the production side of the economy, it is always possible to define demand in a way to get supply-demand equality at any production side equilibrium amounts. Therefore, it is not possible to talk about factor-price equalization without specifying demand in the economy. Using L-P diagrams, the paper demonstrates how both factor-price equalization and non-equalization cases are possible when we look at only the production side of the economy. It is also demonstrated that the equilibrium possibilities will be much larger for factor-price equalization case if the number of commodities is more than the number of factors of production. However, the larger possibilities do not refer to different real equilibria, but only to indeterminacy in production. When demand is introduced in the economy and supply-demand equality constraints are respected, we see that factor-prices might or might not be equalized depending on factor endowments, production functions and demand. The paper demonstrates this by introducing a model with 2 countries, 2 factors of production, 3 goods and CES utility function. Finally, using comparative statistics on this simple model, the conditions under which the likelihood of factor-price equalization increases are determined.
Non-line-of-sight tracking of people at long range
A remote-sensing system that can determine the position of hidden objects has
applications in many critical real-life scenarios, such as search and rescue
missions and safe autonomous driving. Previous work has shown the ability to
range and image objects hidden from the direct line of sight, employing
advanced optical imaging technologies aimed at small objects at short range. In
this work we demonstrate a long-range tracking system based on single laser
illumination and single-pixel single-photon detection. This enables us to track
one or more people hidden from view at a stand-off distance of over 50~m. These
results pave the way towards next generation LiDAR systems that will
reconstruct not only the direct-view scene but also the main elements hidden
behind walls or corners
Improved analysis of the bounds from the electroweak precision tests on the 4-site model
We present a new complete analysis of the electroweak precision observables
within the recently proposed 4-site Higgsless model, which is based on the
SU(2)_L x SU(2)_1 x SU(2)_2 x U(1)_Y gauge symmetry and predicts six extra
gauge bosons, W_{1,2} and Z_{1,2}. Within the epsilon_i (i=1,2,3,b)
parametrization, we compute for the first time the EWPT bounds via a complete
numerical algorithm going beyond commonly used approximations. Both
epsilon_{1,3} impose strong constraints. Hence, it is mandatory to consider
them jointly when extracting EWPT bounds and to fully take in to account the
correlations among the electroweak precison measurements. The phenomenological
consequence is that the extra gauge bosons must be heavier than 250 GeV. Their
couplings to SM fermions, even if bounded, might be of the same order of
magnitude than the SM ones. In contrast to other Higgsless models, the 4-site
model is not fermiophobic. The new gauge bosons could thus be discovered in the
favoured Drell-Yan channel already during the present run of the LHC
experiment.Comment: Latex file, 35 pages, 10 figures, corrected typos, published versio
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