12,366 research outputs found
The Halo Mass Function from Excursion Set Theory with a Non-Gaussian Trispectrum
A sizeable level of non-Gaussianity in the primordial cosmological
perturbations may be induced by a large trispectrum, i.e. by a large connected
four-point correlation function. We compute the effect of a primordial
non-Gaussian trispectrum on the halo mass function, within excursion set
theory. We use the formalism that we have developed in a previous series of
papers and which allows us to take into account the fact that, in the presence
of non-Gaussianity, the stochastic evolution of the smoothed density field, as
a function of the smoothing scale, is non-markovian. In the large mass limit,
the leading-order term that we find agrees with the leading-order term of the
results found in the literature using a more heuristic Press-Schecther
(PS)-type approach. Our approach however also allows us to evaluate
consistently the subleading terms, which depend not only on the four-point
cumulant but also on derivatives of the four-point correlator, and which cannot
be obtained within non-Gaussian extensions of PS theory. We perform explicitly
the computation up to next-to-leading order.Comment: LaTeX file, 15 page
A hydrodynamical homotopy co-momentum map and a multisymplectic interpretation of higher order linking numbers
In this article a homotopy co-momentum map (\`a la
Callies-Fr\'egier-Rogers-Zambon) trangressing to the standard hydrodynamical
co-momentum map of Arnol'd, Marsden and Weinstein and others is constructed and
then generalized to a special class of Riemannian manifolds. Also, a covariant
phase space interpretation of the coadjoint orbits associated to the Euler
evolution for perfect fluids and in particular of Brylinski's manifold of
smooth oriented knots is discussed. As an application of the above homotopy
co-momentum map, a reinterpretation of the (Massey) higher order linking
numbers in terms of conserved quantities within the multisymplectic framework
is provided and knot theoretic analogues of first integrals in involution are
determined.Comment: 21 pages, 3 figures. The present version focuses on the connections
between multisymplectic geometry, hydrodynamics and vortices. The derivation
of the HOMFLYPT polynomial via geometric quantization has been proposed as a
separate preprint, see "Derivation of the HOMFLYPT knot polynomial via
helicity and geometric quantization ", arXiv:1910.xxx
Transition to ballistic regime for heat transport in helium II
The size-dependent and flux-dependent effective thermal conductivity of
narrow capillaries filled with superfluid helium is analyzed from a
thermodynamic continuum perspective. The classical Landau evaluation of the
effective thermal conductivity of quiescent superfluid, or the Gorter-Mellinck
regime of turbulent superfluids, are extended to describe the transition to
ballistic regime in narrow channels wherein the radius is comparable to (or
smaller than) the phonon mean-free path in superfluid helium. To do so
we start from an extended equation for the heat flux incorporating non-local
terms, and take into consideration a heat slip flow along the walls of the
tube. This leads from an effective thermal conductivity proportional to
(Landau regime) to another one proportional to (ballistic regime). We
consider two kinds of flows: along cylindrical pipes and along two infinite
parallel plates.Comment: 16 page
Scaling in soft spheres: fragility invariance on the repulsive potential softness
We address the question of the dependence of the fragility of glass forming
supercooled liquids on the softness of an interacting potential by performing
numerical simulation of a binary mixture of soft spheres with different power n
of the interparticle repulsive potential. We show that the temperature
dependence of the diffusion coefficients for various collapses onto a
universal curve, supporting the unexpected view that fragility is not related
to the hard core repulsion. We also find that the configurational entropy
correlates with the slowing down of the dynamics for all studied n.Comment: 4 pages, 4 figure
Modelling Italian potential output and the output gap
The aim of the paper is to estimate a reliable quarterly time-series of potential output for the Italian economy, exploiting four alternative approaches: a Bayesian unobserved component method, a univariate time-varying autoregressive model, a production function approach and a structural VAR. Based on a wide range of evaluation criteria, all methods generate output gaps that accurately describe the Italian business cycle over the past three decades. All output gap measures are subject to non-negligible revisions when new data become available. Nonetheless they still prove to be informative about the current cyclical phase and, unlike the evidence reported in most of the literature, helpful at predicting inflation compared with simple benchmarks. We assess also the performance of output gap estimates obtained by combining the four original indicators, using either equal weights or Bayesian averaging, showing that the resulting measures (i) are less sensitive to revisions; (ii) are at least as good as the originals at tracking business cycle fluctuations; (iii) are more accurate as inflation predictors.potential output, business cycle, Phillips curve, output gap
Modelling the evolution of transcription factor binding preferences in complex eukaryotes
Transcription factors (TFs) exert their regulatory action by binding to DNA
with specific sequence preferences. However, different TFs can partially share
their binding sequences due to their common evolutionary origin. This
`redundancy' of binding defines a way of organizing TFs in `motif families' by
grouping TFs with similar binding preferences. Since these ultimately define
the TF target genes, the motif family organization entails information about
the structure of transcriptional regulation as it has been shaped by evolution.
Focusing on the human TF repertoire, we show that a one-parameter evolutionary
model of the Birth-Death-Innovation type can explain the TF empirical
ripartition in motif families, and allows to highlight the relevant
evolutionary forces at the origin of this organization. Moreover, the model
allows to pinpoint few deviations from the neutral scenario it assumes: three
over-expanded families (including HOX and FOX genes), a set of `singleton' TFs
for which duplication seems to be selected against, and a higher-than-average
rate of diversification of the binding preferences of TFs with a Zinc Finger
DNA binding domain. Finally, a comparison of the TF motif family organization
in different eukaryotic species suggests an increase of redundancy of binding
with organism complexity.Comment: 14 pages, 5 figures. Minor changes. Final version, accepted for
publicatio
Random Access in Massive MIMO by Exploiting Timing Offsets and Excess Antennas
Massive MIMO systems, where base stations are equipped with hundreds of
antennas, are an attractive way to handle the rapid growth of data traffic. As
the number of user equipments (UEs) increases, the initial access and handover
in contemporary networks will be flooded by user collisions. In this paper, a
random access protocol is proposed that resolves collisions and performs timing
estimation by simply utilizing the large number of antennas envisioned in
Massive MIMO networks. UEs entering the network perform spreading in both time
and frequency domains, and their timing offsets are estimated at the base
station in closed-form using a subspace decomposition approach. This
information is used to compute channel estimates that are subsequently employed
by the base station to communicate with the detected UEs. The favorable
propagation conditions of Massive MIMO suppress interference among UEs whereas
the inherent timing misalignments improve the detection capabilities of the
protocol. Numerical results are used to validate the performance of the
proposed procedure in cellular networks under uncorrelated and correlated
fading channels. With UEs that may simultaneously become active
with probability 1\% and a total of frequency-time codes (in a given
random access block), it turns out that, with antennas, the proposed
procedure successfully detects a given UE with probability 75\% while providing
reliable timing estimates.Comment: 30 pages, 6 figures, 1 table, submitted to Transactions on
Communication
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