10,662 research outputs found
Trispectrum versus Bispectrum in Single-Field Inflation
In the standard slow-roll inflationary cosmology, quantum fluctuations in a
single field, the inflaton, generate approximately Gaussian primordial density
perturbations. At present, the bispectrum and trispectrum of the density
perturbations have not been observed and the probability distribution for these
perturbations is consistent with Gaussianity. However, Planck satellite data
will bring a new level of precision to bear on this issue, and it is possible
that evidence for non-Gaussian effects in the primordial distribution will be
discovered. One possibility is that a trispectrum will be observed without
evidence for a non-zero bispectrum. It is not difficult for this to occur in
inflationary models where quantum fluctuations in a field other than the
inflaton contribute to the density perturbations. A natural question to ask is
whether such an observation would rule out the standard scenarios. We explore
this issue and find that it is possible to construct single-field models in
which inflaton-generated primordial density perturbations have an observable
trispectrum, but a bispectrum that is too small to be observed by the Planck
satellite. However, an awkward fine tuning seems to be unavoidable.Comment: 15 pages, 3 figures; journal versio
Dynamical excitations in the collision of 2D Bose-Einstein condensates
We carry out simulations of the collision of two components of an
adiabatically divided, quasi-2D BEC. We identify under, over and critically
damped regimes in the dipole oscillations of the components according to the
balance of internal and centre-of-mass (c.m.) energies of the components and
investigate the creation of internal excitations. We distinguish the behaviour
of this system from previous studies of quasi-1D BEC's. In particular we note
that the nature of the internal excitations is only essentially sensitive to an
initial phase difference between the components in the overdamped regime.Comment: 17 pages, 9 figure
Knowing You Beyond Race: The Importance of Individual Feature Encoding in the Other-Race Effect
A commentary on Why some faces won’t be remembered: brain potentials illuminate successful versus unsuccessful encoding for same-race and other-race face
AI Solutions for MDS: Artificial Intelligence Techniques for Misuse Detection and Localisation in Telecommunication Environments
This report considers the application of Articial Intelligence (AI) techniques to
the problem of misuse detection and misuse localisation within telecommunications
environments. A broad survey of techniques is provided, that covers inter alia
rule based systems, model-based systems, case based reasoning, pattern matching,
clustering and feature extraction, articial neural networks, genetic algorithms, arti
cial immune systems, agent based systems, data mining and a variety of hybrid
approaches. The report then considers the central issue of event correlation, that
is at the heart of many misuse detection and localisation systems. The notion of
being able to infer misuse by the correlation of individual temporally distributed
events within a multiple data stream environment is explored, and a range of techniques,
covering model based approaches, `programmed' AI and machine learning
paradigms. It is found that, in general, correlation is best achieved via rule based approaches,
but that these suffer from a number of drawbacks, such as the difculty of
developing and maintaining an appropriate knowledge base, and the lack of ability
to generalise from known misuses to new unseen misuses. Two distinct approaches
are evident. One attempts to encode knowledge of known misuses, typically within
rules, and use this to screen events. This approach cannot generally detect misuses
for which it has not been programmed, i.e. it is prone to issuing false negatives.
The other attempts to `learn' the features of event patterns that constitute normal
behaviour, and, by observing patterns that do not match expected behaviour, detect
when a misuse has occurred. This approach is prone to issuing false positives,
i.e. inferring misuse from innocent patterns of behaviour that the system was not
trained to recognise. Contemporary approaches are seen to favour hybridisation,
often combining detection or localisation mechanisms for both abnormal and normal
behaviour, the former to capture known cases of misuse, the latter to capture
unknown cases. In some systems, these mechanisms even work together to update
each other to increase detection rates and lower false positive rates. It is concluded
that hybridisation offers the most promising future direction, but that a rule or state
based component is likely to remain, being the most natural approach to the correlation
of complex events. The challenge, then, is to mitigate the weaknesses of
canonical programmed systems such that learning, generalisation and adaptation
are more readily facilitated
A Bayesian Estimate of the Primordial Helium Abundance
We introduce a new statistical method to estimate the primordial helium
abundance, Y_p from observed abundances in a sample of galaxies which have
experienced stellar helium enrichment. Rather than using linear regression on
metal abundance we construct a likelihood function using a Bayesian prior,
where the key assumption is that the true helium abundance must always exceed
the primordial value. Using a sample of measurements compiled from the
literature we find estimates of Y_p between 0.221 and 0.236, depending on the
specific subsample and prior adopted, consistent with previous estimates either
from a linear extrapolation of the helium abundance with respect to
metallicity, or from the helium abundance of the lowest metallicity HII region,
I Zw 18. We also find an upper limit which is insensitive to the specific
subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95%
confidence, favoring a low cosmic baryon density and a high primordial
deuterium abundance. The main uncertainty is not the model of stellar
enrichment but possible common systematic biases in the estimate of Y in each
individual HII region.Comment: 14 pages, latex, 3 ps figure
The Anderson transition: time reversal symmetry and universality
We report a finite size scaling study of the Anderson transition. Different
scaling functions and different values for the critical exponent have been
found, consistent with the existence of the orthogonal and unitary universality
classes which occur in the field theory description of the transition. The
critical conductance distribution at the Anderson transition has also been
investigated and different distributions for the orthogonal and unitary classes
obtained.Comment: To appear in Physical Review Letters. Latex 4 pages with 4 figure
Field location and player roles as constraints on emergent 1-vs-1 interpersonal patterns of play in football
This study examined effects of player roles on interpersonal patterns of coordination that sustain decision-making in 1-vs-1 sub-phases of football in different field locations near the goal (left-, middle- and right zone). Participants were fifteen U-16 yrs players from a local competitive amateur team. To measure interpersonal patterns of coordination in the 1-vs-1 dyads we recorded: (i) the relative distance value between each attacker and defender to the centre of the goal, and (ii), the relative angle between the centre of the goal, each defender and attacker. Results revealed how variations in field locations near the goal (left-, middle- and right-zones) constrained the relative distance and relative angle values that emerged between them and the goal. It reveals that relative position of the goal is a key informational variable that sustained participants’ behaviours for dribbling and shooting. Higher values of relative distance and angle were observed in the middle zone, compared to other zones. Players’ roles also constitute a constraint on the interpersonal coordination for dribbling and shooting. Additionally, it seems that players’ foot preference constrains the dynamics of interpersonal patterns of coordination between participants, especially in left and right zones. The findings suggest that to increase participants’ opportunities for action, coaches should account with field positions, players’ roles and preference foot
Early-type stars observed in the ESO UVES Paranal Observatory Project - V. Time-variable interstellar absorption
The structure and properties of the diffuse interstellar medium (ISM) on
small scales, sub-au to 1 pc, are poorly understood. We compare interstellar
absorption-lines, observed towards a selection of O- and B-type stars at two or
more epochs, to search for variations over time caused by the transverse motion
of each star combined with changes in the structure in the foreground ISM. Two
sets of data were used: 83 VLT- UVES spectra with approximately 6 yr between
epochs and 21 McDonald observatory 2.7m telescope echelle spectra with 6 - 20
yr between epochs, over a range of scales from 0 - 360 au. The interstellar
absorption-lines observed at the two epochs were subtracted and searched for
any residuals due to changes in the foreground ISM. Of the 104 sightlines
investigated with typically five or more components in Na I D, possible
temporal variation was identified in five UVES spectra (six components), in Ca
II, Ca I and/or Na I absorption-lines. The variations detected range from 7\%
to a factor of 3.6 in column density. No variation was found in any other
interstellar species. Most sightlines show no variation, with 3{\sigma} upper
limits to changes of the order 0.1 - 0.3 dex in Ca II and Na I. These
variations observed imply that fine-scale structure is present in the ISM, but
at the resolution available in this study, is not very common at visible
wavelengths. A determination of the electron densities and lower limits to the
total number density of a sample of the sightlines implies that there is no
striking difference between these parameters in sightlines with, and sightlines
without, varying components.Comment: 19 pages, 11 figures, accepted for publication in MNRA
CP-odd Phase Correlations and Electric Dipole Moments
We revisit the constraints imposed by electric dipole moments (EDMs) of
nucleons and heavy atoms on new CP-violating sources within supersymmetric
theories. We point out that certain two-loop renormalization group corrections
induce significant mixing between the basis-invariant CP-odd phases. In the
framework of the constrained minimal supersymmetric standard model (CMSSM), the
CP-odd invariant related to the soft trilinear A-phase at the GUT scale,
theta_A, induces non-trivial and distinct CP-odd phases for the three gaugino
masses at the weak scale. The latter give one-loop contributions to EDMs
enhanced by tan beta, and can provide the dominant contribution to the electron
EDM induced by theta_A. We perform a detailed analysis of the EDM constraints
within the CMSSM, exhibiting the reach, in terms of sparticle spectra, which
may be obtained assuming generic phases, as well as the limits on the CP-odd
phases for some specific parameter points where detailed phenomenological
studies are available. We also illustrate how this reach will expand with
results from the next generation of experiments which are currently in
development.Comment: 31 pages, 21 eps figures; v2: additional remarks on 2-loop threshold
corrections and references added; v3: typos corrected, to appear in Phys.
Rev.
Computational Studies of Intramolecular Hydrogen Atom Transfers in the Ăź-Hydroxyethylperoxy and Ăź -Hydroxyethoxy Radicals
The Ăź-hydroxyethylperoxy (I) and Ăź-hydroxyethoxy (III) radicals are prototypes of species that can undergo hydrogen atom transfer across their intramolecular hydrogen bonds. These reactions may play an important role in both the atmosphere and in combustion systems. We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2). Our multi-coefficient Gaussian-3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol. These predictions agree rather well with the MPW1K and BB1K density functional theory predictions but disagree with predictions based on B3LYP energies or geometries. Our RRKM/master equation simulations suggest that almost 50% of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain. For Reaction 1 at 298 K, the variational TST rate constant is ~30% lower than the conventional TST result, and the microcanonical optimized multidimensional tunneling (OMT) method predicts that tunneling accelerates the reaction by a factor of 3. TST calculations on Reaction 2 reveal no variational effect and a 298 K OMT transmission coefficient of 105. The Eckart method overestimates transmission coefficients for both reactions. [ACS abstract]http://pubs.acs.org/cgi-bin/abstract.cgi/jpcafh/asap/abs/jp0704113.htm
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