7,556 research outputs found
A Renormalizable 4-Dimensional Tensor Field Theory
We prove that an integrated version of the Gurau colored tensor model
supplemented with the usual Bosonic propagator on is renormalizable to
all orders in perturbation theory. The model is of the type expected for
quantization of space-time in 4D Euclidean gravity and is the first example of
a renormalizable model of this kind. Its vertex and propagator are
four-stranded like in 4D group field theories, but without gauge averaging on
the strands. Surprisingly perhaps, the model is of the rather than of
the type, since two different -type interactions are
log-divergent, i.e. marginal in the renormalization group sense. The
renormalization proof relies on a multiscale analysis. It identifies all
divergent graphs through a power counting theorem. These divergent graphs have
internal and external structure of a particular kind called melonic. Melonic
graphs dominate the 1/N expansion of colored tensor models and generalize the
planar ribbon graphs of matrix models. A new locality principle is established
for this category of graphs which allows to renormalize their divergences
through counterterms of the form of the bare Lagrangian interactions. The model
also has an unexpected anomalous log-divergent term, which
can be interpreted as the generation of a scalar matter field out of pure
gravity.Comment: 44 pages, 11 figures, typos corrected, figures added, improved
versio
Maximally rotating waves in AdS and on spheres
We study the cubic wave equation in AdS_(d+1) (and a closely related cubic
wave equation on S^3) in a weakly nonlinear regime. Via time-averaging, these
systems are accurately described by simplified infinite-dimensional quartic
Hamiltonian systems, whose structure is mandated by the fully resonant spectrum
of linearized perturbations. The maximally rotating sector, comprising only the
modes of maximal angular momentum at each frequency level, consistently
decouples in the weakly nonlinear regime. The Hamiltonian systems obtained by
this decoupling display remarkable periodic return behaviors closely analogous
to what has been demonstrated in recent literature for a few other related
equations (the cubic Szego equation, the conformal flow, the LLL equation).
This suggests a powerful underlying analytic structure, such as integrability.
We comment on the connection of our considerations to the Gross-Pitaevskii
equation for harmonically trapped Bose-Einstein condensates.Comment: 17 page
Privacy-preserving Publication of Mobility Data with High Utility
An increasing amount of mobility data is being collected every day by
different means, e.g., by mobile phone operators. This data is sometimes
published after the application of simple anonymization techniques, which might
lead to severe privacy threats. We propose in this paper a new solution whose
novelty is twofold. Firstly, we introduce an algorithm designed to hide places
where a user stops during her journey (namely points of interest), by enforcing
a constant speed along her trajectory. Secondly, we leverage places where users
meet to take a chance to swap their trajectories and therefore confuse an
attacker.Comment: 2015 35th IEEE International Conference on Distributed Computed
System
Book review: risk, power and inequality in the 21st century by Dean Curran
In Risk, Power and Inequality in the 21st Century, Dean Curran offers a reassessment of the relationship between risk and inequality, drawing on such examples as the 2008 financial crisis and climate change to show how class is integral to better understanding risk society. While the book could explore modalities of inequality – such as race and gender – in more detail, this is a rigorous text that responds to the formative work of Ulrich Beck to offer a valuable new contribution to theorisations of risk today, finds Ben Vincent
Time Distortion Anonymization for the Publication of Mobility Data with High Utility
An increasing amount of mobility data is being collected every day by
different means, such as mobile applications or crowd-sensing campaigns. This
data is sometimes published after the application of simple anonymization
techniques (e.g., putting an identifier instead of the users' names), which
might lead to severe threats to the privacy of the participating users.
Literature contains more sophisticated anonymization techniques, often based on
adding noise to the spatial data. However, these techniques either compromise
the privacy if the added noise is too little or the utility of the data if the
added noise is too strong. We investigate in this paper an alternative
solution, which builds on time distortion instead of spatial distortion.
Specifically, our contribution lies in (1) the introduction of the concept of
time distortion to anonymize mobility datasets (2) Promesse, a protection
mechanism implementing this concept (3) a practical study of Promesse compared
to two representative spatial distortion mechanisms, namely Wait For Me, which
enforces k-anonymity, and Geo-Indistinguishability, which enforces differential
privacy. We evaluate our mechanism practically using three real-life datasets.
Our results show that time distortion reduces the number of points of interest
that can be retrieved by an adversary to under 3 %, while the introduced
spatial error is almost null and the distortion introduced on the results of
range queries is kept under 13 % on average.Comment: in 14th IEEE International Conference on Trust, Security and Privacy
in Computing and Communications, Aug 2015, Helsinki, Finlan
Platinum thickness dependence of the inverse spin-Hall voltage from spin pumping in a hybrid YIG/Pt system
We show the first experimental observation of the platinum (Pt) thickness
dependence in a hybrid YIG/Pt system of the inverse spin-Hall effect from spin
pumping, over a large frequency range and for different rf powers. From the
measurement of the dc voltage () at the resonant condition
and the resistance () of the Pt layer, a strong enhancement of the ratio
has been observed, which is not in agreement with previous
studies on the NiFe/Pt system. The origin of this behaviour is still unclear
and cannot be explained by the spin transport model that we have used.Comment: 4 pages, 3 figure
Conformal flow on and weak field integrability in AdS
We consider the conformally invariant cubic wave equation on the Einstein
cylinder for small rotationally symmetric
initial data. This simple equation captures many key challenges of nonlinear
wave dynamics in confining geometries, while a conformal transformation relates
it to a self-interacting conformally coupled scalar in four-dimensional anti-de
Sitter spacetime (AdS) and connects it to various questions of AdS
stability. We construct an effective infinite-dimensional time-averaged
dynamical system accurately approximating the original equation in the weak
field regime. It turns out that this effective system, which we call the
conformal flow, exhibits some remarkable features, such as low-dimensional
invariant subspaces, a wealth of stationary states (for which energy does not
flow between the modes), as well as solutions with nontrivial exactly periodic
energy flows. Based on these observations and close parallels to the cubic
Szego equation, which was shown by Gerard and Grellier to be Lax-integrable, it
is tempting to conjecture that the conformal flow and the corresponding weak
field dynamics in AdS are integrable as well.Comment: 22 pages, v2: minor revisions, several references added, v3: typos
corrected, v4: typos corrected, one reference added, matches version accepted
by CM
Vanishing beta function for Grosse-Wulkenhaar model in a magnetic field
We prove that the beta function of the Grosse-Wulkenhaar model including a
magnetic field vanishes at all order of perturbations. We compute the
renormalization group flow of the relevant dynamic parameters and find a
non-Gaussian infrared fixed point. Some consequences of these results are
discussed.Comment: 14 pages, 5 figure
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