17,060 research outputs found
Type IIA/M-theory Moduli fixing in a Class of Orientifold Models
We present the study of type II A flux vacua and their M-theory duals for
compactification on a class of Calabi-Yau orientifolds. The Kaehler potential
is derived from toroidal compactifications and the superpotential contains a
contribution from non-Abelian gauge degrees of freedoms. We obtain complete
stabilisation of the moduli. We found one supersymmetric minimum and several
non supersymmetric ones. Consistency of the analysis constrains the parameters
of the models in a finite region containing a finite, although very large,
number of flux vacua. From the M-theory side, we found some differences in the
distributions of the physical quantities with respect to the M-theory ensemble
studied by Acharya et al. In particular, it is easier to find small
supersymmetry breaking scale.Comment: 37 pages, 4 figures, LaTeX. Extended one Section, added reference
The Wavelet Trie: Maintaining an Indexed Sequence of Strings in Compressed Space
An indexed sequence of strings is a data structure for storing a string
sequence that supports random access, searching, range counting and analytics
operations, both for exact matches and prefix search. String sequences lie at
the core of column-oriented databases, log processing, and other storage and
query tasks. In these applications each string can appear several times and the
order of the strings in the sequence is relevant. The prefix structure of the
strings is relevant as well: common prefixes are sought in strings to extract
interesting features from the sequence. Moreover, space-efficiency is highly
desirable as it translates directly into higher performance, since more data
can fit in fast memory.
We introduce and study the problem of compressed indexed sequence of strings,
representing indexed sequences of strings in nearly-optimal compressed space,
both in the static and dynamic settings, while preserving provably good
performance for the supported operations.
We present a new data structure for this problem, the Wavelet Trie, which
combines the classical Patricia Trie with the Wavelet Tree, a succinct data
structure for storing a compressed sequence. The resulting Wavelet Trie
smoothly adapts to a sequence of strings that changes over time. It improves on
the state-of-the-art compressed data structures by supporting a dynamic
alphabet (i.e. the set of distinct strings) and prefix queries, both crucial
requirements in the aforementioned applications, and on traditional indexes by
reducing space occupancy to close to the entropy of the sequence
Heat kernel for Newton-Cartan trace anomalies
We compute the leading part of the trace anomaly for a free non-relativistic
scalar in 2+1 dimensions coupled to a background Newton-Cartan metric. The
anomaly is proportional to 1/m, where m is the mass of the scalar. We comment
on the implications of a conjectured a-theorem for non-relativistic theories
with boost invariance.Comment: 18 page
Predictive brains: forethought and the levels of explanation
Is any unified theory of brain function possible? Following a line of thought dat- ing back to the early cybernetics (see, e.g., Cordeschi, 2002), Clark (in press) has proposed the action-oriented Hierarchical Predictive Coding (HPC) as the account to be pursued in the effort of gain- ing the âGrand Unified Theory of the Mindââor âpainting the big picture,â as Edelman (2012) put it. Such line of thought is indeed appealing, but to be effectively pursued it should be confronted with experimental findings and explana- tory capabilities (Edelman, 2012).
The point we are making in this note is that a brain with predictive capa- bilities is certainly necessary to endow the agent situated in the environment with forethought or foresight, a crucial issue to outline the unified account advocated by Clark. But the capacity for fore- thought is deeply entangled with the capacity for emotions and when emotions are brought into the game, cogni- tive functions become part of a large-scale functional brain network. However, for such complex networks a consistent view of hierarchical organization in large-scale functional networks has yet to emerge (Bressler and Menon, 2010), whilst heterarchical organization is likely to play a strategic role (Berntson et al., 2012). This raises the necessity of a multilevel approach that embraces causal relations across levels of explanation in either direc- tion (bottomâup or topâdown), endorsing mutual calibration of constructs across levels (Berntson et al., 2012). Which, in turn, calls for a revised perspective on Marrâs levels of analysis framework (Marr, 1982). In the following we highlight some drawbacks of Clarkâs proposal in address- ing the above issues
Linear models for thin plates of polymer gels
Within the linearized three-dimensional theory of polymer gels, we consider a
sequence of problems formulated on a family of cylindrical domains whose height
tends to zero. We assume that the fluid pressure is controlled at the top and
bottom faces of the cylinder, and we consider two different scaling regimes for
the diffusivity tensor. Through asymptotic-analysis techniques we obtain two
plate models where the transverse displacement is governed by a plate equation
with an extra contribution from the fluid pressure. In the limit obtained
within the first scaling regime the fluid pressure is affine across the
thickness and hence it is determined by its instantaneous trace on the top and
bottom faces. In the second model, instead, the value of the fluid pressure is
governed by a three-dimensional diffusion equation
Exact Gravitational Dual of a Plasma Ball
We present an exact solution for a black hole localized near an infrared wall
in four-dimensional anti-deSitter space. By computing the holographic stress
tensor we show that the CFT dual of the black hole is a 2+1-dimensional ball
(i.e., a disk) of plasma at finite temperature, surrounded by vacuum. This
confirms some earlier conjectures about plasma balls in AdS/CFT. We also
estimate the value of the surface tension for the ball. The solution displays a
number of peculiarities, most notably a non-trivial curvature of the boundary
geometry, as well as other properties associated to the vanishing deconfinement
temperature of the set up. We discuss how these features are related to
specific physics at the infrared and ultraviolet boundaries for this solution,
and should not be generic properties of plasma balls.Comment: 23 pages, 3 figure
A numerical method to calculate the muon relaxation function in the presence of diffusion
We present an accurate and efficient method to calculate the effect of random
fluctuations of the local field at the muon, for instance in the case muon
diffusion, within the framework of the strong collision approximation. The
method is based on a reformulation of the Markovian process over a discretized
time base, leading to a summation equation for the muon polarization function
which is solved by discrete Fourier transform. The latter is formally
analogous, though not identical, to the integral equation of the original
continuous-time model, solved by Laplace transform. With real-case parameter
values, the solution of the discrete-time strong collision model is found to
approximate the continuous-time solution with excellent accuracy even with a
coarse-grained time sampling. Its calculation by the fast Fourier transform
algorithm is very efficient and suitable for real time fitting of experimental
data even on a slow computer.Comment: 7 pages, 3 figures. Submitted to Journal of Physics: Condensed Matte
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