7,350 research outputs found
Mean Field Methods for a Special Class of Belief Networks
The chief aim of this paper is to propose mean-field approximations for a
broad class of Belief networks, of which sigmoid and noisy-or networks can be
seen as special cases. The approximations are based on a powerful mean-field
theory suggested by Plefka. We show that Saul, Jaakkola and Jordan' s approach
is the first order approximation in Plefka's approach, via a variational
derivation. The application of Plefka's theory to belief networks is not
computationally tractable. To tackle this problem we propose new approximations
based on Taylor series. Small scale experiments show that the proposed schemes
are attractive
Brane Intersections in the Presence of a Worldvolume Electric Field
The study of brane intersections has provided important insights into a
possible non-commutative structure of spacetime geometry. In this paper we
focus on the D1D3 system. We compare the D1 and D3 descriptions of the
interesection and search for non-static solutions of the D3D1 funnel
equations in the presence of a worldvolume electric field. We find that the D1
and D3 descriptions do not agree. We find time dependent solutions that are a
natural generalization of those found without the electric field.Comment: 10 page
Spin-glass-like state in GdCu: role of phase separation and magnetic frustration
We report investigations on the ground state magnetic properties of
intermetallic compound GdCu through dc magnetization measurements. GdCu
undergoes first order martensitic type structural transition over a wide
temperature window of coexisting phases. The high temperature cubic and the low
temperature orthorhombic phases have different magnetic character and they show
antiferromagnetic and helimagnetic orderings below 145 K and 45 K respectively.
We observe clear signature of a glassy magnetic phase below the helimagnetic
ordering temperature, which is marked by thermomagnetic irreversibility, aging
and memory effects. The glassy magnetic phase in GdCu is found to be rather
intriguing with its origin lies in the interfacial frustration due to distinct
magnetic character of the coexisting phases.Comment: Physical Review B 83, 134427 (2011
On Time-dependent Collapsing Branes and Fuzzy Odd-dimensional Spheres
We study the time-dependent dynamics of a collection of N
collapsing/expanding D0-branes in type IIA String Theory. We show that the
fuzzy-S^3 and S^5 provide time-dependent solutions to the Matrix Model of
D0-branes and its DBI generalisation. Some intriguing cancellations in the
calculation of the non-abelian DBI Matrix actions result in the fuzzy-S^3 and
S^5 having the same dynamics at large-N. For the Matrix model, we find analytic
solutions describing the time-dependent radius, in terms of Jacobi elliptic
functions. Investigation of the physical properties of these configurations
shows that there are no bounces for the trajectory of the collapse at large-N.
We also write down a set of useful identities for fuzzy-S^3, fuzzy-S^5 and
general fuzzy odd-spheres.Comment: 35 pages, latex; v2: discussion in Appendix B on the large-N limit of
the associator is modified, main results of paper unchange
Upper bounds on all R-parity-violating \lambda\lambda'' combinations from proton stability
In an R-parity-violating supersymmetric theory, we derive upper bounds on all
the \lambda''_{ijk}\lambda_{i'j'k'}-type combinations from the consideration of
proton stability, where \lambda''_{ijk} are baryon-number-violating couplings
involving three baryonic fields and \lambda_{i'j'k'} are
lepton-number-violating couplings involving three leptonic fields.Comment: 5 pages, Latex, uses axodraw.sty; minor changes in the text. Final
versio
Weak Field Black Hole Formation in Asymptotically AdS Spacetimes
We use the AdS/CFT correspondence to study the thermalization of a strongly
coupled conformal field theory that is forced out of its vacuum by a source
that couples to a marginal operator. The source is taken to be of small
amplitude and finite duration, but is otherwise an arbitrary function of time.
When the field theory lives on , the source sets up a
translationally invariant wave in the dual gravitational description. This wave
propagates radially inwards in space and collapses to form a black
brane. Outside its horizon the bulk spacetime for this collapse process may
systematically be constructed in an expansion in the amplitude of the source
function, and takes the Vaidya form at leading order in the source amplitude.
This solution is dual to a remarkably rapid and intriguingly scale dependent
thermalization process in the field theory. When the field theory lives on a
sphere the resultant wave either slowly scatters into a thermal gas (dual to a
glueball type phase in the boundary theory) or rapidly collapses into a black
hole (dual to a plasma type phase in the field theory) depending on the time
scale and amplitude of the source function. The transition between these two
behaviors is sharp and can be tuned to the Choptuik scaling solution in
.Comment: 50 pages + appendices, 6 figures, v2: Minor revisions, references
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