100 research outputs found
Strings in Horizons, Dissipation and a Possible Interpretation of the Hagedorn Temperature
We consider the entanglement of closed bosonic strings intersecting the event
horizon of a Rindler spacetime and, by using some simplified (rather
semiclassical) arguments and some elements of the string field theory, we show
the existence of a critical temperature beyond which closed strings
\emph{cannot be in thermal equilibrium}. The order of magnitude of this
critical value coincides with the Hagedorn temperature, which suggests an
interpretation consistent with the fact of having a partition function which is
bad defined for temperatures higher than it. Possible implications of the
present approach on the microscopical structure of stretched horizons are also
pointed out.Comment: A detailed description of string boundary states in a Rindler horizon
was added, and their relation with the stretched horizon microscopic
structure was emphasized. References added. To appear in Eur. Phys. J.
On the Non-renormalization of the AdS Radius
We show that the relation between the 't Hooft coupling and the radius of AdS
is not renormalized at one-loop in the sigma model perturbation theory. We
prove this by computing the quantum effective action for the superstring on
AdS_5 x S^5 and showing that it does not receive any finite alpha' corrections.
We also show that the central charge of the interacting worldsheet conformal
field theory vanishes at one-loop.Comment: 13 pages, harvmac. v2: refs added, version to be published on JHE
Observer dependent D-brane for strings propagating in pp-wave time dependent background
We study type IIB superstring in the pp-wave time-dependent background, which
has a singularity at . We show that this background can provide a toy
model to study some ideas related to the stretched horizon paradigm and the
complementary principle of black holes. To this end, we construct a unitary
Bogoliubov generator which relates the asymptotically flat string Hilbert
space, defined at , to the finite time Hilbert space. For
asymptotically flat observers, the closed string vacuum close to the
singularity appears as a boundary state which is in fact a D-brane described in
the closed string channel. However, observers who go with the string towards to
the singularity see the original vacuum.Comment: 12 pages, revtex 4, added references, corrected mistake
XAS signatures of Am(III) adsorbed onto magnetite and maghemite
Trivalent americium was adsorbed on magnetite and maghemite under similar chemical conditions and the local environment probed by EXAFS spectroscopy. In both samples, partially hydrated Am(III) binds the surface but slightly different surface complexes were identified. On Fe3O4, Am(III) forms monomeric tridentate surface complexes similar to that reported for Pu(III) at the (111) surface. In contrast, the lower number of detected Fe atoms may suggest that Am(III) forms monomeric bidentate surface complexes on γ-Fe2O3. Alternatively, the lower Fe coordination number can also be due to the presence of vacancies in maghemite. XPS data imply very similar binding environments for Am at both Fe oxide surfaces
PP-Wave Light-Cone Free String Field Theory at Finite Temperature
In this paper, a real-time formulation of light-cone pp-wave string field
theory at finite temperature is presented. This is achieved by developing the
thermo field dynamics (TFD) formalism in a second quantized string scenario.
The equilibrirum thermodynamic quantities for a pp-wave ideal string gas are
derived directly from expectation values on the second quantized string thermal
vacuum. Also, we derive the real-time thermal pp-wave closed string propagator.
In the flat space limit it is shown that this propagator can be written in
terms of Theta functions, exactly as the zero temperature one. At the end, we
show how supestrings interactions can be introduced, making this approach
suitable to study the BMN dictionary at finite temperature.Comment: 27 pages, revtex
Quantum Current Algebra for the Superstring
The sigma model describing the dynamics of the superstring in the background can be constructed using the coset
. A basic set of operators in this two
dimensional conformal field theory is composed by the left invariant currents.
Since these currents are not (anti) holomorphic, their OPE's is not determined
by symmetry principles and its computation should be performed perturbatively.
Using the pure spinor sigma model for this background, we compute the one-loop
correction to these OPE's. We also compute the OPE's of the left invariant
currents with the energy momentum tensor at tree level and one loop.Comment: 28 pages, 12 figures, v2: typos corrected
Real-time biomechanical modeling of the liver using Machine Learning models trained on Finite Element Method simulations
[EN] The development of accurate real-time models of the biomechanical behavior of different organs and tissues still poses a challenge in the field of biomechanical engineering. In the case of the liver, specifically, such a model would constitute a great leap forward in the implementation of complex applications such as surgical simulators, computed-assisted surgery or guided tumor irradiation.
In this work, a relatively novel approach for developing such a model is presented. It consists in the use of a machine learning algorithm, which provides real-time inference, trained on tens of thousands of simulations of the biomechanical behavior of the liver carried out by the finite element method on more than 100 different liver geometries.
Considering a target accuracy threshold of 3 mm for the Euclidean Error, four different scenarios were modeled and assessed: a single liver with an arbitrary force applied (99.96% of samples within the accepted error range), a single liver with two simultaneous forces applied (99.84% samples in range), a single liver with different material properties and an arbitrary force applied (98.46% samples in range), and a much more general model capable of modeling the behavior of any liver with an arbitrary force applied (99.01% samples in range for the median liver).
The results show that the Machine Learning models perform extremely well on all the scenarios, managing to keep the Mean Euclidean Error under 1 mm in all cases. Furthermore, the proposed model achieves working frequencies above 100Hz on modest hardware (with frequencies above 1000Hz being easily achievable on more powerful GPUs) thus fulfilling the real-time requirements. These results constitute a remarkable improvement in this field and may involve a prompt implementation in clinical practice.This work has been funded by the Spanish Ministry of Economy and Competitiveness (MINECO) through research projects TIN2014-52033-R, also supported by European FEDER funds.Pellicer-Valero, OJ.; Rupérez Moreno, MJ.; Martinez-Sanchis, S.; Martín-Guerrero, JD. (2020). Real-time biomechanical modeling of the liver using Machine Learning models trained on Finite Element Method simulations. 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