654 research outputs found
Integrable Supersymmetric Fluid Mechanics from Superstrings
Following the construction of a model for the planar supersymmetric Chaplygin
gas, supersymmetric fluid mechanics in (1+1)-dimensions is obtained from the
light-cone parametrized Nambu-Goto superstring in (2+1)-dimensions. The lineal
model is completely integrable and can be formulated neatly using Riemann
coordinates. Infinite towers of conserved charges and supercharges are
exhibited. They form irreducible representations of a dynamical (hidden)
SO(2,1) symmetry group.Comment: 11 pages, LaTex; typographical errors correcte
Effects of age differences in memory formation on neural mechanisms of consolidation and retrieval
Oscillatory mechanisms of successful memory formation in younger and older adults are related to structural integrity
Compactified N = 1 supersymmetric Yang-Mills theory on the lattice: continuity and the disappearance of the deconfinement transition
Dressing Up the Kink
Many quantum field theoretical models possess non-trivial solutions which are
stable for topological reasons. We construct a self-consistent example for a
self-interacting scalar field--the quantum (or dressed) kink--using a two
particle irreducible effective action in the Hartree approximation. This new
solution includes quantum fluctuations determined self-consistently and
nonperturbatively at the 1-loop resummed level and allowed to backreact on the
classical mean-field profile. This dressed kink is static under the familiar
Hartree equations for the time evolution of quantum fields. Because the quantum
fluctuation spectrum is lower lying in the presence of the defect, the quantum
kink has a lower rest energy than its classical counterpart. However its energy
is higher than well-known strict 1-loop results, where backreaction and
fluctuation self-interactions are omitted. We also show that the quantum kink
exists at finite temperature and that its profile broadens as temperature is
increased until it eventually disappears.Comment: 13 pages, latex, 3 eps figures; revised with yet additional
references, minor rewordin
The Center of Lifespan Psychology at the Max Planck Institute for Human Development : Conceptual agenda and illustration of research activities
The Distribution and Excitation of CH₃CN in a Solar Nebula Analog
Cometary studies suggest that the organic composition of the early Solar Nebula was rich in complex nitrile species such CH3CN. Recent ALMA detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas-phase measurements to cometary abundances first requires constraints on formation chemistry and distributions of these species. We present here the detection of seven spatially resolved transitions of CH3CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis, we find a disk-averaged column density of cm−2 and a rotational temperature of K. A radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the CH3CN emission originates from a layer at z/r ~ 0.3. Through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate CH3CN formation and that in situ disk chemistry is sufficient to explain the observed CH3CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a solar system cometary abundance of CH3CN relative to H2O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula
Vibrational contribution to the thermodynamics of nanosized precipitates: vacancy-copper clusters in bcc-Fe
Within the harmonic approximation, the effects of lattice vibration on the
thermodynamics of nano-sized coherent clusters in bcc-Fe consisting of
vacancies and/or copper are investigated. A combination of on-lattice simulated
annealing based on Metropolis Monte Carlo simulations and off-lattice
relaxation by Molecular Dynamics is applied to obtain the most stable cluster
configurations at T = 0 K. The most recent interatomic potential built within
the framework of the embedded atom method for the Fe-Cu system is used. The
vibrational part of the total free energy of defect clusters in bcc-Fe is
calculated using their phonon density of states. The total free energy of pure
bcc-Fe and fcc-Cu as well as the total formation free energy and the total
binding free energy of the vacancy-copper clusters are determined for finite
temperatures. Our results are compared with the available data from previous
investigations performed using empirical many-body interatomic potentials and
first-principle methods. For further applications in rate theory and object
kinetic Monte Carlo simulations, the vibrational effects evaluated in the
present study are included in the previously derived analytical fits based on
the classical capillary model.Comment: 6 figure
Locus coeruleus integrity is related to tau burden and memory loss in autosomal-dominant Alzheimer’s disease (Version posted online February 2, 2021)
A constrained, total-variation minimization algorithm for low-intensity X-ray CT
Purpose: We develop an iterative image-reconstruction algorithm for
application to low-intensity computed tomography (CT) projection data, which is
based on constrained, total-variation (TV) minimization. The algorithm design
focuses on recovering structure on length scales comparable to a detector-bin
width.
Method: Recovering the resolution on the scale of a detector bin, requires
that pixel size be much smaller than the bin width. The resulting image array
contains many more pixels than data, and this undersampling is overcome with a
combination of Fourier upsampling of each projection and the use of
constrained, TV-minimization, as suggested by compressive sensing. The
presented pseudo-code for solving constrained, TV-minimization is designed to
yield an accurate solution to this optimization problem within 100 iterations.
Results: The proposed image-reconstruction algorithm is applied to a
low-intensity scan of a rabbit with a thin wire, to test resolution. The
proposed algorithm is compared with filtered back-projection (FBP).
Conclusion: The algorithm may have some advantage over FBP in that the
resulting noise-level is lowered at equivalent contrast levels of the wire.Comment: This article has been submitted to "Medical Physics" on 9/13/201
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