155 research outputs found
D-Brane Recoil and Supersymmetry Breaking as a Relaxation Process
We propose a new mechanism for the formation of conical singularities on
D-branes by means of recoil resulting from scattering of closed string states
propagating in the (large) transverse dimensions. By viewing the (spatial part
of the) four-dimensional world as a 3-brane with large transverse dimensions
the above mechanism can lead to supersymmetry obstruction at the TeV scale. The
vacuum remains supersymmetric while the mass spectrum picks up a supersymmetry
obstructing mass splitting. The state with ``broken'' supersymmetry is not an
equilibrium ground state, but is rather an excited state of the D-brane which
relaxes to the supersymmetric ground state asymptotically in (cosmic) time.Comment: 9 pages revtex, uses axodraw style (Arguments clarified, citations
added; no change in conclusions.
A direct route to cyclic organic nanostructures via ring-expansion metathesis polymerization of a dendronized macromonomer
Cyclic organic nanostructures were prepared via ring-expansion metathesis polymerization of a dendronized norbornene macromonomer. The strategy provides a direct, efficient route to nanoscale rings in a single operation. AFM imaging confirmed toroidal features having diameters of ca. 35â40 nm
Cortical and subcortical alterations associated with precision visuomotor behavior in individuals with autism spectrum disorder
In addition to core deficits in social-communication abilities and repetitive behaviors and interests, many 2 patients with autism spectrum disorder (ASD) experience developmental comorbidities, including 3 sensorimotor issues. Sensorimotor issues are common in ASD and associated with more severe clinical 4 symptoms. Importantly, sensorimotor behaviors are precisely quantifiable and highly translational, 5 offering promising targets for neurophysiological studies of ASD. We used functional MRI to identify 6 brain regions associated with sensorimotor behavior using a visually-guided precision gripping task in 7 individuals with ASD (N=20) and age-, IQ-, and handedness-matched controls (N=18). During 8 visuomotor behavior, individuals with ASD showed greater force variability than controls. BOLD signal 9 for multiple cortical and subcortical regions was associated with force variability, including motor and 10 premotor cortex, posterior parietal cortex, extrastriate cortex, putamen, and cerebellum. Activation in 11 right premotor cortex scaled with sensorimotor variability in controls, but not in ASD. Individuals with 12 ASD showed greater activation than controls in left putamen and left cerebellar lobule VIIb and activation 13 in these regions was associated with more severe clinically-rated symptoms of ASD. Together, these 14 results suggest that greater sensorimotor variability in ASD is associated with altered cortical-striatal 15 processes supporting action selection and cortical-cerebellar circuits involved in feedback-guided reactive 16 adjustments of motor output. Our findings also indicate that atypical organization of visuomotor cortical 17 circuits may result in heightened reliance on subcortical circuits typically dedicated to motor skill 18 acquisition. Overall, these results provide new evidence that sensorimotor alterations in ASD involve 19 aberrant cortical and subcortical organization that may contribute to key clinical issues in patients. 20
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New and noteworthy: This is the first known study to examine functional brain activation during 22 precision visuomotor behavior in autism spectrum disorder (ASD). We replicate previous findings of 23 elevated force variability in ASD and find these deficits are associated with atypical function of ventral 24 premotor cortex, putamen, and posterolateral cerebellum, indicating cortical-striatal processes supporting 25 action selection and cortical-cerebellar circuits involved in feedback-guided reactive adjustments of motor 26 output may be key targets for understanding the neurobiology of ASD.NICHD 055751NIMH R01 12743-01NCATS TL1 TR002368,Kansas Center for Autism Research and Training (K-CART) Research Investment Council Strategic Initiative Gran
Uncertainty Analysis of Experimental Discharge Coefficients in Additively Manufactured Liquid Injector Elements
Screening of two additively manufactured liquid injector designs was conducted in the UAH high pressure spray facility. Four variants of each geometry with slightly different dimensions were obtained from eleven separate commercial additive manufacturing services. The devices were manufactured from Inconel 625 using the selective laser melting (SLM) powder bed process. The devices were cold flowed with water over a range of relevant pressure drops (75 psi to 1500 psi) to produce water flow rates from 0.037 to 1.75 lbm/s into ambient back pressure. Discharge coefficients determined from the testing along with the associated uncertainties provide insight into characteristic flow performance variabilities that can be expected from the SLM process for similar geometries
D-Brane Recoil and Supersymmetry Obstruction
We discuss a model in which our universe is pictured as a recoiling Dirichlet
brane: we find that a proper treatment of the recoil leads naturally to
supersymmetry obstruction on the four-dimensional world. An essential feature
of our approach is the fact that the underlying worldsheet sigma model is
non-critical, and the Liouville mode plays the role of the target time. Also,
the extra bulk dimensions are viewed as sigma model couplings, and as such have
to be averaged by appropriate summation over worldsheet genera. The recoiling
brane is in an excited state rather than its ground state, to which it relaxes
asymptotically in time, restoring supersymmetry. We also find that the
excitation energy, which is considered as the observable effective cosmological
`constant' on the brane, is naturally small and can accommodate upper bounds
from observations.Comment: 9 pages, no figure
Schumacher's quantum data compression as a quantum computation
An explicit algorithm for performing Schumacher's noiseless compression of
quantum bits is given. This algorithm is based on a combinatorial expression
for a particular bijection among binary strings. The algorithm, which adheres
to the rules of reversible programming, is expressed in a high-level pseudocode
language. It is implemented using two- and three-bit primitive
reversible operations, where is the length of the qubit strings to be
compressed. Also, the algorithm makes use of auxiliary qubits; however,
space-saving techniques based on those proposed by Bennett are developed which
reduce this workspace to while increasing the running time by
less than a factor of two.Comment: 37 pages, no figure
A Liquid Model Analogue for Black Hole Thermodynamics
We are able to characterize a 2--dimensional classical fluid sharing some of
the same thermodynamic state functions as the Schwarzschild black hole. This
phenomenological correspondence between black holes and fluids is established
by means of the model liquid's pair-correlation function and the two-body
atomic interaction potential. These latter two functions are calculated exactly
in terms of the black hole internal (quasilocal) energy and the isothermal
compressibility. We find the existence of a ``screening" like effect for the
components of the liquid.Comment: 20 pages and 6 Encapsulated PostScript figure
Optomechanical Cooling of a Macroscopic Oscillator by Homodyne Feedback
We propose a simple optomechanical model in which a mechanical oscillator
quadrature could be "cooled" well below its equilibrium temperature by applying
a suitable feedback to drive the orthogonal quadrature by means of the homodyne
current of the radiation field used to probe its position.Comment: 9 pages, RevTeX, Figures available from authors, to appear in Phys.
Rev. Let
The Forum: Fall 2006/Spring 2007
Fall 2006 / Spring 2007 journal of the Honors Program at the University of North Dakota. The issue includes stories, poems, essays and art by undergraduate students.https://commons.und.edu/und-books/1060/thumbnail.jp
Exact Absorption Probability in the Extremal Six-Dimensional Dyonic String Background
We show that the minimally coupled massless scalar wave equation in the
background of an six-dimensional extremal dyonic string (or D1-D5 brane
intersection) is exactly solvable, in terms of Mathieu functions. Using this
fact, we calculate absorption probabilities for these scalar waves, and present
the explicit results for the first few low energy corrections to the
leading-order expressions. For a specific tuning of the dyonic charges one can
reach a domain where the low energy absorption probability goes to zero with
inverse powers of the logarithm of the energy. This is a dividing domain
between the regime where the low energy absorption probability approaches zero
with positive powers of energy and the regime where the probability is an
oscillatory function of the logarithm of the energy. By the conjectured AdS/CFT
correspondence, these results shed novel light on the strongly coupled
two-dimensional field theory away from its infrared conformally invariant fixed
point (the strongly coupled ``non-critical'' string).Comment: Latex (3 times), 23 page
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