569 research outputs found
Stabilization of Sub-Millimeter Dimensions: The New Guise of the Hierarchy Problem
A new framework for solving the hierarchy problem was recently proposed which
does not rely on low energy supersymmetry or technicolor. The fundamental
Planck mass is at a \tev and the observed weakness of gravity at long
distances is due the existence of new sub-millimeter spatial dimensions. In
this picture the standard model fields are localized to a -dimensional
wall or ``3-brane''. The hierarchy problem becomes isomorphic to the problem of
the largeness of the extra dimensions. This is in turn inextricably linked to
the cosmological constant problem, suggesting the possibility of a common
solution. The radii of the extra dimensions must be prevented from both
expanding to too great a size, and collapsing to the fundamental Planck length
\tev^{-1}. In this paper we propose a number of mechanisms addressing this
question. We argue that a positive bulk cosmological constant can
stabilize the internal manifold against expansion, and that the value of
is not unstable to radiative corrections provided that the
supersymmetries of string theory are broken by dynamics on our 3-brane. We
further argue that the extra dimensions can be stabilized against collapse in a
phenomenologically successful way by either of two methods: 1) Large,
topologically conserved quantum numbers associated with higher-form bulk U(1)
gauge fields, such as the naturally occurring Ramond-Ramond gauge fields, or
the winding number of bulk scalar fields. 2) The brane-lattice-crystallization
of a large number of 3-branes in the bulk. These mechanisms are consistent with
theoretical, laboratory, and cosmological considerations such as the absence of
large time variations in Newton's constant during and after primordial
nucleosynthesis, and millimeter-scale tests of gravity.Comment: Corrected referencing to important earlier work by Sundrum, errors
fixed, additional discussion on radion phenomenology, conclusions unchanged,
23 pages, LaTe
Spatial and frequency domain effects of defects in 1D photonic crystal
The aim of this paper is to present the analysis of influence of defects in
1D photonic crystal (PC) on the density of states and simultaneously
spontaneous emission, in both spatial and frequency domains. In our
investigations we use an analytic model of 1D PC with defects. Our analysis
reveals how presence of a defect causes a defect mode to appear. We show that a
defect in 1D PC has local character, being negligible in regions of PC situated
far from the defected elementary cell. We also analyze the effect of multiple
defects, which lead to photonic band gap splitting.Comment: presented at International Workshop on Physics of Photonic Crystals
and Metamaterials, Brussels, Belgium, 12-13.06.200
Heart period variability during vagal nerve stimulation
Vagal nerve stimulation is an emerging therapy for epilepsy, yet little is known regarding the effects of this stimulation on heart period variability. We selected 10 patients (two female, eight male) who were receiving high-frequency, high-intensity left vagal nerve stimulation for intractable epilepsy. Electrocardiogram data were recorded for a 7 min baseline, 2.5 min of stimulation and a 7 min post-stimulation period. We found no significant changes in average heart period, instantaneous changes of successive R-to-R intervals greater than 50 ms or fractal dimension. We also found no significant changes in the total power in the 0.0-0.04 Hz, 0.04-0.12 Hz and 0.2-0.4 Hz bands with stimulation of the left vagus nerve. This study suggests that left vagal nerve stimulation has little acute effect on the cardiac rhythm or heart period variability
Neutrino Masses from Large Extra Dimensions
Recently it was proposed that the standard model (SM) degrees of freedom
reside on a -dimensional wall or ``3-brane'' embedded in a
higher-dimensional spacetime. Furthermore, in this picture it is possible for
the fundamental Planck mass \mst to be as small as the weak scale \mst\simeq
O(\tev) and the observed weakness of gravity at long distances is due the
existence of new sub-millimeter spatial dimensions. We show that in this
picture it is natural to expect neutrino masses to occur in the 10^{-1} -
10^{-4}\ev range, despite the lack of any fundamental scale higher than
\mst. Such suppressed neutrino masses are not the result of a see-saw, but
have intrinsically higher-dimensional explanations. We explore two
possibilities. The first mechanism identifies any massless bulk fermions as
right-handed neutrinos. These give naturally small Dirac masses for the same
reason that gravity is weak at long distances in this framework. The second
mechanism takes advantage of the large {\it infrared} desert: the space in the
extra dimensions. Here, small Majorana neutrino masses are generated by
breaking lepton number on distant branes.Comment: 17 pages, late
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Cosmology of Brane Models with Radion Stabilization
We analyze the cosmology of the Randall-Sundrum model and that of compact
brane models in general in the presence of a radius stabilization mechanism. We
find that the expansion of our universe is generically in agreement with the
expected effective four dimensional description. The constraint (which is
responsible for the appearance of non-conventional cosmologies in these models)
that must be imposed on the matter densities on the two branes in the theory
without a stabilized radius is a consequence of requiring a static solution
even in the absence of stabilization. Such constraints disappear in the
presence of a stablizing potential, and the ordinary FRW
(Friedmann-Robertson-Walker) equations are reproduced, with the expansion
driven by the sum of the physical values of the energy densities on the two
branes and in the bulk. For the case of the Randall-Sundrum model we examine
the kinematics of the radion field, and find that corrections to the standard
FRW equations are small for temperatures below the weak scale. We find that the
radion field has renormalizable and unsuppressed couplings to Standard Model
particles after electroweak symmetry breaking. These couplings may have
important implications for collider searches. We comment on the possibility
that matter off the TeV brane could serve as a dark matter candidate.Comment: 35 pages, Late
Bottom-Up and Top-Down Processes in Emotion Generation: Common and Distinct Neural Mechanisms
Emotions are generally thought to arise through the interaction of bottom-up and top-down processes. However, prior work has not delineated their relative contributions. In a sample of 20 females, we used functional magnetic resonance imaging to compare the neural correlates of negative emotions generated by the bottom-up perception of aversive images and by the top-down interpretation of neutral images as aversive. We found that (a) both types of responses activated the amygdala, although bottom-up responses did so more strongly; (b) bottom-up responses activated systems for attending to and encoding perceptual and affective stimulus properties, whereas top-down responses activated prefrontal regions that represent high-level cognitive interpretations; and (c) self-reported affect correlated with activity in the amygdala during bottom-up responding and with activity in the medial prefrontal cortex during top-down responding. These findings provide a neural foundation for emotion theories that posit multiple kinds of appraisal processes and help to clarify mechanisms underlying clinically relevant forms of emotion dysregulation.National Institutes of Health (U.S.) (Grant MH58147)National Institutes of Health (U.S.) (Grant MH076137
Mesoscopic models for DNA stretching under force: new results and comparison to experiments
Single molecule experiments on B-DNA stretching have revealed one or two
structural transitions, when increasing the external force. They are
characterized by a sudden increase of DNA contour length and a decrease of the
bending rigidity. It has been proposed that the first transition, at forces of
60--80 pN, is a transition from B to S-DNA, viewed as a stretched duplex DNA,
while the second one, at stronger forces, is a strand peeling resulting in
single stranded DNAs (ssDNA), similar to thermal denaturation. But due to
experimental conditions these two transitions can overlap, for instance for
poly(dA-dT). We derive analytical formula using a coupled discrete worm like
chain-Ising model. Our model takes into account bending rigidity, discreteness
of the chain, linear and non-linear (for ssDNA) bond stretching. In the limit
of zero force, this model simplifies into a coupled model already developed by
us for studying thermal DNA melting, establishing a connexion with previous
fitting parameter values for denaturation profiles. We find that: (i) ssDNA is
fitted, using an analytical formula, over a nanoNewton range with only three
free parameters, the contour length, the bending modulus and the monomer size;
(ii) a surprisingly good fit on this force range is possible only by choosing a
monomer size of 0.2 nm, almost 4 times smaller than the ssDNA nucleobase
length; (iii) mesoscopic models are not able to fit B to ssDNA (or S to ss)
transitions; (iv) an analytical formula for fitting B to S transitions is
derived in the strong force approximation and for long DNAs, which is in
excellent agreement with exact transfer matrix calculations; (v) this formula
fits perfectly well poly(dG-dC) and -DNA force-extension curves with
consistent parameter values; (vi) a coherent picture, where S to ssDNA
transitions are much more sensitive to base-pair sequence than the B to S one,
emerges.Comment: 14 pages, 9 figure
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