6,999 research outputs found
Persistent Current of Free Electrons in the Plane
Predictions of Akkermans et al. are essentially changed when the Krein
spectral displacement operator is regularized by means of zeta function.
Instead of piecewise constant persistent current of free electrons on the plane
one has a current which varies linearly with the flux and is antisymmetric with
regard to all time preserving values of including . Different
self-adjoint extensions of the problem and role of the resonance are discussed.Comment: (Comment on "Relation between Persistent Currents and the Scattering
Matrix", Phys. Rev. Lett. {\bf 66}, 76 (1991)) plain latex, 4pp., IPNO/TH
94-2
Moduli Stabilization with Long Winding Strings
Stabilizing all of the modulus fields coming from compactifications of string
theory on internal manifolds is one of the outstanding challenges for string
cosmology. Here, in a simple example of toroidal compactification, we study the
dynamics of the moduli fields corresponding to the size and shape of the torus
along with the ambient flux and long strings winding both internal directions.
It is known that a string gas containing states with non-vanishing winding and
momentum number in one internal direction can stabilize the radius of this
internal circle to be at self-dual radius. We show that a gas of long strings
winding all internal directions can stabilize all moduli, except the dilaton
which is stabilized by hand, in this simple example.Comment: title changed, improved presentation; reference added. 18 pages, JHEP
styl
On graviton non-Gaussianities during inflation
We consider the most general three point function for gravitational waves
produced during a period of exactly de Sitter expansion. The de Sitter
isometries constrain the possible shapes to only three: two preserving parity
and one violating parity. These isometries imply that these correlation
functions should be conformal invariant. One of the shapes is produced by the
ordinary gravity action. The other shape is produced by a higher derivative
correction and could be as large as the gravity contribution. The parity
violating shape does not contribute to the bispectrum [1106.3228, 1108.0175],
even though it is present in the wavefunction. We also introduce a spinor
helicity formalism to describe de Sitter gravitational waves with circular
polarization. These results also apply to correlation functions in Anti-de
Sitter space. They also describe the general form of stress tensor correlation
functions, in momentum space, in a three dimensional conformal field theory.
Here all three shapes can arise, including the parity violating one.Comment: 51 pages, v2: Corrected statement about parity violation in the
gravitational wave bispectrum. Some other changes and references adde
Torsion Constraints in the Randall--Sundrum Scenario
Torsion appears due to fermions coupled to gravity and leads to the strongest
particle physics bounds on flat extra dimensions. In this work, we consider
torsion constraints in the case of a warped extra dimension with brane and bulk
fermions. From current data we obtain a 3-sigma bound on the TeV--brane mass
scale scale \Lambda_\pi > 2.2 (10) TeV for the AdS curvature k=1 (0.01) in
(reduced) Planck units. If Dirac or light sterile neutrinos reside on the
brane, the bound increases to 17 (78) TeV.Comment: typos corrected, matches the Phys. Rev. D versio
Neuromagnetic activation and oscillatory dynamics of stimulus-locked processing during naturalistic viewing
Naturalistic stimuli such as watching a movie while in the scanner provide an ecologically valid paradigm that has the potential of extracting valuable information on how the brain processes complex stimuli in realistic visual and auditory contexts. Naturalistic viewing is also easier to conduct with challenging participant groups including patients and children. Given the high temporal resolution of MEG, in the present study, we demonstrate how a short movie clip can be used to map distinguishable activation and connectivity dynamics underlying the processing of specific classes of visual stimuli such as face and hand manipulations, as well as contrasting activation dynamics for auditory words and non-words. MEG data were collected from 22 healthy volunteers (6 females, 3 left handed, mean age – 27.7 ± 5.28 years) during the presentation of naturalistic audiovisual stimuli. The MEG data were split into trials with the onset of the stimuli belonging to classes of interest (words, non-words, faces, hand manipulations). Based on the components of the averaged sensor ERFs time-locked to the visual and auditory stimulus onset, four and three time-windows, respectively, were defined to explore brain activation dynamics. Pseudo-Z, defined as the ratio of the source-projected time-locked power to the projected noise power for each vertex, was computed and used as a proxy of time-locked brain activation. Statistical testing using the mean-centered Partial Least Squares analysis indicated periods where a given visual or auditory stimuli had higher activation. Based on peak pseudo-Z differences between the visual conditions, time-frequency resolved analyses were performed to assess beta band desynchronization in motor-related areas, and inter-trial phase synchronization between face processing areas. Our results provide the first evidence that activation and connectivity dynamics in canonical brain regions associated with the processing of particular classes of visual and auditory stimuli can be reliably mapped using MEG during presentation of naturalistic stimuli. Given the strength of MEG for brain mapping in temporal and frequency domains, the use of naturalistic stimuli may open new techniques in analyzing brain dynamics during ecologically valid sensation and perception
A Crucial Test for Color-Octet Production Mechanism in Z^0 Decays
The direct production rates of -wave charmonia in the decays of is
evaluated. The color-octet production processes are shown to have distinctively large branching ratios, the same order
of magnitude as that of prodution, as compared with other -wave
charmonium production mechanisms. This may suggest a crucial channel to test
the color-octet mechanism as well as to observe the -wave charmonium states
in decays. In addition, a signal for the charmonium as strong as
or with large transverse momentum at the Tevatron should
also be observed.Comment: 14 pages in LaTex (3 figures in PS-file
The Cosmology of Massless String Modes
We consider the spacetime dynamics of a gas of closed strings in the context
of General Relativity in a background of arbitrary spatial dimensions. Our
motivation is primarily late time String Gas Cosmology, where such a spacetime
picture has to emerge after the dilaton has stabilized. We find that after
accounting for the thermodynamics of a gas of strings, only string modes which
are massless at the self-dual radius are relevant, and that they lead to a
dynamics which is qualitatively different from that induced by the modes
usually considered in the literature. In the context of an ansatz with three
large spatial dimensions and an arbitrary number of small extra dimensions, we
obtain isotropic stabilization of these extra dimensions at the self-dual
radius. This stabilization occurs for fixed dilaton, and is induced by the
special string states we focus on. The three large dimensions undergo a regular
Friedmann-Robertson-Walker expansion. We also show that this framework for
late-time cosmology is consistent with observational bounds.Comment: 15 pages, no figures, references added (again
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