9,034 research outputs found
THE SENSITIVITY TO NEW PHYSICS OF A LEP SCAN IN 1995
We study the implications of possible off-peak measurements in the 1995 LEP
run, in regard to probing physics beyond the Standard Model. To do so, we
determine the accuracy with which various nonstandard couplings can be expected
to be measured in the three different scan scenarios recently discussed by
Clarke and Wyatt. We find that each scan scenario allows greater sensitivity to
a different set of new physics couplings. Oblique parameters are best measured
with the longest scan, while nonstandard fermion couplings to the Z tend to be
better constrained (albeit only marginally) if all of the 1995 LEP measurements
are taken on the Z peak.Comment: Plain TeX, 9 pages, no figures. We have streamlined our presentation
by omitting observables of our Class B. All else is completely unchanged
Connection behaviour and the robustness of steel-framed structures in fire
The full-scale fire tests at Cardington in the 1990s, and the collapse of at least one of the WTC buildings in 2001, illustrated that connections are potentially the most vulnerable parts of a structure in fire. Fracture of connections causes structural discontinuities and reduces the robustness provided by alternative load paths. An understanding of connection performance is essential to the assessment of structural robustness, and so to structural design against progressive collapse. The forces and deformations to which connectionscan be subjected during a fire differ significantly from those assumed in general design. The internal forces i generally start with moment and shear at ambient temperature, then superposing compression in the initial stages of a fire, which finally changes to catenary tension at high temperatures. If a connection does not have sufficient resistance or ductility to accommodate simultaneous large rotations and normal forces, then connections may fracture, leading to extensive damage or progressive collapse of the structure. Practical assessment of the robustness of steel connections in fire will inevitably rely largely on numerical modelling, but this is unlikely to include general-purpose finite element modelling, because of the complexity of such models. The most promising alternative is the component method, a practical approach which can be included within global three-dimensional frame analysis. The connection is represented by an assembly of individual components with known mechanical properties. Component characterization must include high-deflection elevated-temperature behaviour, and represent it up to fracture.In reality a connection may either be able to regain its stability after the initial fracture of one (or a few) components, or the first failure may trigger a cascade of failures of other components, leading to complete detachment of the supported member. Numerical modelling must be capable of predicting the sequence of failures of components, rather than considering the first loss of stability as signifying building failure. It is necessary to use a dynamic analysis, so that loss of stability and re-stabilization can be tracked, includingthe movements of disengaging members and the loadsharing mechanisms which maintain integrity and stability within the remaining structure, until total collapse occurs
Comment on "Wandering minds: The default network and stimulus-independent thought"
Mason et al. (Reports, 19 January 2007, p. 393) attributed activity in certain regions of the "resting" brain to the occurrence of mind-wandering. However, previous research has demonstrated the difficulty of distinguishing this type of stimulus-independent thought from stimulus-oriented thought (e.g., watchfulness). Consideration of both possibilities is required to resolve this ambiguity
Modulated Reheating and Large Non-Gaussianity in String Cosmology
A generic feature of the known string inflationary models is that the same
physics that makes the inflaton lighter than the Hubble scale during inflation
often also makes other scalars this light. These scalars can acquire
isocurvature fluctuations during inflation, and given that their VEVs determine
the mass spectrum and the coupling constants of the effective low-energy field
theory, these fluctuations give rise to couplings and masses that are modulated
from one Hubble patch to another. These seem just what is required to obtain
primordial adiabatic fluctuations through conversion into density perturbations
through the `modulation mechanism', wherein reheating takes place with
different efficiency in different regions of our Universe. Fluctuations
generated in this way can generically produce non-gaussianity larger than
obtained in single-field slow-roll inflation; potentially observable in the
near future. We provide here the first explicit example of the modulation
mechanism at work in string cosmology, within the framework of LARGE Volume
Type-IIB string flux compactifications. The inflationary dynamics involves two
light Kaehler moduli: a fibre divisor plays the role of the inflaton whose
decay rate to visible sector degrees of freedom is modulated by the primordial
fluctuations of a blow-up mode (which is made light by the use of
poly-instanton corrections). We find the challenges of embedding the mechanism
into a concrete UV completion constrains the properties of the non-gaussianity
that is found, since for generic values of the underlying parameters, the model
predicts a local bi-spectrum with fNL of order `a few'. However, a moderate
tuning of the parameters gives also rise to explicit examples with fNL O(20)
potentially observable by the Planck satellite.Comment: 42 pages, 2 figure
Coriolis force corrections to g-mode spectrum in 1D MHD model
The corrections to g-mode frequencies caused by the presence of a central
magnetic field and rotation of the Sun are calculated. The calculations are
carried out in the simple one dimensional magnetohydrodynamical model using the
approximations which allow one to find the purely analytical spectra of
magneto-gravity waves beyond the scope of the JWKB approximation and avoid in a
small background magnetic field the appearance of the cusp resonance which
locks a wave within the radiative zone. These analytic results are compared
with the satellite observations of the g-mode frequency shifts which are of the
order one per cent as given in the GOLF experiment at the SoHO board. The main
contribution turns out to be the magnetic frequency shift in the strong
magnetic field which obeys the used approximations. In particular, the fixed
magnetic field strength 700 KG results in the mentioned value of the frequency
shift for the g-mode of the radial order n=-10. The rotational shift due to the
Coriolis force appears to be small and does not exceed a fracton of per cent,
\alpha_\Omega < 0.003.Comment: RevTeX4, 9 pages, 4 eps figures; accepted for publication in
Astronomy Reports (Astronomicheskii Zhurnal
Sculpting the Extra Dimensions: Inflation from Codimension-2 Brane Back-reaction
We construct an inflationary model in 6D supergravity that is based on
explicit time-dependent solutions to the full higher-dimensional field
equations, back-reacting to the presence of a 4D inflaton rolling on a
space-filling codimension-2 source brane. Fluxes in the bulk stabilize all
moduli except the `breathing' modulus (that is generically present in
higher-dimensional supergravities). Back-reaction to the inflaton roll causes
the 4D Einstein-frame on-brane geometry to expand, a(t) ~ t^p, as well as
exciting the breathing mode and causing the two off-brane dimensions to expand,
r(t) ~ t^q. The model evades the general no-go theorems precluding 4D de Sitter
solutions, since adjustments to the brane-localized inflaton potential allow
the power p to be dialed to be arbitrarily large, with the 4D geometry becoming
de Sitter in the limit p -> infinity (in which case q = 0). Slow-roll solutions
give accelerated expansion with p large but finite, and q = 1/2. Because the
extra dimensions expand during inflation, the present-day 6D gravity scale can
be much smaller than it was when primordial fluctuations were generated -
potentially allowing TeV gravity now to be consistent with the much higher
gravity scale required at horizon-exit for observable primordial gravity waves.
Because p >> q, the 4 on-brane dimensions expand more quickly than the 2
off-brane ones, providing a framework for understanding why the observed four
dimensions are presently so much larger than the internal two. If uplifted to a
10D framework with 4 dimensions stabilized, the 6D evolution described here
could describe how two of the six extra dimensions evolve to become much larger
than the others, as a consequence of the enormous expansion of the 4 large
dimensions we can see.Comment: 27 pages + appendices, 2 figure
Consequences of Zeeman Degeneracy for van der Waals Blockade between Rydberg Atoms
We analyze the effects of Zeeman degeneracies on the long-range interactions
between like Rydberg atoms, with particular emphasis on applications to quantum
information processing using van der Waals blockade. We present a general
analysis of how degeneracies affect the primary error sources in blockade
experiments, emphasizing that blockade errors are sensitive primarily to the
weakest possible atom-atom interactions between the degenerate states, not the
mean interaction strength. We present explicit calculations of the van der
Waals potentials in the limit where the fine-structure interaction is large
compared to the atom-atom interactions. The results are presented for all
potential angular momentum channels invoving s, p, and d states. For most
channels there are one or more combinations of Zeeman levels that have
extremely small dipole-dipole interactions and are therefore poor candidates
for effective blockade experiments. Channels with promising properties are
identified and discussed. We also present numerical calculations of Rb and Cs
dipole matrix elements and relevant energy levels using quantum defect theory,
allowing for convenient quantitative estimates of the van der Waals
interactions to be made for principal quantum numbers up to 100. Finally, we
combine the blockade and van der Waals results to quantitatively analyze the
angular distribution of the blockade shift and its consequence for angular
momentum channels and geometries of particular interest for blockade
experiments with Rb.Comment: 16 figure
A self-tuning mechanism in (3+p)d gravity-scalar theory
We present a new type of self-tuning mechanism for ()d brane world
models in the framework of gravity-scalar theory. This new type of self-tuning
mechanism exhibits a remarkable feature. In the limit , being
the string coupling, the geometry of bulk spacetime remains virtually unchanged
by an introduction of the Standard Model(SM)-brane, and consequently it is
virtually unaffected by quantum fluctuations of SM fields with support on the
SM-brane. Such a feature can be obtained by introducing Neveu-Schwarz(NS)-brane
as a background brane on which our SM-brane is to be set. Indeed, field
equations naturally suggest the existence of the background NS-brane. Among the
given such models, of the most interest is the case with , where
represents the bulk cosmological constant. This model contains a pair
of coincident branes (of the SM- and the NS-branes), one of which is a
codimension-2 brane placed at the origin of 2d transverse space (), another a codimension-1 brane placed at the edge of .
These two branes are (anti) T-duals of each other, and one of them may be
identified as our SM-brane plus the background NS-brane. In the presence of the
background NS-brane (and in the absence of ), the 2d transverse space
becomes an orbifold with an appropriate deficit angle.
But this is only possible if the ()d Planck scale and the string
scale () are of the same order, which
accords with the hierarchy assumption \cite{1,2,3} that the electroweak scale
is the only short distance scale existing in nature
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