2,255 research outputs found
"Barber pole turbulence" in large aspect ratio Taylor-Couette flow
Investigations of counter-rotating Taylor-Couette flow (TCF) in the narrow
gap limit are conducted in a very large aspect ratio apparatus. The phase
diagram is presented and compared to that obtained by Andereck et al. The
spiral turbulence regime is studied by varying both internal and external
Reynolds numbers. Spiral turbulence is shown to emerge from the fully turbulent
regime via a continuous transition appearing first as a modulated turbulent
state, which eventually relaxes locally to the laminar flow. The connection
with the intermittent regimes of the plane Couette flow (pCf) is discussed
On the one-loop Kahler potential in five-dimensional brane-world supergravity
We present an on-shell formulation of 5d gauged supergravity coupled to
chiral matter multiplets localized at the orbifold fixed points. The brane
action is constructed via the Noether method. In such set-up we compute
one-loop corrections to the Kahler potential of the effective 4d supergravity
and compare the result with previous computations based on the off-shell
formalism. The results agree at lowest order in brane sources, however at
higher order there are differences. We explain this discrepancy by an ambiguity
in resolving singularities associated with the presence of infinitely thin
branes.Comment: 20 page
Combinatorial screening yields discovery of 29 metal oxide photoanodes for solar fuel generation
Combinatorial synthesis combined with high throughput electrochemistry enabled discovery of 29 ternary oxide photoanodes, 15 with visible light response for oxygen evolution. YâFeâ
Oââ and trigonal VâCoOâ emerge as particularly promising candidates due to their photorepsonse at sub-2.4 eV illumination
Adaptive latitudinal variation in Common Blackbird Turdus merula nest characteristics
Nest construction is taxonomically widespread, yet our understanding of adaptive
intraspecific variation in nest design remains poor. Nest characteristics are
expected to vary adaptively in response to predictable variation in spring temperatures
over large spatial scales, yet such variation in nest design remains largely
overlooked, particularly amongst open-cup-nesting birds. Here, we systematically
examined the effects of latitudinal variation in spring temperatures and precipitation
on the morphology, volume, composition, and insulatory properties of
open-cup-nesting Common Blackbirdsâ Turdus merula nests to test the hypothesis
that birds living in cooler environments at more northerly latitudes would build
better insulated nests than conspecifics living in warmer environments at more
southerly latitudes. As spring temperatures increased with decreasing latitude, the
external diameter of nests decreased. However, as nest wall thickness also
decreased, there was no variation in the diameter of the internal nest cups. Only
the mass of dry grasses within nests decreased with warmer temperatures at lower
latitudes. The insulatory properties of nests declined with warmer temperatures at
lower latitudes and nests containing greater amounts of dry grasses had higher insulatory
properties. The insulatory properties of nests decreased with warmer temperatures
at lower latitudes, via changes in morphology (wall thickness) and
composition (dry grasses). Meanwhile, spring precipitation did not vary with latitude,
and none of the nest characteristics varied with spring precipitation. This suggests
that Common Blackbirds nesting at higher latitudes were building nests with
thicker walls in order to counteract the cooler temperatures. We have provided evidence
that the nest construction behavior of open-cup-nesting birds systematically
varies in response to large-scale spatial variation in spring temperatures
Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?
All-organic-based photovoltaic solar cells have attracted considerable
attention because of their low-cost processing and short energy payback time.
In such systems the primary dissociation of an optical excitation into a pair
of photocarriers has been recently shown to be extremely rapid and efficient,
but the physical reason for this remains unclear. Here, two-dimensional
photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy,
is used to probe the ultrafast coherent decay of photoexcitations into
charge-producing states in a polymer:fullerene based solar cell. The
two-dimensional photocurrent spectra are interpreted by introducing a
theoretical model for the description of the coupling of the electronic states
of the system to an external environment and to the applied laser fields. The
experimental data show no cross-peaks in the two-dimensional photocurrent
spectra, as predicted by the model for coherence times between the exciton and
the photocurrent producing states of 20\,fs or less
Vacuum Stability in Split Susy and Little Higgs Models
We study the stability of the effective higgs potential in the split
supersymmetry and Little Higgs models. In particular, we study the effects of
higher dimensional operators in the effective potential on the higgs mass
predictions. We find that the size and sign of the higher dimensional operators
can significantly change the higgs mass required to maintain vacuum stability
in Split Susy models. In the Little Higgs models the effects of higher
dimensional operators can be large because of a relatively lower cut-off scale.
Working with a specific model we find that a contribution from the higher
dimensional operator with coefficient of O(1) can destabilize the vacuum.Comment: Latex 22 pages, 3 figures. Added discussion, published versio
Little Hierarchy, Little Higgses, and a Little Symmetry
Little Higgs theories are an attempt to address the little hierarchy problem,
i.e., the tension between the naturalness of the electroweak scale and the
precision measurements showing no evidence for new physics up to 5-10 TeV. In
little Higgs theories, the Higgs mass-squareds are protected to the one-loop
order from the quadratic divergence. This allows the cutoff to be raised up to
\~10 TeV, beyond the scales probed by the precision data. However, strong
constraints can still arise from the contributions of the new TeV scale
particles and hence re-introduces the fine-tuning problem. In this paper we
show that a new symmetry, denoted as T-parity, under which all heavy gauge
bosons and scalar triplets are odd, can remove all the tree-level contributions
to the electroweak observables and therefore makes the little Higgs theories
completely natural. The T-parity can be manifestly implemented in a majority of
little Higgs models by following the most general construction of the low
energy effective theory a la Callan, Coleman, Wess and Zumino. In particular,
we discuss in detail how to implement the T-parity in the littlest Higgs model
based on SU(5)/SO(5). The symmetry breaking scale f can be even lower than 500
GeV if the contributions from the unknown UV physics at the cutoff are somewhat
small. The existence of -parity has drastic impacts on the phenomenology of
the little Higgs theories. The T-odd particles need to be pair-produced and
will cascade down to the lightest T-odd particle (LTP) which is stable. A
neutral LTP gives rise to missing energy signals at the colliders which can
mimic supersymmetry. It can also serve as a good dark matter candidate.Comment: 20 pages, 2 figures, RevTeX; v2: Yukawa sector in the SU(5)/SO(5)
model slightly modified. Also added comments on the Dirac mass term for the
fermionic doublet partner; v3: clarifying comments on the modified Yukawa
sector. version to appear on JHE
Precision Electroweak Observables in the Minimal Moose Little Higgs Model
Little Higgs theories, in which the Higgs particle is realized as the
pseudo-Goldstone boson of an approximate global chiral symmetry have generated
much interest as possible alternatives to weak scale supersymmetry. In this
paper we analyze precision electroweak observables in the Minimal Moose model
and find that in order to be consistent with current experimental bounds, the
gauge structure of this theory needs to be modified. We then look for viable
regions of parameter space in the modified theory by calculating the various
contributions to the S and T parameters.Comment: v2: 17 pages, 9 figures. Typeset in JHEP style. Added a references
and two figures showing parameter space for each of two reference points.
Corrected typo
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