123,466 research outputs found
Pion Form Factor in the Factorization Formalism
Based on the light-cone (LC) framework and the factorization formalism,
the transverse momentum effects and the different helicity components'
contributions to the pion form factor are recalculated. In
particular, the contribution to the pion form factor from the higher helicity
components (), which come from the spin-space Wigner
rotation, are analyzed in the soft and hard energy regions respectively. Our
results show that the right power behavior of the hard contribution from the
higher helicity components can only be obtained by fully keeping the
dependence in the hard amplitude, and that the dependence in LC wave
function affects the hard and soft contributions substantially. As an example,
we employ a model LC wave function to calculate the pion form factor and then
compare the numerical predictions with the experimental data. It is shown that
the soft contribution is less important at the intermediate energy region.Comment: 21 pages, 4 figure
Optimized Double-well quantum interferometry with Gaussian squeezed-states
A Mach-Zender interferometer with a gaussian number-difference squeezed input
state can exhibit sub-shot-noise phase resolution over a large phase-interval.
We obtain the optimal level of squeezing for a given phase-interval
and particle number , with the resulting phase-estimation
uncertainty smoothly approaching as approaches 10/N,
achieved with highly squeezed states near the Fock regime. We then analyze an
adaptive measurement scheme which allows any phase on to be
measured with a precision of requiring only a few measurements, even
for very large . We obtain an asymptotic scaling law of , resulting in a final
precision of . This scheme can be readily implemented in a
double-well Bose-Einstein condensate system, as the optimal input states can be
obtained by adiabatic manipulation of the double-well ground state.Comment: updated versio
Assessment of closure coefficients for compressible-flow turbulence models
A critical assessment is made of the closure coefficients used for turbulence length scale in existing models of the transport equation, with reference to the extension of these models to compressible flow. It is shown that to satisfy the compressible 'law of the wall', the model coefficients must actually be functions of density gradients. The magnitude of the errors that result from neglecting this dependence on density varies with the variable used to specify the length scale. Among the models investigated, the k-omega model yields the best performance, although it is not completely free from errors associated with density terms. Models designed to reduce the density-gradient effect to an insignificant level are proposed
Stability of parallel/perpendicular domain boundaries in lamellar block copolymers under oscillatory shear
We introduce a model constitutive law for the dissipative stress tensor of
lamellar phases to account for low frequency and long wavelength flows. Given
the uniaxial symmetry of these phases, we argue that the stress tensor must be
the same as that of a nematic but with the local order parameter being the
slowly varying lamellar wavevector. This assumption leads to a dependence of
the effective dynamic viscosity on orientation of the lamellar phase. We then
consider a model configuration comprising a domain boundary separating
laterally unbounded domains of so called parallel and perpendicularly oriented
lamellae in a uniform, oscillatory, shear flow, and show that the configuration
can be hydrodynamically unstable for the constitutive law chosen. It is argued
that this instability and the secondary flows it creates can be used to infer a
possible mechanism for orientation selection in shear experiments.Comment: 26 pages, 10 figure
Orientation selection in lamellar phases by oscillatory shears
In order to address the selection mechanism that is responsible for the
unique lamellar orientation observed in block copolymers under oscillatory
shears, we use a constitutive law for the dissipative part of the stress tensor
that respects the uniaxial symmetry of a lamellar phase. An interface
separating two domains oriented parallel and perpendicular to the shear is
shown to be hydrodynamically unstable, a situation analogous to the thin layer
instability of stratified fluids under shear. The resulting secondary flows
break the degeneracy between parallel and perpendicular lamellar orientation,
leading to a preferred perpendicular orientation in certain ranges of
parameters of the polymer and of the shear.Comment: 4 pages, 3 figure
Investigation of a novel elastic-mechanical wheel transmission under light duty conditions
A novel 'Elastic Engagement and Friction Coupled' (EEFC) mechanical transmission has been proposed recently in which the power is transmitted through elastic tines on the surfaces of the driving and driven wheels. This study introduces new variations of EEFC mechanical wheel transmission ( broadly emulating a gear-pair) with small contact areas for use under light duty conditions. Because a drive of this type inevitably has a strong statistical component, theoretical analysis of the geometrical and mechanical relationships has been attempted by using linear modeling and empirical weightings. Several simple forms of the EEFC wheel transmission are tested under limiting ( slip) conditions for transmission force and transmission coefficients against normal load. Normalized standard deviation of these parameters is used to summarize noise performance. Models and experiments are in reasonable agreement, suggesting that the model parameters reflect important design considerations. EEFC transmissions appear well suited to force regimes of a few tenths of a newton and to have potential for use in, for example, millimetre-scale robots
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