46,499 research outputs found
Crystal nuclei templated nanostructured membranes prepared by solvent crystallization and polymer migration
Currently, production of porous polymeric membranes for filtration is predominated by the phase-separation process. However, this method has reached its technological limit, and there have been no significant breakthrough over the last decade. Here we show, using polyvinylidene fluoride as a sample polymer, a new concept of membrane manufacturing by combining oriented green solvent crystallization and polymer migration is able to obtain high performance membranes with pure water permeation flux substantially higher than those with similar pore size prepared by conventional phase-separation processes. The new manufacturing procedure is governed by fewer operating parameters and is, thus, easier to control with reproducible results. Apart from the high water permeation flux, the prepared membranes also show excellent stable flux after fouling and superior mechanical properties of high pressure load and better abrasion resistance. These findings demonstrate the promise of a new concept for green manufacturing nanostructured polymeric membranes with high performances
Unified Halo-Independent Formalism From Convex Hulls for Direct Dark Matter Searches
Using the Fenchel-Eggleston theorem for convex hulls (an extension of the
Caratheodory theorem), we prove that any likelihood can be maximized by either
a dark matter 1- speed distribution in Earth's frame or 2- Galactic
velocity distribution , consisting of a sum of delta
functions. The former case applies only to time-averaged rate measurements and
the maximum number of delta functions is , where is the total number of data entries. The second case applies to any
harmonic expansion coefficient of the time-dependent rate and the maximum
number of terms is . Using time-averaged rates, the
aforementioned form of results in a piecewise constant unmodulated halo
function (which is an integral of the speed
distribution) with at most downward steps. The authors had
previously proven this result for likelihoods comprised of at least one
extended likelihood, and found the best-fit halo function to be unique. This
uniqueness, however, cannot be guaranteed in the more general analysis applied
to arbitrary likelihoods. Thus we introduce a method for determining whether
there exists a unique best-fit halo function, and provide a procedure for
constructing either a pointwise confidence band, if the best-fit halo function
is unique, or a degeneracy band, if it is not. Using measurements of modulation
amplitudes, the aforementioned form of , which is a sum
of Galactic streams, yields a periodic time-dependent halo function
which at any fixed time is a piecewise
constant function of with at most downward steps.
In this case, we explain how to construct pointwise confidence and degeneracy
bands from the time-averaged halo function. Finally, we show that requiring an
isotropic ...Comment: v2: Published version. Text altered, conclusions unchanged. v1: 30
pages, 7 figure
Assessing Compatibility of Direct Detection Data: Halo-Independent Global Likelihood Analyses
We present two different halo-independent methods to assess the compatibility
of several direct dark matter detection data sets for a given dark matter model
using a global likelihood consisting of at least one extended likelihood and an
arbitrary number of Gaussian or Poisson likelihoods. In the first method we
find the global best fit halo function (we prove that it is a unique piecewise
constant function with a number of down steps smaller than or equal to a
maximum number that we compute) and construct a two-sided pointwise confidence
band at any desired confidence level, which can then be compared with those
derived from the extended likelihood alone to assess the joint compatibility of
the data. In the second method we define a "constrained parameter
goodness-of-fit" test statistic, whose -value we then use to define a
"plausibility region" (e.g. where ). For any halo function not
entirely contained within the plausibility region, the level of compatibility
of the data is very low (e.g. ). We illustrate these methods by
applying them to CDMS-II-Si and SuperCDMS data, assuming dark matter particles
with elastic spin-independent isospin-conserving interactions or exothermic
spin-independent isospin-violating interactions.Comment: 31 pages, 6 figures. V2: Modified several paragraphs to improve
clarify. Modified Fig. 5 and added Fig. 6 to further illustrate methods of
Section 5. Added proof of uniqueness of best fit halo function in Appendix
Leading Chiral Contributions to the Spin Structure of the Proton
The leading chiral contributions to the quark and gluon components of the
proton spin are calculated using heavy-baryon chiral perturbation theory.
Similar calculations are done for the moments of the generalized parton
distributions relevant to the quark and gluon angular momentum densities. These
results provide useful insight about the role of pions in the spin structure of
the nucleon, and can serve as a guidance for extrapolating lattice QCD
calculations at large quark masses to the chiral limit.Comment: 8 pages, 2 figures; a typo in Ref. 7 correcte
Quark Orbital-Angular-Momentum Distribution in the Nucleon
We introduce gauge-invariant quark and gluon angular momentum distributions
after making a generalization of the angular momentum density operators. From
the quark angular momentum distribution, we define the gauge-invariant and
leading-twist quark {\it orbital} angular momentum distribution . The
latter can be extracted from data on the polarized and unpolarized quark
distributions and the off-forward distribution in the forward limit. We
comment upon the evolution equations obeyed by this as well as other orbital
distributions considered in the literature.Comment: 8 pages, latex, no figures, minor corrections mad
Disentangling positivity constraints for generalized parton distributions
Positivity constraints are derived for the generalized parton distributions
(GPDs) of spin-1/2 hadrons. The analysis covers the full set of eight twist-2
GPDs. Several new inequalities are obtained which constrain GPDs by various
combinations of usual (forward) unpolarized and polarized parton distributions
including the transversity distribution.Comment: 9 pages (REVTEX), typos correcte
Electromagnetic fields in a 3D cavity and in a waveguide with oscillating walls
We consider classical and quantum electromagnetic fields in a
three-dimensional (3D) cavity and in a waveguide with oscillating boundaries of
the frequency . The photons created by the parametric resonance are
distributed in the wave number space around along the axis of the
oscillation. When classical waves propagate along the waveguide in the one
direction, we observe the amplification of the original waves and another wave
generation in the opposite direction by the oscillation of side walls. This can
be understood as the classical counterpart of the photon production. In the
case of two opposite walls oscillating with the same frequency but with a phase
difference, the interferences are shown to occur due to the phase difference in
the photon numbers and in the intensity of the generated waves.Comment: 8 pages revTeX including 1 eps fi
Nucleon-Quarkonium Elastic Scattering and the Gluon Contribution to Nucleon Spin
It is shown that the amplitude for the scattering of a heavy quarkonium
system from a nucleon near threshold is completely determined by the fraction
of angular momentum, as well as linear momentum, carried by gluons in the
nucleon. A form for the quarkonium-nucleon non-relativistic potential is
derived.Comment: 4 pages, no figures. Author's e-mail: [email protected]
A model of driven and decaying magnetic turbulence in a cylinder
Using mean-field theory, we compute the evolution of the magnetic field in a
cylinder with outer perfectly conducting boundaries, an imposed axial magnetic
and electric field. The thus injected magnetic helicity in the system can be
redistributed by magnetic helicity fluxes down the gradient of the local
current helicity of the small-scale magnetic field. A weak reversal of the
axial magnetic field is found to be a consequence of the magnetic helicity flux
in the system. Such fluxes are known to alleviate so-called catastrophic
quenching of the {\alpha}-effect in astrophysical applications. Application to
the reversed field pinch in plasma confinement devices is discussed.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
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