160,613 research outputs found
An efficient, simple dialyzer
Easily assembled, efficient, countercurrent, sandwich-type barrier dialyzer was developed. Dialyzer contains six blood chambers that provide 500 sq cm membrane area. Design membranes are cuprammonium cellulose film. Unit performance was compared with thirteen other dialyzers
Tensor-polarized quark and antiquark distribution functions in a spin-one hadron
To understand orbital-angular-momentum contributions is becoming crucial for
clarifying nucleon-spin issue in the parton level. Twist-two structure
functions b_1 and b_2 for spin-one hadrons could probe orbital-angular-momentum
effects, which reflect a different aspect from current studies for the spin-1/2
nucleon, since they should vanish if internal constituents are in the S state.
These structure functions are related to tensor structure in spin-one hadrons.
Studies of such tensor structure will open a new field of high-energy spin
physics. The structure functions b_1 and b_2 are described by tensor-polarized
quark and antiquark distributions delta_T-q and delta_T-qbar. Using HERMES data
on the b_1 structure function for the deuteron, we made an analysis of
extracting the distributions delta_T-q and delta_T-qbar in a simple x-dependent
functional form. Optimum distributions are proposed for the tensor-polarized
valence and antiquark distribution functions from the analysis. A finite tensor
polarization is obtained for antiquarks if we impose a constraint that the
first moments of tensor-polarized valence-quark distributions vanish. It is
interesting to investigate a physics mechanism to create a finite
tensor-polarized antiquark distribution.Comment: 4 pages, LaTeX, 2 eps figures, Phys. Rev. D in pres
Electromagnetic Form Factors and Charge Densities From Hadrons to Nuclei
A simple exact covariant model in which a scalar particle is modeled as a
bound state of two different particles is used to elucidate relativistic
aspects of electromagnetic form factors. The model form factor is computed
using an exact covariant calculation of the lowest-order triangle diagram and
shown to be the same as that obtained using light-front techniques. The meaning
of transverse density is explained using coordinate space variables, allowing
us to identify a true mean-square transverse size directly related to the form
factor. We show that the rest-frame charge distribution is generally not
observable because of the failure to uphold current conservation. Neutral
systems of two charged constituents are shown to obey the lore that the heavier
one is generally closer to the transverse origin than the lighter one. It is
argued that the negative central charge density of the neutron arises, in
pion-cloud models, from pions of high longitudinal momentum. The
non-relativistic limit is defined precisely and the ratio of the binding energy
to that of the mass of the lightest constituent is shown to govern the
influence of relativistic effects. The exact relativistic formula for the form
factor reduces to the familiar one of the three-dimensional Fourier transform
of a square of a wave function for a very limited range of parameters. For
masses that mimic the quark-di-quark model of the nucleon we find substantial
relativistic corrections for any value of . A schematic model of the
lowest s-states of nuclei is used to find that relativistic effects decrease
the form factor for light nuclei but increase the form factor for heavy nuclei.
Furthermore, these states are strongly influenced by relativity.Comment: 18 pages, 11 figure
Shapes of the Nucleon
Previously defined spin-dependent quark densities that are matrix elements of
specific density operators in proton states of definite spin-polarization
generally have an infinite variety of non-spherical shapes. The present
application is concerned with both charge and matter densities. We show that
the Gross & Agbakpe model nucleon harbors an interesting variety of
non-spherical shapes.Comment: 8 pages 3 figure
The Effects of Quantum Entropy on the Bag Constant
The effects of quantum entropy on the bag constant are studied at low
temperatures and small chemical potentials. The inclusion of the quantum
entropy of the quarks in the equation of state provides the hadronic bag with
an additional heat which causes a decrease in the effective latent heat inside
the bag. We have considered two types of baryonic bags, and
. In both cases we have found that the bag constant without the
quantum entropy almost does not change with the temperature and the quark
chemical potential. The contribution from the quantum entropy to the equation
of state clearly decreases the value of the bag constant.Comment: 7 pages, 2 figures (two parts each
The influence of strange quarks on QCD phase diagram and chemical freeze-out: Results from the hadron resonance gas model
We confront the lattice results on QCD phase diagram for two and three
flavors with the hadron resonance gas model. Taking into account the
truncations in the Taylor-expansion of energy density done on the
lattice at finite chemical potential , we find that the hadron resonance
gas model under the condition of constant describes very well the
lattice phase diagram. We also calculate the chemical freeze-out curve
according to the entropy density . The -values are taken from lattice QCD
simulations with two and three flavors. We find that this condition is
excellent in reproducing the experimentally estimated parameters of the
chemical freeze-out.Comment: 5 pages, 3 figures and 1 table Talk given at VIIIth international
conference on ''Strangeness in Quark Matter'' (SQM 2004), Cape Town, South
Africa, Sep. 15-20 200
Conditions driving chemical freeze-out
We propose the entropy density as the thermodynamic condition driving best
the chemical freeze-out in heavy-ion collisions. Taking its value from lattice
calculations at zero chemical potential, we find that it is excellent in
reproducing the experimentally estimated freeze-out parameters. The two
characteristic endpoints in the freeze-out diagram are reproduced as well.Comment: 8 pages, 5 eps figure
Mapping Atomic Motions with Electrons: Toward the Quantum Limit to Imaging Chemistry
Recent advances in ultrafast electron and X-ray diffraction have pushed imaging of structural dynamics into the femtosecond time domain, that is, the fundamental time scale of atomic motion. New physics can be reached beyond the scope of traditional diffraction or reciprocal space imaging. By exploiting the high time resolution, it has been possible to directly observe the collapse of nearly innumerable possible nuclear motions to a few key reaction modes that direct chemistry. It is this reduction in dimensionality in the transition state region that makes chemistry a transferable concept, with the same class of reactions being applicable to synthetic strategies to nearly arbitrary levels of complexity. The ability to image the underlying key reaction modes has been achieved with resolution to relative changes in atomic positions to better than 0.01 Ă
, that is, comparable to thermal motions. We have effectively reached the fundamental space-time limit with respect to the reaction energetics and imaging the acting forces. In the process of ensemble measured structural changes, we have missed the quantum aspects of chemistry. This perspective reviews the current state of the art in imaging chemistry in action and poses the challenge to access quantum information on the dynamics. There is the possibility with the present ultrabright electron and X-ray sources, at least in principle, to do tomographic reconstruction of quantum states in the form of a Wigner function and density matrix for the vibrational, rotational, and electronic degrees of freedom. Accessing this quantum information constitutes the ultimate demand on the spatial and temporal resolution of reciprocal space imaging of chemistry. Given the much shorter wavelength and corresponding intrinsically higher spatial resolution of current electron sources over X-rays, this Perspective will focus on electrons to provide an overview of the challenge on both the theory and the experimental fronts to extract the quantum aspects of molecular dynamics
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