1,334 research outputs found
Thermal and cryogenic design study for space infrared telescope facility (SIRTF)
A study was conducted to determine the ability of an all superfluid helium design to meet the performance requirements of background limited to 200 micrometer, and a two year lifetime for a one meter class free flying infrared observatory. Both a 98 deg and 28.5 deg inclination orbits were examined, and aperture shade designs were developed for both orbits. A unique forebaffle cooling design significantly reduces the sensitivity to aperture heat loads. With certain restrictions on observing modes, the study determined that an all superfluid helium Dewar will meet the temperature and lifetime requirements. A dual cryogen SFHe/SH2 system was also investigated for the 28.5 deg orbit and found to provide a more constant forebaffle temperature but with only a slight improvement in lifetime
Forcing and Velocity Correlations in a Vibrated Granular Monolayer
The role of forcing on the dynamics of a vertically shaken granular monolayer
is investigated. Using a flat plate, surprising negative velocity correlations
are measured. A mechanism for this anti-correlation is proposed with support
from both experimental results and molecular dynamics simulations. Using a
rough plate, velocity correlations are positive, and the velocity distribution
evolves from a gaussian at very low densities to a broader distribution at high
densities. These results are interpreted as a balance between stochastic
forcing, interparticle collisions, and friction with the plate.Comment: 4 pages, 5 figure
Renormalization constants of local operators for Wilson type improved fermions
Perturbative and non-perturbative results are presented on the
renormalization constants of the quark field and the vector, axial-vector,
pseudoscalar, scalar and tensor currents. The perturbative computation, carried
out at one-loop level and up to second order in the lattice spacing, is
performed for a fermion action, which includes the clover term and the twisted
mass parameter yielding results that are applicable for unimproved Wilson
fermions, as well as for improved clover and twisted mass fermions. We consider
ten variants of the Symanzik improved gauge action corresponding to ten
different values of the plaquette coefficients. Non-perturbative results are
obtained using the twisted mass Wilson fermion formulation employing two
degenerate dynamical quarks and the tree-level Symanzik improved gluon action.
The simulations are performed for pion masses in the range of 480 MeV to 260
MeV and at three values of the lattice spacing, a, corresponding to beta=3.9,
4.05, 4.20. For each renormalization factor computed non-perturbatively we
subtract its perturbative O(a^2) terms so that we eliminate part of the cut-off
artifacts. The renormalization constants are converted to MS-bar at a scale of
mu=2 GeV. The perturbative results depend on a large number of parameters and
are made easily accessible to the reader by including them in the distribution
package of this paper, as a Mathematica input file.Comment: 36 pages, 11 figures and 6 tables. The results are included in
electronic form (Mathematica files
Multivalent Recognition of Peptides by Modular Self-Assembled Receptors
Developing nontraditional approaches to the synthesis and characterization of multivalent compounds is critical to our efforts to study and interface with biological systems and to build new noncovalent materials. This paper demonstrates a biomimetic approach to the construction of discrete, modular, multivalent receptors via molecular self-assembly in aqueous solution. Scaffolds presenting 1−3 viologen groups recruit a respective 1−3 copies of the synthetic host, cucurbit[8]uril, in a noncooperative manner and with a consistent equilibrium association constant (Ka) value of 2 × 106 M−1 per binding site. The assembled mono-, di-, and trivalent receptors bind to their cognate target peptides containing 1−3 Trp residues with Ka values in the range 1.7 × 104−4.7 × 106 M−1 and in predetermined mono- or multivalent binding modes with 31−280-fold enhancements in affinity and additive enthalpies due to multivalency. The extent of valency was determined directly by measuring the visible charge-transfer absorptivity due to the viologen−indole pair. The predictable behavior of this system and its ease of synthesis and analysis make it well suited to serve as a model for multivalent binding and for the multivalent recognition of peptides by design
Comparison between overlap and twisted mass fermions towards the chiral limit
We compare overlap fermions, which are chirally invariant, and Wilson twisted
mass fermions in the approach to the chiral limit. Our quenched simulations
reveal that with both formulations of lattice fermions pion masses of O(250
MeV) can be reached in practical simulations. Our comparison is done at a fixed
lattice spacing a=0.123 fm. Several quantities are measured, such as hadron
masses and pseudoscalar decay constants.Comment: Lattice2004(chiral
{\eta} and {\eta}' mesons from Nf=2+1+1 twisted mass lattice QCD
We determine mass and mixing angles of eta and eta' states using Nf=2+1+1
Wilson twisted mass lattice QCD. We describe how those flavour singlet states
need to be treated in this lattice formulation. Results are presented for three
values of the lattice spacing, a=0.061 fm, a=0.078 fm and a=0.086 fm, with
light quark masses corresponding to values of the charged pion mass in a range
of 230 to 500 MeV and fixed bare strange and charm quark mass values. We obtain
557(15)(45) MeV for the eta mass (first error statistical, second systematic)
and 44(5) degrees for the mixing angle in the quark flavour basis,
corresponding to -10(5) degrees in the octet-singlet basis.Comment: 28 pages, 9 figures, version to appear in JHEP, extended discussion
of autocorrelation times and comparison to results available in the
literature, added a comment for FS-effects and clarified the description of
our blocking procedur
Health promotion: From malaria control to elimination
Here we reflect on the achievement of some of the diverse activities that have brought malaria under control, highlight key challenges and propose specific health promotion interventions required to move South Africa’s malaria programme from control to elimination
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