559 research outputs found
Feasibility study of using the overlap-Dirac operator for hadron spectroscopy
We investigate a number of algorithms that calculate the quark propagators
for the overlap-Dirac fermion operator. The QCD simulations were performed at
beta = 5.9 with a lattice volume of 16**3*32.Comment: LATTICE99(Chiral Fermions) 3 pages, 2 figures. A problem with the
incorrect definition of the overlap mass has been corrected. The results have
changed, but not the conclusions of the pape
Getting into the thick (and thin) of it
The heart is a highly regulated system in which a combination of mechanisms work together to match cardiac output to the needs of the body. Loss of this regulation characterizes many cardiomyopathies, where muscle is either hypo- or hypercontractile. For many years, x-ray diffraction has been used to study the structural basis of myofilament length-dependent activation..
Pseudoscalar singlet physics with staggered fermions
We report on progress in measuring disconnected correlators associated with
pseudoscalar flavor-singlet mesons. This will eventually allow us to compute
the masses of the eta and eta' mesons. Flavor-singlet physics also presents an
interesting test of the staggered fermion formulation, as disconnected
correlators are sensitive to whether the same action governs both sea quarks
and valence quarks. It can also help test the validity of the ``fourth-root
trick'' used in unquenched lattice calculations where the number of flavors
.Comment: Talk presented at Lattice 2005 (Hadron spectrum and quark masses), 6
pages, 3 figure
Scalar glueball and meson spectroscopy in unquenched lattice QCD with improved staggered quarks
We present results of an exploratory study of singlet scalar states in
unquenched QCD using both glueball and meson operators. Results for non-singlet
non-strange scalar mesons are also presented. We use Asqtad improved staggered
fermions and gauge configurations generated by the MILC collaboration at
lattice spacings of .12 and .09 fm. In this formulation, the glueball mass is
not significantly different from the quenched value at finite lattice spacing.
Significant taste violations are present in the scalar sector. At light quark
masses, decay channels complicate the mass determinations. There is some
evidence that the non-strange singlet meson lies below the non-singlet meson.Comment: Lattice 2005 (hadron spectrum and quark masses), 6 pages, 4 figure
Glueball mass measurements from improved staggered fermion simulations
We present the first 2+1 flavour spectrum measurements of glueball states
using high statistics simulations with improved staggered fermions. We find a
spectrum consistent with quenched measurements of scalar, pseudoscalar
andtensor glueball states. The measurements were made using 5000 configurations
at a lattice spacing of 0.123 fm and pion mass of 280 MeV and 3000
configurations at 0.092 fm with a pion mass of 360 MeV. We see some evidence of
coupling to 2 pion states. We compare our results with the experimental
glueball candidate spectrum as well as quenched glueball estimates.Comment: 22 pages, 19 figures and 8 tables, minor additions on mixing
post-refere
Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle
The thick (myosin-containing) filaments of vertebrate skeletal muscle are arranged in a hexagonal lattice, interleaved with an array of thin (actin-containing) filaments with which they interact to produce contraction. X-ray diffraction and EM have shown that there are two types of thick filament lattice. In the simple lattice, all filaments have the same orientation about their long axis, while in the superlattice, nearest neighbors have rotations differing by 0 degrees or 60 degrees . Tetrapods (amphibians, reptiles, birds, and mammals) typically have only a superlattice, while the simple lattice is confined to fish. We have performed x-ray diffraction and electron microscopy of the soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat and found that while the EDL has a superlattice as expected, the SOL has a simple lattice. The EDL and SOL of the rat are unusual in being essentially pure fast and slow muscles, respectively. The mixed fiber content of most tetrapod muscles and/or lattice disorder may explain why the simple lattice has not been apparent in these vertebrates before. This is supported by only weak simple lattice diffraction in the x-ray pattern of mouse SOL, which has a greater mix of fiber types than rat SOL. We conclude that the simple lattice might be common in tetrapods. The correlation between fiber type and filament lattice arrangement suggests that the lattice arrangement may contribute to the functional properties of a muscle
Relaxed tarantula skeletal muscle has two ATP energy-saving mechanisms
Myosin molecules in the relaxed thick filaments of striated muscle have a helical arrangement in which the heads of each molecule interact with each other, forming the interacting-heads motif (IHM). In relaxed mammalian skeletal muscle, this helical ordering occurs only at temperatures \u3e 20 degrees C and is disrupted when temperature is decreased. Recent x-ray diffraction studies of live tarantula skeletal muscle have suggested that the two myosin heads of the IHM (blocked heads [BHs] and free heads [FHs]) have very different roles and dynamics during contraction. Here, we explore temperature-induced changes in the BHs and FHs in relaxed tarantula skeletal muscle. We find a change with decreasing temperature that is similar to that in mammals, while increasing temperature induces a different behavior in the heads. At 22.5 degrees C, the BHs and FHs containing ADP.Pi are fully helically organized, but they become progressively disordered as temperature is lowered or raised. Our interpretation suggests that at low temperature, while the BHs remain ordered the FHs become disordered due to transition of the heads to a straight conformation containing Mg.ATP. Above 27.5 degrees C, the nucleotide remains as ADP.Pi, but while BHs remain ordered, half of the FHs become progressively disordered, released semipermanently at a midway distance to the thin filaments while the remaining FHs are docked as swaying heads. We propose a thermosensing mechanism for tarantula skeletal muscle to explain these changes. Our results suggest that tarantula skeletal muscle thick filaments, in addition to having a superrelaxation-based ATP energy-saving mechanism in the range of 8.5-40 degrees C, also exhibit energy saving at lower temperatures ( \u3c 22.5 degrees C), similar to the proposed refractory state in mammals
- …