609 research outputs found
Effects of spatial size, lattice doubling and source operator on the hadron spectrum with dynamical staggered quarks
We have extended our previous study of the lattice QCD spectrum with 2
flavors of staggered dynamical quarks at and and 0.01
to larger lattices, with better statistics and with additional sources for the
propagators. The additional sources allowed us to estimate the mass
and to measure the masses of all mesons whose operators are local in time.
These mesons show good evidence for flavor symmetry restoration, except for the
masses of the Goldstone and non-Goldstone pions. PCAC is observed in that
, and is estimated. Use of undoubled lattices
removes problems with the pion propagator found in our earlier work. Previously
we found a large change in the nucleon mass at a quark mass of when
we increased the spatial size from 12 to 16. No such effect is observed at the
larger quark mass, . Two kinds of wall source were used, and we
have found difficulties in getting consistent results for the nucleon mass
between the two sources.Comment: 30 pages PostScript fil
QCD thermodynamics with two flavors of Wilson quarks at N_t=6
We report on a study of hadron thermodynamics with two flavors of Wilson
quarks on 12^3x6 lattices. We have studied the crossover between the high and
low temperature regimes for three values of the hopping parameter, kappa=0.16,
0.17, and 0.18. At each of these values of kappa we have carried out spectrum
calculations on 12^3x24 lattices for two values of the gauge coupling in the
vicinity of the crossover in order to set an energy scale for our
thermodynamics calculations and to determine the critical value of the gauge
coupling for which the pion and quark masses vanish. For kappa=0.17 and 0.18 we
find coexistence between the high and low temperature regimes over 1,000
simulation time units indicating either that the equilibration time is
extremely long or that there is a possibility of a first order phase
transition. The pion mass is large at the crossover values of the gauge
coupling, but the crossover curve has moved closer to the critical curve along
which the pion and quark masses vanish, than it was on lattices with four time
slices. In addition, values of the dimensionless quantity T_c/m_rho are in
closer agreement with those for staggered quarks than was the case at N_t=4. (A
POSTSCRIPT VERSION OF THIS PAPER IS AVAILABLE BY ANONYMOUS FTP FROM
sarek.physics.ucsb.edu (128.111.8.250) IN THE FILE pub/wilson_thermo.ps)Comment: 24 page
Hadron Spectrum in QCD with Valence Wilson Fermions and Dynamical Staggered Fermions at $6/g^2=5.6
We present an analysis of hadronic spectroscopy for Wilson valence quarks
with dynamical staggered fermions at lattice coupling at
sea quark mass and 0.025, and of Wilson valence quarks in quenched
approximation at and 5.95, both on lattices. We
make comparisons with our previous results with dynamical staggered fermions at
the same parameter values but on lattices doubled in the temporal
direction.Comment: 32 page
Light Quark Masses from Lattice QCD
We present estimates of the masses of light quarks using lattice data. Our
main results are based on a global analysis of all the published data for
Wilson, Sheikholeslami-Wohlert (clover), and staggered fermions, both in the
quenched approximation and with dynamical flavors. We find that the
values of masses with the various formulations agree after extrapolation to the
continuum limit for the theory. Our best estimates, in the MSbar scheme
at , are \mbar=3.4 +- 0.4 +- 0.3 MeV and in the quenched approximation. The results, \mbar = 2.7 +- 0.3 +-
0.3 MeV and , are preliminary. (A linear
extrapolation in would further reduce these estimates for the physical
case of three dynamical flavors.) These estimates are smaller than
phenomenological estimates based on sum rules, but maintain the ratios
predicted by chiral perturbation theory. The new results have a significant
impact on the extraction of from the Standard Model. Using
the same lattice data we estimate the quark condensate using the
Gell-Mann-Oakes-Renner relation. Again the three formulations give consistent
results after extrapolation to , and the value turns out to be
correspondingly larger, roughly preserving m_s \vev{\bar \psi \psi}.Comment: 32 pages. Package submitted in uufiles format: unpack and tex
paper.tex. Modified "axis" source for figures also included. Latex2e
document. Uncomment hyperref if available. This is the final published
versio
Properties of the a1 Meson from Lattice QCD
We determine the mass and decay constant of the meson using Monte Carlo
simulation of lattice QCD. We find MeV and , in good agreement with experiment.Comment: 9 page uu-encoded compressed postscript file. version appearing in
Phys. Rev. Lett. 74 (1995) 459
Baryon number non-conservation and phase transitions at preheating
Certain inflation models undergo pre-heating, in which inflaton oscillations
can drive parametric resonance instabilities. We discuss several phenomena
stemming from such instabilities, especially in weak-scale models; generically,
these involve energizing a resonant system so that it can evade tunneling by
crossing barriers classically. One possibility is a spontaneous change of phase
from a lower-energy vacuum state to one of higher energy, as exemplified by an
asymmetric double-well potential with different masses in each well. If the
lower well is in resonance with oscillations of the potential, a system can be
driven resonantly to the upper well and stay there (except for tunneling) if
the upper well is not resonant. Another example occurs in hybrid inflation
models where the Higgs field is resonant; the Higgs oscillations can be
transferred to electroweak (EW) gauge potentials, leading to rapid transitions
over sphaleron barriers and consequent B+L violation. Given an appropriate
CP-violating seed, we find that preheating can drive a time-varying condensate
of Chern-Simons number over large spatial scales; this condensate evolves by
oscillation as well as decay into modes with shorter spatial gradients,
eventually ending up as a condensate of sphalerons. We study these examples
numerically and to some extent analytically. The emphasis in the present paper
is on the generic mechanisms, and not on specific preheating models; these will
be discussed in a later paper.Comment: 10 pages, 7 figures included, revtex, epsf, references adde
Thermodynamics of lattice QCD with two light quark flavours on a 16^3 x 8 lattice II
We have extended our earlier simulations of the high temperature behaviour of
lattice QCD with two light flavours of staggered quarks on a
lattice to lower quark mass (m_q=0.00625). The transition from hadronic matter
to a quark-gluon plasma is observed at corresponding to a
temperature of MeV. We present measurements of observables
which probe the nature of the quark-gluon plasma and serve to distinguish it
from hadronic matter. Although the transition is quite abrupt, we have seen no
indications that it is first order.Comment: 23 pages, RevteX, 6 encapsulated postscript figure
Two-Flavor Staggered Fermion Thermodynamics at N_t = 12
We present results of an ongoing study of the nature of the high temperature
crossover in QCD with two light fermion flavors. These results are obtained
with the conventional staggered fermion action at the smallest lattice spacing
to date---approximately 0.1 fm. Of particular interest are a study of the
temperature of the crossover a determination of the induced baryon charge and
baryon susceptibility, the scalar susceptibility, and the chiral order
parameter, used to test models of critical behavior associated with chiral
symmetry restoration. From our new data and published results for N_t = 4, 6,
and 8, we determine the QCD magnetic equation of state from the chiral order
parameter using O(4) and mean field critical exponents and compare it with the
corresponding equation of state obtained from an O(4) spin model and mean field
theory. We also present a scaling analysis of the Polyakov loop, suggesting a
temperature dependent ``constituent quark free energy.''Comment: LaTeX 25 pages, 15 Postscript figure
Revealing new perspectives on the regulation of gene expression by the trans-splicing mechanism in Shistosoma mansoni.
Schistosomiasis is the second most prevalent neglected tropical disease caused by a Trematode flatworm from the genus Schistosoma. Schistosomes possess a complex life cycle requiring numerous and intricate mechanisms to rule transcriptional and post-transcriptional gene regulation. Thus, it is believed that spliced leader (SL) trans-splicing could play an important role in the parasite biology. The purpose of this study was is to shed light on the function of the trans-splicing mechanism in S. mansoni by searching gene categories that could be target of this process and attempting to silence transcripts harboring the SL sequence. Here, we analyze different life stages cDNA libraries enriched on SL-transcripts of S. mansoni. Our results show that, contrary to previous hypothesis, trans-spliced transcripts are associated to specific gene categories, such as carbohydrate metabolic process, RNA metabolism and DNA repair. Several trans-spliced transcripts were verified in at least two different stages of the S. mansoni life cycle. The marjority of the SL-transcripts are shared among males, females and mixed-sex adults, being followed by SL-transcripts shared by all adults and schistosomula. Among the analyzed life stages, the one presenting the lower number of shared genes is the egg. The results also reveal a set of SL-transcripts that are observed only in one stage of the life cycle and could represent transcripts that undergo stage-specific trans-splicing. Finally, we demonstrated that trans-splicing knockdown in sprocysts caused a reduction on the levels of all tested trans-spliced transcripts, but surprisingly the only phenotypic effect observed was a diminished larvae size. Financial support: CNPq, FAPEMIGX-MEETING 2011
Non-compact Lattice QED with Two Charges: Phase Diagram and Renormalization Group Flow
The phase diagram of non-compact lattice QED in four dimensions with
staggered fermions of charges 1 and is investigated. The renormalized
charges are determined and found to be in agreement with perturbation theory.
This is an indication that there is no continuum limit with non-vanishing
renormalized gauge coupling, and that the theory has a validity bound for every
finite value of the renormalized coupling. The renormalization group flow of
the charges is investigated and an estimate for the validity bound as a
function of the cut-off is obtained. Generalizing this estimate to all fermions
in the Standard Model,it is found that a cut-off at the Planck scale implies
that has to be less than . Due to spontaneous chiral symmetry
breaking, strongly bound fermion-antifermion composite states are generated.
Their spectrum is discussed.Comment: 35 pages, LATEX, 26 PostScript figures (uuencoded
- âŠ