149 research outputs found
Quenched Chiral Perturbation Theory for Vector Mesons
We develop quenched chiral perturbation theory for vector mesons made of
light quarks, in the limit where the vector meson masses are much larger than
the pion mass. We use this theory to extract the leading nonanalytic dependence
of the vector meson masses on the masses of the light quarks. By comparing with
analogous quantities computed in ordinary chiral perturbation theory, we
estimate the size of quenching effects, observing that in general they can be
quite large. This estimate is relevant to lattice simulations, where the
mass is often used to set the lattice spacing.Comment: 18 pages, 8 figures, uses REVTeX and epsf.st
Enhanced chiral logarithms in partially quenched QCD
I discuss the properties of pions in ``partially quenched'' theories, i.e.
those in which the valence and sea quark masses, and , are
different. I point out that for lattice fermions which retain some chiral
symmetry on the lattice, e.g. staggered fermions, the leading order prediction
of the chiral expansion is that the mass of the pion depends only on , and
is independent of . This surprising result is shown to receive corrections
from loop effects which are of relative size , and which thus
diverge when the valence quark mass vanishes. Using partially quenched chiral
perturbation theory, I calculate the full one-loop correction to the mass and
decay constant of pions composed of two non-degenerate quarks, and suggest
various combinations for which the prediction is independent of the unknown
coefficients of the analytic terms in the chiral Lagrangian. These results can
also be tested with Wilson fermions if one uses a non-perturbative definition
of the quark mass.Comment: 14 pages, 3 figures, uses psfig. Typos in eqs (18)-(20) corrected
(alpha_4 is replaced by alpha_4/2
Phylogenetic Signals of Salinity and Season in Bacterial Community Composition Across the Salinity Gradient of the Baltic Sea
Understanding the key processes that control bacterial community composition has enabled predictions of bacterial distribution and function within ecosystems. In this study, we used the Baltic Sea as a model system to quantify the phylogenetic signal of salinity and season with respect to bacterioplankton community composition. The abundances of 16S rRNA gene amplicon sequencing reads were analyzed from samples obtained from similar geographic locations in July and February along a brackish to marine salinity gradient in the Baltic Sea. While there was no distinct pattern of bacterial richness at different salinities, the number of bacterial phylotypes in winter was significantly higher than in summer. Bacterial community composition in brackish vs. marine conditions, and in July vs. February was significantly different. Non-metric multidimensional scaling showed that bacterial community composition was primarily separated according to salinity and secondly according to seasonal differences at all taxonomic ranks tested. Similarly, quantitative phylogenetic clustering implicated a phylogenetic signal for both salinity and seasonality. Our results support that global patterns of bacterial community composition with respect to salinity and season are the result of phylogenetically clustered ecological preferences with stronger imprints from salinity
A Lattice QCD Analysis of the Strangeness Magnetic Moment of the Nucleon
The outcome of the SAMPLE Experiment suggests that the strange-quark
contribution to the nucleon magnetic moment, G_M^s(0), may be greater than
zero. This result is very difficult to reconcile with expectations based on the
successful baryon magnetic-moment phenomenology of the constituent quark model.
We show that careful consideration of chiral symmetry reveals some rather
unexpected properties of QCD. In particular, it is found that the valence
u-quark contribution to the magnetic moment of the neutron can differ by more
than 50% from its contribution to the Xi^0 magnetic moment. This hitherto
unforeseen result leads to the value G_M^s(0) = -0.16 +/- 0.18 with a
systematic error, arising from the relatively large strange quark mass used in
existing lattice calculations, that would tend to shift G_M^s(0) towards small
positive values.Comment: RevTeX, 20 pages, 12 figure
Restless pions: orbifold boundary conditions and noise suppression in lattice QCD
The study of one or more baryons in lattice QCD is severely hindered by the
exponential decay in time of the signal-to-noise ratio. The rate at which the
signal-to-noise decreases is a function of the the pion mass. More precisely,
it depends on the minimum allowed pion energy in the box, which, for periodic
boundary conditions, is equal to its mass. We propose a set of boundary
conditions, given by a "parity orbifold'' construction, which eliminates the
zero momentum pion modes, raising the minimum pion energy without altering the
QCD ground state, and thereby improving the signal-to-noise ratio of
(multi)-baryon correlation functions at long Euclidean times. We discuss
variations of these "restless pions" boundary conditions and focus on their
impact on the study of nuclear forces.Comment: 15 pages, 4 figure
Chiral Logs in Quenched QCD
The quenched chiral logs are examined on a lattice with
Iwasaki gauge action and overlap fermions. The pion decay constant is
used to set the lattice spacing, . With pion mass as low
as , we see the quenched chiral logs clearly in
and , the pseudoscalar decay constant. We analyze the data
to determine how low the pion mass needs to be in order for the quenched
one-loop chiral perturbation theory (PT) to apply. With the constrained
curve-fitting method, we are able to extract the quenched chiral log parameter
together with other low-energy parameters. Only for do we obtain a consistent and stable fit with a constant
which we determine to be 0.24(3)(4) (at the chiral scale ). By comparing to the lattice, we estimate the
finite volume effect to be about 2.7% for the smallest pion mass. We also
fitted the pion mass to the form for the re-summed cactus diagrams and found
that its applicable region is extended farther than the range for the one-loop
formula, perhaps up to MeV. The scale independent
is determined to be 0.20(3) in this case. We study the quenched
non-analytic terms in the nucleon mass and find that the coefficient
in the nucleon mass is consistent with the prediction of one-loop PT\@.
We also obtain the low energy constant from . We conclude from
this study that it is imperative to cover only the range of data with the pion
mass less than in order to examine the chiral behavior of
the hadron masses and decay constants in quenched QCD and match them with
quenched one-loop PT\@.Comment: 37 pages and 24 figures, pion masses are fitted to the form for the
re-summed cactus diagrams, figures added, to appear in PR
Quenched hadron spectroscopy with improved staggered quark action
We investigate light hadron spectroscopy with an improved quenched staggered
quark action. We compare the results obtained with an improved gauge plus an
improved quark action, an improved gauge plus standard quark action, and the
standard gauge plus standard quark action. Most of the improvement in the
spectroscopy results is due to the improved gauge sector. However, the improved
quark action substantially reduces violations of Lorentz invariance, as
evidenced by the meson dispersion relations.Comment: New references adde
Quenched Chiral Perturbation Theory for Heavy Baryons
Heavy baryon chiral perturbation theory is extended to include the effects of
quenching. In this framework the leading nonanalytic dependence of the heavy
baryon masses on the light quark masses is studied. The size of quenching
effects is estimated by comparing the results of quenched and ordinary chiral
perturbation theories. It is found that in general they can be large. This
estimate is relevant to lattice simulations of the heavy baryon masses.Comment: 14 pages, 5 figures, uses REVTe
Baryon Spectroscopy in Lattice QCD
We review recent developments in the study of excited baryon spectroscopy in
lattice QCD. After introducing the basic methods used to extract masses from
correlation functions, we discuss various interpolating fields and lattice
actions commonly used in the literature. We present a survey of results of
recent calculations of excited baryons in quenched QCD, and outline possible
future directions in the study of baryon spectra.Comment: Contribution to Lecture Notes in Physics on Lattice Hadron Physics,
43 pages, 11 figures, 3 table
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