2,900 research outputs found
Monophyly of brachiopods and phoronids: reconciliation of molecular evidence with Linnaean classification (the subphylum Phoroniformea nov.)
Molecular phylogenetic analyses of aligned 18S rDNA gene sequences from articulate and inarticulate brachiopods representing all major extant lineages, an enhanced set of phoronids and several unrelated protostome taxa, confirm previous indications that in such data, brachiopod and phoronids form a well-supported clade that (on previous evidence) is unambiguously affiliated with protostomes rather than deuterostomes. Within the brachiopod-phoronid clade, an association between phoronids and inarticulate brachiopods is moderately well supported, whilst a close relationship between phoronids and craniid inarticulates is weakly indicated. Brachiopod-phoronid monophyly is reconciled with the most recent Linnaean classification of brachiopods by abolition of the phylum Phoronida and rediagnosis of the phylum Brachiopoda to include tubiculous, shell-less forms. Recognition that brachiopods and phoronids are close genealogical allies of protostome phyla such as molluscs and annelids, but are much more distantly related to deuterostome phyla such as echinoderms and chordates, implies either (or both) that the morphology and ontogeny of blastopore, mesoderm and coelom formation have been widely misreported or misinterpreted, or that these characters have been subject to extensive homoplasy. This inference, if true, undermines virtually all morphology-based reconstructions of phylogeny made during the past century or more
Relations Among Correlation Functions in the High Temperature Phase of QCD with Broken SU(3)
Group-theoretic arguments are used to determine the dependence of two-point
correlators of quark bilinears on the current quark masses. The leading
difference between and correlators is found to be of order
times a U(1)-violating correlator. These general
arguments are consistent with Schaefer's observation that if
U(1) violation persists to high enough temperatures
then the strange can be lighter than the non-strange one.Comment: 8 page
Pion and kaon physics with improved staggered quarks
We compute pseudoscalar meson masses and decay constants using staggered
quarks on lattices with three flavors of sea quarks and lattice spacings
fm and fm. We fit partially quenched results to
``staggered chiral perturbation theory'' formulae, thereby taking into account
the effects of taste-symmetry violations. Chiral logarithms are observed. From
the fits we calculate and , extract Gasser-Leutwyler parameters of
the chiral Lagrangian, and (modulo rather large perturbative errors) find the
light and strange quark masses.Comment: Lattice2003(spectrum); 3 pages, 1 eps figur
Recommended from our members
Change detection in children with autism: an auditory event-related fMRI study
Autism involves impairments in communication and social interaction, as well as high levels of repetitive, stereotypic and ritualistic behaviours, and extreme resistance to change. This latter dimension, whilst required for a diagnosis, has received less research attention. We hypothesise that this extreme resistance to change in autism is rooted in atypical processing of unexpected stimuli. We tested this using auditory event-related fMRI to determine regional brain activity associated with passive detection of infrequently occurring frequency-deviant and complex novel sounds in a no-task condition. Participants were twelve 10 to 15-year-old children with autism, and a group of 12 age- and sex-matched healthy controls
Complete next-to-leading order calculation for pion production in nucleon-nucleon collisions at threshold
Based on a counting scheme that explicitly takes into account the large
momentum sqrt(M m_pi) characteristic for pion production in nucleon-nucleon
collisions we calculate all diagrams for the reaction NN --> NN pi at threshold
up to next-to-leading order. At this order there are no free parameters and the
size of the next-to-leading order contributions is in line with the expectation
from power counting. The sum of loop corrections at that order vanishes for the
process pp --> pp pi^0 at threshold. The total contribution at next-to-leading
order from loop diagrams that include the delta degree of freedom vanishes at
threshold in both reaction channels pp --> pp pi^0, pn pi^+.Comment: 9 pages, 4 figure
Physical Results from Unphysical Simulations
We calculate various properties of pseudoscalar mesons in partially quenched
QCD using chiral perturbation theory through next-to-leading order. Our results
can be used to extrapolate to QCD from partially quenched simulations, as long
as the latter use three light dynamical quarks. In other words, one can use
unphysical simulations to extract physical quantities - in this case the quark
masses, meson decay constants, and the Gasser-Leutwyler parameters L_4-L_8. Our
proposal for determining L_7 makes explicit use of an unphysical (yet
measurable) effect of partially quenched theories, namely the double-pole that
appears in certain two-point correlation functions. Most of our calculations
are done for sea quarks having up to three different masses, except for our
result for L_7, which is derived for degenerate sea quarks.Comment: 26 pages, 12 figures (discussion on discretization errors at end of
sec. IV clarified; minor improvements in presentation; results unchanged
Chiral Corrections to Lattice Calculations of Charge Radii
Logarithmic divergences in pion and proton charge radii associated with
chiral loops are investigated to assess systematic uncertainties in current
lattice determinations of charge radii. The chiral corrections offer a possible
solution to the long standing problem of why present lattice calculations yield
proton and pion radii which are similar in size.Comment: PostScript file only. Ten pages. Figures included. U. of MD Preprint
#92-19
Is the up-quark massless?
We report on determinations of the low-energy constants alpha5 and alpha8 in
the effective chiral Lagrangian at O(p^4), using lattice simulations with N_f=2
flavours of dynamical quarks. Precise knowledge of these constants is required
to test the hypothesis whether or not the up-quark is massless. Our results are
obtained by studying the quark mass dependence of suitably defined ratios of
pseudoscalar meson masses and matrix elements. Although comparisons with an
earlier study in the quenched approximation reveal small qualitative
differences in the quark mass behaviour, numerical estimates for alpha5 and
alpha8 show only a weak dependence on the number of dynamical quark flavours.
Our results disfavour the possibility of a massless up-quark, provided that the
quark mass dependence in the physical three-flavour case is not fundamentally
different from the two-flavour case studied here.Comment: references added, typos correcte
Partially quenched chiral perturbation theory without
This paper completes the argument that lattice simulations of partially
quenched QCD can provide quantitative information about QCD itself, with the
aid of partially quenched chiral perturbation theory. A barrier to doing this
has been the inclusion of , the partially quenched generalization of
the , in previous calculations in the partially quenched effective
theory. This invalidates the low energy perturbative expansion, gives rise to
many new unknown parameters, and makes it impossible to reliably calculate the
relation between the partially quenched theory and low energy QCD. We show that
it is straightforward and natural to formulate partially quenched chiral
perturbation theory without , and that the resulting theory contains
the effective theory for QCD without the . We also show that previous
results, obtained including , can be reinterpreted as applying to the
theory without . We contrast the situation with that in the quenched
effective theory, where we explain why it is necessary to include .
We also compare the derivation of chiral perturbation theory in partially
quenched QCD with the standard derivation in unquenched QCD. We find that the
former cannot be justified as rigorously as the latter, because of the absence
of a physical Hilbert space. Finally, we present an encouraging result:
unphysical double poles in certain correlation functions in partially quenched
chiral perturbation theory can be shown to be a property of the underlying
theory, given only the symmetries and some plausible assumptions.Comment: 45 pages, no figure
Chiral perturbation theory calculation for pn -> dpipi at threshold
We investigate the reaction pn -> dpipi in the framework of Chiral
Perturbation Theory. For the first time a complete calculation of the leading
order contributions is presented. We identify various diagrams that are of
equal importance as compared to those recognized in earlier works. The diagrams
at leading order behave as expected by the power counting. Also for the first
time the nucleon-nucleon interaction in the initial, intermediate and final
state is included consistently and found to be very important. This study
provides a theoretical basis for a controlled evaluation of the non-resonant
contributions in two-pion production reactions in nucleon-nucleon collisions.Comment: 24 pages, 3 figures, 3 table
- …