231 research outputs found
Hemisphere-scale differences in conifer evolutionary dynamics
Fundamental differences in the distribution of oceans and landmasses in the Northern and Southern Hemispheres potentially impact patterns of biological diversity in the two areas. The evolutionary history of conifers provides an opportunity to explore these dynamics, because the majority of extant conifer species belong to lineages that have been broadly confined to the Northern or Southern Hemisphere during the Cenozoic. Incorporating genetic information with a critical review of fossil evidence, we developed an age-calibrated phylogeny sampling ∼80% of living conifer species. Most extant conifer species diverged recently during the Neogene within clades that generally were established during the later Mesozoic, but lineages that diversified mainly in the Southern Hemisphere show a significantly older distribution of divergence ages than their counterparts in the Northern Hemisphere. Our tree topology and divergence times also are best fit by diversification models in which Northern Hemisphere conifer lineages have higher rates of species turnover than Southern Hemisphere lineages. The abundance of recent divergences in northern clades may reflect complex patterns of migration and range shifts during climatic cycles over the later Neogene leading to elevated rates of speciation and extinction, whereas the scattered persistence of mild, wetter habitats in the Southern Hemisphere may have favored the survival of older lineages
O(a^2) cutoff effects in lattice Wilson fermion simulations
In this paper we propose to interpret the large discretization artifacts
affecting the neutral pion mass in maximally twisted lattice QCD simulations as
O(a^2) effects whose magnitude is roughly proportional to the modulus square of
the (continuum) matrix element of the pseudoscalar density operator between
vacuum and one-pion state. The numerical size of this quantity is determined by
the dynamical mechanism of spontaneous chiral symmetry breaking and turns out
to be substantially larger than its natural magnitude set by the value of
Lambda_QCD.Comment: 38 pages, 1 figure, 2 table
On the Possibility of Large Axion Decay Constants
The decay constant of the QCD axion is required by observation to be small
compared to the Planck scale. In theories of "natural inflation," and certain
proposed anthropic solutions of the cosmological constant problem, it would be
interesting to obtain a large decay constant for axion-like fields from
microscopic physics. String theory is the only context in which one can
sensibly address this question. Here we survey a number of periodic fields in
string theory in a variety of string vacua. In some examples, the decay
constant can be parameterically larger than the Planck scale but the effective
action then contains appreciable harmonics of order . As a result,
these fields are no better inflaton candidates than Planck scale axions.Comment: 17 pages, no figures, minor change mad
Hadron Spectrum with Wilson fermions
We present results of a high statistics study of the quenched spectrum using
Wilson fermions at on lattices. We calculate the
masses of mesons and baryons composed of both degenerate and non-degenerate
quarks. Using non-degenerate quark combinations allows us to study baryon mass
splittings in detail. We find significant deviations from the lowest order
chiral expansion, deviations that are consistent with the expectations of
quenched chiral perturbation theory. We find that there is a
systematic error in the extracted value of , depending on the meson mass
ratio used to set its value. Using the largest estimate of we find that
the extrapolated octet mass-splittings are in agreement with the experimental
values, as is , while the decuplet splittings are 30% smaller
than experiment. Combining our results with data from the GF11 collaboration we
find considerable ambiguity in the extrapolation to the continuum limit. Our
preferred values are and , suggesting that the quenched approximation is good to only . We also analyze the discretization errors in heavy quark
masses.Comment: 52 pages. Tex. Modified "axis" source for figures also included.
Needs macro packages lanlmac and epsf. Uses hyperbasics if available.
Significant number of typographical errors correcte
Annihilation, Rescattering, and CP Asymmetries in B Meson Decays
A number of meson decays may proceed only through participation of the
spectator quark, whether through amplitudes proportional to or via
rescattering from other less-suppressed amplitudes. An expected hierarchy of
amplitudes in the absence of rescattering will be violated by rescattering
corrections. Such violations could point the way toward channels in which
final-state interactions could be important. Cases in which final state phases
can lead to large CP asymmetries are pointed out.Comment: 9 page
Precision Prediction for the Big-Bang Abundance of Primordial Helium
Within the standard models of particle physics and cosmology we have
calculated the big-bang prediction for the primordial abundance of \he to a
theoretical uncertainty of less than 0.1 \pct ,
improving the current theoretical precision by a factor of 10. At this accuracy
the uncertainty in the abundance is dominated by the experimental uncertainty
in the neutron mean lifetime, . The following
physical effects were included in the calculation: the zero and
finite-temperature radiative, Coulomb and finite-nucleon-mass corrections to
the weak rates; order- quantum-electrodynamic correction to the plasma
density, electron mass, and neutrino temperature; and incomplete neutrino
decoupling. New results for the finite-temperature radiative correction and the
QED plasma correction were used. In addition, we wrote a new and independent
nucleosynthesis code designed to control numerical errors to be less than
0.1\pct. Our predictions for the \EL[4]{He} abundance are presented in the form
of an accurate fitting formula. Summarizing our work in one number, . Further,
the baryon density inferred from the Burles-Tytler determination of the
primordial D abundance, , leads to the
prediction: . This ``prediction'' and an accurate measurement of the primeval \he
abundance will allow an important consistency test of primordial
nucleosynthesis.Comment: Replaced fitting formulas - new versions differ by small but
significant amount. Other minor changes. 30 pages, 17 figures, 5 table
Effective chiral Lagrangians for spin-1 mesons
The commonly used types of effective theory for vector mesons are reviewed
and their relationships clarified. They are shown to correspond to different
choices of field for spin-1 particles and the rules for transforming between
them are described. The importance of respecting chiral symmetry is stressed.
The choice of fields that transform homogeneously under the nonlinear
realisation of chiral symmetry imposes no preconceptions about the types of
coupling for the mesons. This representation thus provides a convenient
framework for relating different theories. It is also used to elucidate the
nature of the assumptions in specific hidden-gauge and massive Yang-Mills
models that have been widely used.Comment: 46 pages (RevTeX
The Distribution and Origins of Ancient Leprosy
Human leprosy is primarily caused by Mycobacterium leprae, but also by the related ‘M. lepromatosis’. Ancient leprosy can be recognised in archaeological materials by the paleopathology associated with multi-bacillary or lepromatous forms of the disease. Whole M. leprae genomes have been obtained from human skeletons, and diagnostic aDNA fragments have been recovered. The derived M. leprae phylogenies, based on single nucleotide polymorphisms, mirror past human migrations, as M. leprae is usually an obligate pathogen. The detection of M. leprae in historical leprosy cases is assisted by the hydrophobic M. leprae cell envelope, which is composed of unusual lipids that can be used as specific biomarkers. Lipid biomarkers are more stable than aDNA and can be detected directly without amplification. Indigenous human leprosy is extinct in Western Europe, but recently, both M. leprae and ‘M. lepromatosis’ were found in British red squirrels. Leprosy may also be found in nine-banded armadillos (Dasypus novemcinctus) where it can cause a zoonotic human infection. Certain leprosy-like diseases, caused by uncultivable species in cats, for example, may be related to M. leprae. The closest extant relatives of leprosy bacilli are probably members of the M. haemophilum taxon, emerging pathogens with genomic and lipid biomarker similarities
QCD Sum Rules for the Isospin-Breaking Axial Correlator with Correct Chiral Behavior
We revisit the QCD sum-rule treatment of the isospin-breaking axial
correlator in light of the recent claim that a previous treatment produced
results incompatible with known chiral constraints. The source of the error in
the previous analysis is identified, and a corrected version of the sum-rule
treatment obtained. It is then shown that, using input from chiral perturbation
theory, one may use the resulting sum rule to extract information on the
leading chiral behavior of isospin-breaking parameters associated with the
coupling of excited pseudoscalar resonances to the axial currents. A rather
accurate extraction is possible for the case of the eta'. Demanding stability
of the sum-rule analysis also allows us to improve the upper bound on the
fourth-order low-energy constant, L_7.Comment: 20 pages (RevTeX), 1 figure (epsf
The Brain Activity Map
Neuroscientists have made impressive advances in understanding the microscale function of single neurons and the macroscale activity of the human brain. One can probe molecular and biophysical aspects of individual neurons and also view the human brain in action with magnetic resonance imaging (MRI) or magnetoencephalography (MEG). However, the mechanisms of perception, cognition, and action remain mysterious because they emerge from the real-time interactions of large sets of neurons in densely interconnected, widespread neural circuits
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