380 research outputs found
On the statistical machinery of alien species
Many species of plants are found in regions to which they are alien and their
global distribution has been found to exhibit several remarkable
patterns,characterised by exponential functions of the kind that could arise
through versions of MacArthur's broken stick. We show here that these various
patterns are all quantitatively reproduced by a simple algorithm, in terms of a
single parameter- a single stick to be broken. This algorithm admits a
biological interpretation in terms of niche structures fluctuating with time
and productivity; with sites and species highly idiosyncratic. Technically,
this is an application of statistical mechanics to ecology quite different from
the familiar application to species abundance distributions.Comment: 14 pages, 3 figures. Follows and strengthens arXiv:1004.2271 .
Version 2 has 16 pages and 3 figures. It differs from the original version in
a revised and extended discussion of the biological aspects and a small
change in a parameter to improve agreement with dat
Jet velocity in SS433: its anti-correlation with precession-cone angle and dependence on orbital phase
We present a re-analysis of the optical spectroscopic data on SS433 from the
last quarter-century and demonstrate that these data alone contain systematic
and identifiable deviations from the traditional kinematic model for the jets:
variations in speed, which agree with our analysis of recent radio data; in
precession-cone angle and in phase. We present a simple technique for
separating out the jet speed from the angular properties of the jet axis,
assuming only that the jets are symmetric. With this technique, the archival
optical data reveal that the variations in jet speed and in precession-cone
angle are anti-correlated in the sense that when faster jet bolides are ejected
the cone opening angle is smaller. We also find speed oscillations as a
function of orbital phase.Comment: accepted by ApJ Letter
Maximal variance reduction for stochastic propagators with applications to the static quark spectrum
We study a new method -- maximal variance reduction -- for reducing the
variance of stochastic estimators for quark propagators. We find that while
this method is comparable to usual iterative inversion for light-light mesons,
a considerable improvement is achieved for systems containing at least one
infinitely heavy quark. Such systems are needed for heavy quark effective
theory. As an illustration of the effectiveness of the method we present
results for the masses of the ground state and excited states of
mesons and baryons. We compare these results with the experimental
spectra involving quarks.Comment: 31 pages with 7 postscript file
Mixing of scalar glueballs and flavour-singlet scalar mesons
We discuss in detail the extraction of hadronic mixing strengths from lattice
studies. We apply this to the mixing of a scalar glueball and a scalar meson in
the quenched approximation. We also measure correlations appropriate for
flavour-singlet scalar mesons using dynamical quark configurations from UKQCD.
This enables us to compare the results from the quenched study of the mixing
with the direct determination of the mixed spectrum. Improved methods of
evaluating the disconnected quark diagrams are also presented.Comment: 23 pages, 5 postscript figure
Recommended from our members
Structure-based inhibitors of amyloid beta core suggest a common interface with tau.
Alzheimer's disease (AD) pathology is characterized by plaques of amyloid beta (Aβ) and neurofibrillary tangles of tau. Aβ aggregation is thought to occur at early stages of the disease, and ultimately gives way to the formation of tau tangles which track with cognitive decline in humans. Here, we report the crystal structure of an Aβ core segment determined by MicroED and in it, note characteristics of both fibrillar and oligomeric structure. Using this structure, we designed peptide-based inhibitors that reduce Aβ aggregation and toxicity of already-aggregated species. Unexpectedly, we also found that these inhibitors reduce the efficiency of Aβ-mediated tau aggregation, and moreover reduce aggregation and self-seeding of tau fibrils. The ability of these inhibitors to interfere with both Aβ and tau seeds suggests these fibrils share a common epitope, and supports the hypothesis that cross-seeding is one mechanism by which amyloid is linked to tau aggregation and could promote cognitive decline
The Hyperfine Splitting in Charmonium: Lattice Computations Using the Wilson and Clover Fermion Actions
We compute the hyperfine splitting on the lattice,
using both the Wilson and -improved (clover) actions for quenched quarks.
The computations are performed on a lattice at ,
using the same set of 18 gluon configurations for both fermion actions. We find
that the splitting is 1.83\err{13}{15} times larger with the clover action than
with the Wilson action, demonstrating the sensitivity of the spin-splitting to
the magnetic moment term which is present in the clover action. However, even
with the clover action the result is less than half of the physical
mass-splitting. We also compute the decay constants and
, both of which are considerably larger when computed using
the clover action than with the Wilson action. For example for the ratio
we find 0.32\err{1}{2} with the Wilson action
and with the clover action (the physical value is 0.44(2)).Comment: LaTeX file, 8 pages and two postscript figures. Southampton Preprint:
SHEP 91/92-27 Edinburgh Preprint: 92/51
Symmetry in the changing jets of SS433 and its true distance from us
We present the deepest yet radio image of the Galactic jet source, SS433,
which reveals over two full precession cycles (> 2 x 163 days) of the jet axis.
Systematic and identifiable deviations from the traditional kinematical model
for the jets are found: variations in jet speed, lasting for as long as tens of
days, are necessary to match the detailed structure of each jet. It is
remarkable that these variations are equal and opposite, matching the two jets
simultaneously. This explains certain features of the correlated redshift
residuals found in fits to the kinematic model of SS433 reported in the
literature. Asymmetries in the image caused by light travel time enabled us to
measure the jet speeds of particular points to be within a range from 0.24c to
0.28c, consistent with, yet determined independently from, the speeds derived
from the famous moving optical emission lines. Taken together with the angular
periodicity of the zigzag/corkscrew structure projected on the plane of the sky
(produced by the precession of the jet axis), these measurements determine
beyond all reasonable doubt the distance to SS433 to be 5.5 +/- 0.2 kpc,
significantly different from the distance most recently inferred using neutral
hydrogen measurements together with the current rotation model for the Galaxy.Comment: accepted by ApJ Letter
Quenched Hadrons using Wilson and O(a)-Improved Fermion Actions at beta=6.2
We present the first study of the light hadron spectrum and decay constants
for quenched QCD using an O(a)-improved nearest-neighbour Wilson fermion action
at \beta=6.2. We compare the results with those obtained using the standard
Wilson fermion action, on the same set of 18 gauge field configurations of a
24^3 times 48 lattice. For pseudoscalar meson masses in the range 330-800 MeV,
we find no significant difference between the results for the two actions. The
scales obtained from the string tension and mesonic sector are consistent, but
differ from that derived from baryon masses. The ratio of the pseudoscalar
decay constant to the vector meson mass is roughly independent of quark mass as
observed experimentally, and in approximate agreement with the measured value.Comment: 11 page
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