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Inference of the Distribution of Selection Coefficients for New Nonsynonymous Mutations Using Large Samples.
The distribution of fitness effects (DFE) has considerable importance in population genetics. To date, estimates of the DFE come from studies using a small number of individuals. Thus, estimates of the proportion of moderately to strongly deleterious new mutations may be unreliable because such variants are unlikely to be segregating in the data. Additionally, the true functional form of the DFE is unknown, and estimates of the DFE differ significantly between studies. Here we present a flexible and computationally tractable method, called Fitâaâi, to estimate the DFE of new mutations using the site frequency spectrum from a large number of individuals. We apply our approach to the frequency spectrum of 1300 Europeans from the Exome Sequencing Project ESP6400 data set, 1298 Danes from the LuCamp data set, and 432 Europeans from the 1000 Genomes Project to estimate the DFE of deleterious nonsynonymous mutations. We infer significantly fewer (0.38-0.84 fold) strongly deleterious mutations with selection coefficient |s| > 0.01 and more (1.24-1.43 fold) weakly deleterious mutations with selection coefficient |s| < 0.001 compared to previous estimates. Furthermore, a DFE that is a mixture distribution of a point mass at neutrality plus a gamma distribution fits better than a gamma distribution in two of the three data sets. Our results suggest that nearly neutral forces play a larger role in human evolution than previously thought
Finite-volume two-pion energies and scattering in the quenched approximation
We investigate how L\"uscher's relation between the finite-volume energy of
two pions at rest and pion scattering lengths has to be modified in quenched
QCD. We find that this relation changes drastically, and in particular, that
``enhanced finite-volume corrections" of order and occur at
one loop ( is the linear size of the box), due to the special properties of
the in the quenched approximation. We define quenched pion scattering
lengths, and show that they are linearly divergent in the chiral limit. We
estimate the size of these various effects in some numerical examples, and find
that they can be substantial.Comment: 22 pages, uuencoded, compressed postscript fil
Quenched Chiral Perturbation Theory for Heavy-light Mesons
We formulate quenched chiral perturbation theory for heavy-light mesons
coupled to pions, and calculate the one-loop chiral logarithmic corrections to
, , and . We also calculate these corrections
for ``partially quenched'' theories. In both theories, the chiral logarithms
diverge in the chiral limit, indicating that (partially) quenched theories
should not be used to study this limit. Comparing the chiral logarithms to
those in QCD, we estimate the errors caused by (partial) quenching. By forming
suitable ratios, we can reduce the uncertainties in our estimates.Comment: 22 pages, revtex format, 5 Postscript figure
Thermodynamics of Lattice QCD with Chiral 4-Fermion Interactions
We have studied lattice QCD with an additional, irrelevant 4-fermion
interaction having a U(1)xU(1) chiral symmetry, at finite temperatures. Adding
this 4-fermion term allowed us to work at zero quark mass, which would have
otherwise been impossible. The theory with 2 massless staggered quark flavours
appears to have a first order finite temperature phase transition at N_t=4 for
the value of 4-fermion coupling we have chosen, in contrast to what is expected
for 2-flavour QCD. The pion screening mass is seen to vanish below this
transition, only to become massive and degenerate with the sigma (f_0) above
this transition where the chiral symmetry is restored, as is seen by the
vanishing of the chiral condensate.Comment: 23 pages, 11 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
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
A Lattice Study of the Nucleon Excited States with Domain Wall Fermions
We present results of our numerical calculation of the mass spectrum for
isospin one-half and spin one-half non-strange baryons, i.e. the ground and
excited states of the nucleon, in quenched lattice QCD. We use a new lattice
discretization scheme for fermions, domain wall fermions, which possess almost
exact chiral symmetry at non-zero lattice spacing. We make a systematic
investigation of the negative-parity spectrum by using two distinct
interpolating operators at on a
lattice. The mass estimates extracted from the two operators are consistent
with each other. The observed large mass splitting between this state,
, and the positive-parity ground state, the nucleon N(939), is well
reproduced by our calculations. We have also calculated the mass of the first
positive-parity excited state and found that it is heavier than the
negative-parity excited state for the quark masses studied.Comment: 46 pages, REVTeX, 11 figures included, revised version accepted for
publication in Phys. Rev.
Monte Carlo study of the hull distribution for the q=1 Brauer model
We study a special case of the Brauer model in which every path of the model
has weight q=1. The model has been studied before as a solvable lattice model
and can be viewed as a Lorentz lattice gas. The paths of the model are also
called self-avoiding trails. We consider the model in a triangle with boundary
conditions such that one of the trails must cross the triangle from a corner to
the opposite side. Motivated by similarities between this model, SLE(6) and
critical percolation, we investigate the distribution of the hull generated by
this trail (the set of points on or surrounded by the trail) up to the hitting
time of the side of the triangle opposite the starting point. Our Monte Carlo
results are consistent with the hypothesis that for system size tending to
infinity, the hull distribution is the same as that of a Brownian motion with
perpendicular reflection on the boundary.Comment: 21 pages, 9 figure
Exact Large Deviation Function in the Asymmetric Exclusion Process
By an extension of the Bethe ansatz method used by Gwa and Spohn, we obtain
an exact expression for the large deviation function of the time averaged
current for the fully asymmetric exclusion process in a ring containing
sites and particles. Using this expression we easily recover the exact
diffusion constant obtained earlier and calculate as well some higher
cumulants. The distribution of the deviation of the average current is, in
the limit , skew and decays like for and for . Surprisingly, the
large deviation function has an expression very similar to the pressure (as a
function of the density) of an ideal Bose or Fermi gas in .Comment: 8 pages, in ReVTeX, e-mail addresses: [email protected] and
[email protected]
Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud
The Magellanic Mopra Assessment (MAGMA) is a high angular resolution CO
mapping survey of giant molecular clouds (GMCs) in the Large and Small
Magellanic Clouds using the Mopra Telescope. Here we report on the basic
physical properties of 125 GMCs in the LMC that have been surveyed to date. The
observed clouds exhibit scaling relations that are similar to those determined
for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO
luminosities than Galactic clouds of a similar size. The average mass surface
density of the LMC clouds is 50 Msol/pc2, approximately half that of GMCs in
the inner Milky Way. We compare the properties of GMCs with and without signs
of massive star formation, finding that non-star-forming GMCs have lower peak
CO brightness than star-forming GMCs. We compare the properties of GMCs with
estimates for local interstellar conditions: specifically, we investigate the
HI column density, radiation field, stellar mass surface density and the
external pressure. Very few cloud properties demonstrate a clear dependence on
the environment; the exceptions are significant positive correlations between
i) the HI column density and the GMC velocity dispersion, ii) the stellar mass
surface density and the average peak CO brightness, and iii) the stellar mass
surface density and the CO surface brightness. The molecular mass surface
density of GMCs without signs of massive star formation shows no dependence on
the local radiation field, which is inconsistent with the
photoionization-regulated star formation theory proposed by McKee (1989). We
find some evidence that the mass surface density of the MAGMA clouds increases
with the interstellar pressure, as proposed by Elmegreen (1989), but the
detailed predictions of this model are not fulfilled once estimates for the
local radiation field, metallicity and GMC envelope mass are taken into
account.Comment: 28 pages, 10 figures, accepted by MNRA
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