2,358 research outputs found
Improved Covariance Model Parameter Estimation Using RNA Thermodynamic Properties
Covariance models are a powerful description of non-coding RNA (ncRNA) families that can be used to search nucleotide databases for new members of these ncRNA families. Currently, estimation of the parameters of a covariance model (state transition and emission scores) is based only on the observed frequencies of mutations, insertions, and deletions in known ncRNA sequences. For families with very few known members, this can result in rather uninformative models where the consensus sequence has a good score and most deviations from consensus have a fairly uniform poor score. It is proposed here to combine the traditional observed-frequency information with known information about free energy changes in RNA helix formation and loop length changes. More thermodynamically probable deviations from the consensus sequence will then be favored in database search. The thermodynamic information may be incorporated into the models as informative priors that depend on neighboring consensus nucleotides and on loop lengths
The nucleon spin and momentum decomposition using lattice QCD simulations
We determine within lattice QCD, the nucleon spin carried by valence and sea
quarks, and gluons. The calculation is performed using an ensemble of gauge
configurations with two degenerate light quarks with mass fixed to
approximately reproduce the physical pion mass. We find that the total angular
momentum carried by the quarks in the nucleon is and the gluon contribution is giving a total of consistent with the spin sum. For the quark intrinsic spin contribution
we obtain . All quantities are given in the scheme at
2~GeV. The quark and gluon momentum fractions are also computed and add up to
satisfying the momentum sum.Comment: Version published in PR
Universal distribution of threshold forces at the depinning transition
We study the distribution of threshold forces at the depinning transition for
an elastic system of finite size, driven by an external force in a disordered
medium at zero temperature. Using the functional renormalization group (FRG)
technique, we compute the distribution of pinning forces in the quasi-static
limit. This distribution is universal up to two parameters, the average
critical force, and its width. We discuss possible definitions for threshold
forces in finite-size samples. We show how our results compare to the
distribution of the latter computed recently within a numerical simulation of
the so-called critical configuration.Comment: 12 pages, 7 figures, revtex
Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation
Using the low-energy effective field theory for magnons and holes -- the
condensed matter analog of baryon chiral perturbation theory for pions and
nucleons in QCD -- we study different phases of doped antiferromagnets. We
systematically investigate configurations of the staggered magnetization that
provide a constant background field for doped holes. The most general
configuration of this type is either constant itself or it represents a spiral
in the staggered magnetization. Depending on the values of the low-energy
parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is
energetically favored. The reduction of the staggered magnetization upon doping
is also investigated.Comment: 35 pages, 5 figure
Characterization of CoRoT target fields with BEST: Identification of periodic variable stars in the IR01 field
We report on observations of the CoRoT IR01 field with the Berlin Exoplanet
Search Telescope (BEST). BEST is a small aperture telescope with a wide field
of view (FOV). It is dedicated to search for variable stars within the target
fields of the CoRoT space mission to aid in minimizing false-alarm rates and
identify potential targets for additional science. CoRoT's observational
programm started in February 2007 with the "initial run" field (IR01) observed
for about two months. BEST observed this field for 12 nights spread over three
months in winter 2006. From the total of 30426 stars observed in the IR01 field
3769 were marked as suspected variable stars and 54 from them showed clear
periodicity. From these 19 periodic stars are within the part of the CoRoT FOV
covered in our data set
Two Higgs Doublet Model and Lepton Polarization in the B -> K tau+ tau- Decay
The decay width, forward-backward asymmetry and tau lepton longitudinal and
transversal polarization for the exclusive (B -> K tau^+ tau^-) decay in a two
Higgs doublet model are computed. It is shown that the forward-backward
asymmetry and longitudinal polarization of the tau lepton are very effective
tools for establishing new physics.Comment: 25 pages, 20 figures, LaTeX formatte
Stability and distortions of liquid crystal order in a cell with a heterogeneous substrate
We study stability and distortions of liquid crystal nematic order in a cell
with a random heterogeneous substrate. Modeling this system as a bulk xy model
with quenched disorder confined to a surface, we find that nematic order is
marginally unstable to such surface pinning. We compute the length scale beyond
which nematic distortions become large and calculate orientational correlation
functions using the functional renormalization-group and matching methods,
finding universal logarithmic and double-logarithmic distortions in two and
three dimensions, respectively. We extend these results to a finite-thickness
liquid crystal cell with a second homogeneous substrate, detailing crossovers
as a function of random pinning strength and cell thickness. We conclude with
analysis of experimental signatures of these distortions in a conventional
crossed-polarizer-analyzer light microscopy.Comment: 27 pages, 15 figures, Published in PRE, with minor typos correcte
Measuring functional renormalization group fixed-point functions for pinned manifolds
Exact numerical minimization of interface energies is used to test the
functional renormalization group (FRG) analysis for interfaces pinned by
quenched disorder. The fixed-point function R(u) (the correlator of the
coarse-grained disorder) is computed. In dimensions D=d+1, a linear cusp in
R''(u) is confirmed for random bond (d=1,2,3), random field (d=0,2,3), and
periodic (d=2,3) disorders. The functional shocks that lead to this cusp are
seen. Small, but significant, deviations from 1-loop FRG results are compared
to 2-loop corrections. The cross-correlation for two copies of disorder is
compared with a recent FRG study of chaos.Comment: 4 pages, 4 figure
Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet
We present a numerical study of the spin-1/2 bilayer Heisenberg
antiferromagnet with random interlayer dimer dilution. From the temperature
dependence of the uniform susceptibility and a scaling analysis of the spin
correlation length we deduce the ground state phase diagram as a function of
nonmagnetic impurity concentration p and bilayer coupling g. At the site
percolation threshold, there exists a multicritical point at small but nonzero
bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer
material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be
controlled by the proximity to this new quantum critical point.Comment: minor changes, updated figure
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