6,903 research outputs found
Resistivity of non-Fermi liquid U2Pt2In under pressure
Non-Fermi liquid behaviour in single-crystalline U2Pt2In has been studied by
means of resistivity experiments (I||c) under hydrostatic pressure (P<1.5 GPa).
At ambient pressure the resistivity rho(T) follows a power law rho~T^alpha with
alpha~0.5. Upon applying pressure alpha increases. For P>1 GPa a minimum
develops in rho(T). A study of the field dependence of the minimum confirms its
magnetic origin. The ratio c/a is proposed as the effective control parameter,
rather than the unit cell volume.Comment: 5 pages (incl. 2 figures), submitted to SCES'99, Nagan
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Collins effect and single spin azimuthal asymmetries in the HERMES and COMPASS experiments
Predictions are made for single spin azimuthal asymmetries due to the Collins
effect in pion production from semi-inclusive deeply inelastic scattering off
transversely and longitudinally polarized targets for the HERMES and COMPASS
experiments. The x-dependence of the asymmetries is evaluated using the parton
distribution functions from the chiral quark-soliton model. The overall
normalization of the predicted asymmetries is determined by the information on
the Collins fragmentation function extracted from previous HERMES data on
azimuthal asymmetries from a longitudinally polarized target. The single spin
asymmetries from the transversely polarized proton target are found to be about
20% for positive and neutral pions both at HERMES and COMPASS. For a
longitudinally polarized target we obtain for COMPASS a sin(phi) asymmetry of
1% and a sin(2phi) asymmetry of about 3%.Comment: 10 pages, 9 figures, Ref.[29] adde
Dynamic compartmentalization of bacteria: accurate division in E. coli
Positioning of the midcell division plane within the bacterium E. coli is
controlled by the min system of proteins: MinC, MinD and MinE. These proteins
coherently oscillate from end to end of the bacterium. We present a
reaction--diffusion model describing the diffusion of min proteins along the
bacterium and their transfer between the cytoplasmic membrane and cytoplasm.
Our model spontaneously generates protein oscillations in good agreement with
experiments. We explore the oscillation stability, frequency and wavelength as
a function of protein concentration and bacterial length.Comment: 4 pages, 4 figures, Latex2e, Revtex
Azimuthal single spin asymmetries in SIDIS in the light of chiral symmetry breaking
An attempt is made to understand the z-dependence of the azimuthal single
spin asymmetries observed by the HERMES collaboration in terms of chiral models
based on effective quark and Goldstone boson degrees of freedom. The effects of
respectively neglecting and considering Gaussian intrinsic parton transverse
momenta and the Sivers effect are explored. Predictions for the transverse
target polarization experiment at HERMES are presented.Comment: 14 pages, 18 figure
Unification Picture in Minimal Supersymmetric SU(5) Model with String Remnants
The significant heavy threshold effect is found in the supersymmetric SU(5)
model with two adjoint scalars, one of which is interpreted as a massive string
mode decoupled from the lower-energy particle spectra. This threshold related
with the generic mass splitting of the basic adjoint moduli is shown to alter
properly the running of gauge couplings, thus giving a natural solution to the
string-scale grand unification as prescribed at low energies by LEP precision
measurements and minimal particle content. The further symmetry condition of
the (top-bottom) Yukawa and gauge coupling superunification at a string scale
results in the perfectly working predictions for the top and bottom quark
masses in the absence of any large supersymmetric threshold corrections.Comment: published versio
Electron correlation vs. stabilization: A two-electron model atom in an intense laser pulse
We study numerically stabilization against ionization of a fully correlated
two-electron model atom in an intense laser pulse. We concentrate on two
frequency regimes: very high frequency, where the photon energy exceeds both,
the ionization potential of the outer {\em and} the inner electron, and an
intermediate frequency where, from a ``single active electron''-point of view
the outer electron is expected to stabilize but the inner one is not. Our
results reveal that correlation reduces stabilization when compared to results
from single active electron-calculations. However, despite this destabilizing
effect of electron correlation we still observe a decreasing ionization
probability within a certain intensity domain in the high-frequency case. We
compare our results from the fully correlated simulations with those from
simpler, approximate models. This is useful for future work on ``real''
more-than-one electron atoms, not yet accessible to numerical {\em ab initio}
methods.Comment: 8 pages, 8 figures in an extra ps-file, submitted to Phys. Rev. A,
updated references and shortened introductio
Phenomenology of single spin asymmetries in p(transv. polarized)-p -> pion + X
A phenomenological description of single transverse spin effects in
hadron-hadron inclusive processes is proposed, assuming a generalized
factorization scheme and pQCD hard interactions. The transverse momentum, k_T,
of the quarks inside the hadrons and of the hadrons relatively to the
fragmenting quark, is taken into account in distribution and fragmentation
functions, and leads to possible non zero single spin asymmetries. The role of
k_T and spin dependent quark fragmentations -- the so-called Collins effect --
is investigated in details in p(transv. polarized)-p -> pion + X processes: it
is shown how the experimental data could be described, obtaining an explicit
expression for the spin asymmetry of a polarized fragmenting quark, on which
some comments are made. Predictions for other processes, possible further
applications and experimental tests are discussed.Comment: 20+1 pages, LaTeX, 6 eps figures, uses epsfig.sty. Version v2: Some
sentences rephrased and comments added throughout the paper; one reference
added; no changes in results and figures. Final version to be published in
Phys. Rev.
Accurate genotype imputation in multiparental populations from low-coverage sequence
Many different types of multiparental populations have recently been produced to increase genetic diversity and resolution in QTL mapping. Low-coverage, genotyping-by-sequencing (GBS) technology has become a cost-effective tool in these populations, despite large amounts of missing data in offspring and founders. In this work, we present a general statistical framework for genotype imputation in such experimental crosses from low-coverage GBS data. Generalizing a previously developed hidden Markov model for calculating ancestral origins of offspring DNA, we present an imputation algorithm that does not require parental data and that is applicable to bi-and multiparental populations. Our imputation algorithm allows heterozygosity of parents and offspring as well as error correction in observed genotypes. Further, our approach can combine imputation and genotype calling from sequencing reads, and it also applies to called genotypes from SNP array data. We evaluate our imputation algorithm by simulated and real data sets in four different types of populations: the F2, the advanced intercross recombinant inbred lines, the multiparent advanced generation intercross, and the cross-pollinated population. Because our approach uses marker data and population design information efficiently, the comparisons with previous approaches show that our imputation is accurate at even very low (< 1 Ă—) sequencing depth, in addition to having accurate genotype phasing and error detection.</p
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