3,418 research outputs found
Are there compact heavy four-quark bound states?
We present an exact method to study four-quark systems based on the
hyperspherical harmonics formalism. We apply it to several physical systems of
interest containing two heavy and two light quarks using different quark-quark
potentials. Our conclusions mark the boundaries for the possible existence of
compact, non-molecular, four-quark bound states. While states
may be stable in nature, the stability of states would imply
the existence of quark correlations not taken into account by simple quark
dynamical modelsComment: 10 pages, 1 figure. Accepted for publication in Phys. Rev.
Asteroids in the Inner Solar System II - Observable Properties
This paper presents synthetic observations of long-lived, coorbiting
asteroids of Mercury, Venus, the Earth and Mars. Our sample is constructed by
taking the limiting semimajor axes, differential longitudes and inclinations
for long-lived stability provided by simulations. The intervals are randomly
populated with values to create initial conditions. These orbits are
re-simulated to check that they are stable and then re-sampled every 2.5 years
for 1 million years. The Mercurian sample contains only horseshoe orbits, the
Martian sample only tadpoles. For both Venus and the Earth, the greatest
concentration of objects on the sky occurs close to the classical Lagrange
points at heliocentric ecliptic longitudes of 60 and 300 degrees. The
distributions are broad especially if horseshoes are present in the sample. The
full-width half maximum (FWHM) in heliocentric longitude for Venus is 325
degrees and for the Earth is 328 degrees. The mean and most common velocity of
these coorbiting satellites coincides with the mean motion of the parent
planet, but again the spread is wide with a FWHM for Venus of 27.8 arcsec/hr
and for the Earth of 21.0 arcsec/hr. For Mars, the greatest concentration on
the sky occurs at heliocentric ecliptic latitudes of 12 degrees. The peak of
the velocity distribution occurs at 65 arcsec/hr, significantly less than the
Martian mean motion, while its FWHM is 32.3 arcsec/hr. The case of Mercury is
the hardest of all, as the greatest concentration occurs at heliocentric
longitudes close to the Sun.Comment: 16 pages, 11 figures, Monthly Notices (in press). Higher quality
figures available at
http://www-thphys.physics.ox.ac.uk/users/WynEvans/home.htm
On the existence of exotic and non-exotic multiquark meson states
To obtain an exact solution of a four-body system containing two quarks and
two antiquarks interacting through two-body terms is a cumbersome task that has
been tackled with more or less success during the last decades. We present an
exact method for the study of four-quark systems based on the hyperspherical
harmonics formalism that allows us to solve it without resorting to further
approximations, like for instance the existence of diquark components. We apply
it to systems containing two heavy and two light quarks using different
quark-quark potentials. While states may be stable in nature,
the stability of states would imply the existence of quark
correlations not taken into account by simple quark dynamical models.Comment: 3 pages. Contribution to the 20th European Conference on Few-Body
Problems in Physics, Pisa, Italy. To be published in Few-Body system
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The helicase Ded1p controls use of near-cognate translation initiation codons in 5' UTRs.
The conserved and essential DEAD-box RNA helicase Ded1p from yeast and its mammalian orthologue DDX3 are critical for the initiation of translation1. Mutations in DDX3 are linked to tumorigenesis2-4 and intellectual disability5, and the enzyme is targeted by a range of viruses6. How Ded1p and its orthologues engage RNAs during the initiation of translation is unknown. Here we show, by integrating transcriptome-wide analyses of translation, RNA structure and Ded1p-RNA binding, that the effects of Ded1p on the initiation of translation are connected to near-cognate initiation codons in 5' untranslated regions. Ded1p associates with the translation pre-initiation complex at the mRNA entry channel and repressing the activity of Ded1p leads to the accumulation of RNA structure in 5' untranslated regions, the initiation of translation from near-cognate start codons immediately upstream of these structures and decreased protein synthesis from the corresponding main open reading frames. The data reveal a program for the regulation of translation that links Ded1p, the activation of near-cognate start codons and mRNA structure. This program has a role in meiosis, in which a marked decrease in the levels of Ded1p is accompanied by the activation of the alternative translation initiation sites that are seen when the activity of Ded1p is repressed. Our observations indicate that Ded1p affects translation initiation by controlling the use of near-cognate initiation codons that are proximal to mRNA structure in 5' untranslated regions
Bycatch in a Commercial Lobster Fishery: Effects on Two Benthic Predators, Sea Raven and Longhorn Sculpin
Studying the species‐specific responses to fishing capture is critical for effective management and conservation of bycatch species given that acute stress incurred from capture and handling may ultimately lead to mortality. While species of low commercial value are often overlooked, having accurate information on the effects of capture on all species is necessary for ecosystem‐based management. Sea Raven (SR) Hemitripterus americanus and Longhorn Sculpin (LHS) Myoxocephalus octodecemspinosus are routinely captured in the commercial American lobster Homarus americanus fishery in the Gulf of Maine, and they are discarded due to low commercial value. Despite a lack of economic value, these predatory species play important roles in shaping the benthic communities that they inhabit, highlighting the need to study their stress and mortality due to capture and handling. To help understand the effects of the lobster fishery on these species, the current study evaluated the physical, behavioral, and physiological stress responses of SR and LHS to capture in the state of Maine Zone G commercial lobster fishery. Collectively, our results suggest that although these species appeared to be resilient to capture based on an overt injury assessment, stress responses occurred based on reflex impairment and physiological perturbations, and these responses were species‐specific. Given the prevalence of behavioral and physiological stress in this study, further research into the survival outcomes of SR and LHS following release in the commercial lobster fishery is warranted
A Bima Array Survey of Molecules in Comets Linear (C/2002 T7) and Neat (C/2001 Q4)
We present an interferometric search for large molecules, including methanol,
methyl cyanide, ethyl cyanide, ethanol, and methyl formate in comets LINEAR
(C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland
Association (BIMA) array. In addition, we also searched for transitions of the
simpler molecules CS, SiO, HNC, HN13C and 13CO . We detected transitions of
methanol and CS around Comet LINEAR and one transition of methanol around Comet
NEAT within a synthesized beam of ~20''. We calculated the total column density
and production rate of each molecular species using the variable temperature
and outflow velocity (VTOV) model described by Friedel et al.(2005).Considering
the molecular production rate ratios with respect to water, Comet T7 LINEAR is
more similar to Comet Hale-Bopp while Comet Q4 NEAT is more similar to Comet
Hyakutake. It is unclear, however, due to such a small sample size, whether
there is a clear distinction between a Hale-Bopp and Hyakutake class of comet
or whether comets have a continuous range of molecular production rate ratios.Comment: Accepted for Publication in the Astrophysical Journa
The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol
We present an interferometric and single dish study of small organic species
toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA
interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp,
both the single-dish and interferometer observations of CH3OH indicate an
excitation temperature of 105+/-5 K and an average production rate ratio
Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis
observations of CH3OH suggest a distribution well described by a spherical
outflow and no evidence of significant extended emission. Single-dish
observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of
200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The
non-detection of a previously claimed transition of cometary (CH2OH)2 toward
Comet Hale-Bopp with the 12m telescope indicates a compact distribution of
emission, D<9'' (<8500 km). For the single-dish observations of Comet T7
LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH
production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support
current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear
species distributed into the coma via direct sublimation off cometary ices from
the nucleus with no evidence of significant production in the outer coma.Comment: accepted for publication in Ap
Density of States of Disordered Two-Dimensional Crystals with Half-Filled Band
A diagrammatic method is applied to study the effects of commensurability in
two-dimensional disordered crystalline metals by using the particle-hole
symmetry with respect to the nesting vector P_0={\pm{\pi}/a, {\pi}/a} for a
half-filled electronic band. The density of electronic states (DoS) is shown to
have nontrivial quantum corrections due to both nesting and elastic impurity
scattering processes, as a result the van Hove singularity is preserved in the
center of the band. However, the energy dependence of the DoS is strongly
changed. A small offset from the middle of the band gives rise to disappearence
of quantum corrections to the DoS .Comment: to be published in Physical Review Letter
Short Range Interaction Effects on the Density of States of Disordered Two Dimensional Crystals with a half--filled band
The Density of electronic States (DoS) of a two--dimensional square lattice
with substitutional impurities is calculated in the presence of short--range
electron--electron interactions. In the middle of the energy band, the Bragg
reflections off the Brillouin zone boundary are shown to lead to additional
quantum corrections to the DoS, the sign of which is opposite to the sign of
the Altshuler--Aronov's logarithmic correction. The resulting quantum
correction to the DoS at half--filling is positive, i.e. the DoS increases
logarithmically as the Fermi energy is approached. However, far from the
commensurate points where the Bragg reflections are suppressed, the negative
logarithmic corrections to the DoS survive.Comment: 5 pages 2 figure
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Epigenetic memory in induced pluripotent stem cells.
Somatic cell nuclear transfer and transcription-factor-based reprogramming revert adult cells to an embryonic state, and yield pluripotent stem cells that can generate all tissues. Through different mechanisms and kinetics, these two reprogramming methods reset genomic methylation, an epigenetic modification of DNA that influences gene expression, leading us to hypothesize that the resulting pluripotent stem cells might have different properties. Here we observe that low-passage induced pluripotent stem cells (iPSCs) derived by factor-based reprogramming of adult murine tissues harbour residual DNA methylation signatures characteristic of their somatic tissue of origin, which favours their differentiation along lineages related to the donor cell, while restricting alternative cell fates. Such an 'epigenetic memory' of the donor tissue could be reset by differentiation and serial reprogramming, or by treatment of iPSCs with chromatin-modifying drugs. In contrast, the differentiation and methylation of nuclear-transfer-derived pluripotent stem cells were more similar to classical embryonic stem cells than were iPSCs. Our data indicate that nuclear transfer is more effective at establishing the ground state of pluripotency than factor-based reprogramming, which can leave an epigenetic memory of the tissue of origin that may influence efforts at directed differentiation for applications in disease modelling or treatment
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