399 research outputs found
Band Electronic Structure of One- and Two-Dimensional Pentacene Molecular Crystals
We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Ă
d-spacing polymorph are noticeably larger than for the 14.5 Ă
d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.
Production of -pairs at HERA-
The production of -pairs as a possible measure of the polarized gluon
distribution is studied for proton--nucleon collisions at
\sqrt{s} =40\;\mbox{GeV}^2 (HERA-). Possibilities of
reconstructing the helicity state of at least one of the 's are
critically reviewed. The observation of production asymmetries in the single
polarized mode of HERA- is found to be not feasible.Comment: 8 pages, LATeX, 3 figures availabe as .uu-fil
Thresholds of biodiversity and ecosystem function in a forest ecosystem undergoing dieback
Ecological thresholds, which represent points of rapid change in ecological properties, are of major scientific and societal concern. However, very little research has focused on empirically testing the occurrence of thresholds in temperate terrestrial ecosystems. To address this knowledge gap, we tested whether a number of biodiversity, ecosystem functions and ecosystem condition metrics exhibited thresholds in response to a gradient of forest dieback, measured as changes in basal area of living trees relative to areas that lacked recent dieback. The gradient of dieback was sampled using 12 replicate study areas in a temperate forest ecosystem. Our results provide novel evidence of several thresholds in biodiversity (namely species richness of ectomycorrhizal fungi, epiphytic lichen and ground flora); for ecological condition (e.g. sward height, palatable seedling abundance) and a single threshold
for ecosystem function (i.e. soil respiration rate). Mechanisms for these thresholds are explored. As climate-induced forest dieback is increasing worldwide, both in scale and speed, these results imply that threshold responses may become increasingly widespread
Spin-dependent Parton Distributions from Polarized Structure Function Data
In the past year, polarized deep inelastic scattering experiments at CERN and
SLAC have obtained structure function measurements off proton, neutron and
deuteron targets at a level of precision never before achieved. The
measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and
also to obtain information on the parton distributions in polarized nucleons.
We perform a global leading-order QCD fit to the proton deep inelastic data in
order to extract the spin-dependent parton distributions. By using parametric
forms which are consistent with theoretical expectations at large and small
, we find that the quark distributions are now rather well constrained. We
assume that there is no significant intrinsic polarization of the strange quark
sea. The data are then consistent with a modest amount of the proton's spin
carried by the gluon, although the shape of the gluon distribution is not well
constrained, and several qualitatively different shapes are suggested. The
spin-dependent distributions we obtain can be used as input to phenomenological
studies for future polarized hadron-hadron and lepton-hadron colliders.Comment: 23 pages, DTP/94/3
QCD Corrections to Spin Dependent Drell-Yan and a Global Subtraction Scheme
We present QCD corrections to the Drell-Yan process in the transversely
polarized, longitudinally polarized and unpolarized cases. The analytical
results are presented in a form valid for all -dimensional regularization
schemes. A universal mass factorization scheme is presented in which the
results reduce to those of dimensional reduction. The connection between the
parton distributions and fragmentation functions of dimensional reduction and
those of dimensional regularization is elucidated in a simple manner. Numerical
results are presented for proton-proton collisions at energies relevant to RHIC
(Relativistic Heavy Ion Collider). The perturbative stability of the transverse
and longitudinal asymmetries is investigated.Comment: 18 pages, 8 figures in 1 postscript fil
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
Systems genetics identifies a role for Cacna2d1 regulation in elevated intraocular pressure and glaucoma susceptibility
Glaucoma is a multi-factorial blinding disease in which genetic factors play an important role. Elevated intraocular pressure is a highly heritable risk factor for primary open angle glaucoma and currently the only target for glaucoma therapy. Our study helps to better understand underlying genetic and molecular mechanisms that regulate intraocular pressure, and identifies a new candidate gene, Cacna2d1, that modulates intraocular pressure and a promising therapeutic, pregabalin, which binds to CACNA2D1 protein and lowers intraocular pressure significantly. Because our study utilizes a genetically diverse population of mice with kno
Polarized Parton Distributions in the Nucleon
The distribution of the spin of the nucleon among its constituents can be
parametrized in the form of polarized parton distribution functions for quarks
and gluons. Using all available data on the polarized structure function
, we determine these distributions both at leading and
next-to-leading order in perturbation theory. We suggest three different,
equally possible scenarios for the polarized gluon distribution, which is found
to be only loosely constrained by current experimental data. We examine various
possibilities of measuring polarized parton distributions at future
experiments.Comment: 18 pages, LATEX, 6 figures available as .uu fil
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