30,724 research outputs found
Diffraction and the Pomeron
Recent experimental results on inclusive diffractive scattering and on
exclusive vector meson production are reviewed. The dynamical picture of hard
diffraction emerging in perturbative QCD is highlighted.Comment: 25 pages, 21 postscript figures, contribution to the XIX
International Symposium on Lepton and Photon Interactions at High Energies,
Stanford University, August 9-14, 199
Next-to-Leading Order Hard Scattering Using Fully Unintegrated Parton Distribution Functions
We calculate the next-to-leading order fully unintegrated hard scattering
coefficient for unpolarized gluon-induced deep inelastic scattering using the
logical framework of parton correlation functions developed in previous work.
In our approach, exact four-momentum conservation is maintained throughout the
calculation. Hence, all non-perturbative functions, like parton distribution
functions, depend on all components of parton four-momentum. In contrast to the
usual collinear factorization approach where the hard scattering coefficient
involves generalized functions (such as Dirac -functions), the fully
unintegrated hard scattering coefficient is an ordinary function. Gluon-induced
deep inelastic scattering provides a simple illustration of the application of
the fully unintegrated factorization formalism with a non-trivial hard
scattering coefficient, applied to a phenomenologically interesting case.
Furthermore, the gluon-induced process allows for a parameterization of the
fully unintegrated gluon distribution function.Comment: 22 pages, Typos Fixed, Reference Added, Minor Clarification Adde
Fully Unintegrated Parton Correlation Functions and Factorization in Lowest Order Hard Scattering
Motivated by the need to correct the potentially large kinematic errors in
approximations used in the standard formulation of perturbative QCD, we
reformulate deeply inelastic lepton-proton scattering in terms of gauge
invariant, universal parton correlation functions which depend on all
components of parton four-momentum. Currently, different hard QCD processes are
described by very different perturbative formalisms, each relying on its own
set of kinematical approximations. In this paper we show how to set up
formalism that avoids approximations on final-state momenta, and thus has a
very general domain of applicability. The use of exact kinematics introduces a
number of significant conceptual shifts already at leading order, and tightly
constrains the formalism. We show how to define parton correlation functions
that generalize the concepts of parton density, fragmentation function, and
soft factor. After setting up a general subtraction formalism, we obtain a
factorization theorem. To avoid complications with Ward identities the full
derivation is restricted to abelian gauge theories; even so the resulting
structure is highly suggestive of a similar treatment for non-abelian gauge
theories.Comment: 44 pages, 69 figures typos fixed, clarifications and second appendix
adde
Proton configurations in the hydrogen bonds of KH2PO4 as seen by resonant x-ray diffraction
KH2PO4 (KDP) belongs to the class of hydrogen-bonded ferroelectrics, whose
paraelectric to ferroelectric phase transition is driven by the ordering of the
protons in the hydrogen bonds. We demonstrate that forbidden reflections of
KDP, when measured at an x-ray absorption edge, are highly sensitive to the
asymmetry of proton configurations. The change of average symmetry caused by
the "freezing" of the protons during the phase transition is clearly evidenced.
In the paraelectric phase, we identify in the resonant spectra of the forbidden
reflections a contribution related to the transient proton configurations in
the hydrogen bonds, which violates the high average symmetry of the sites of
the resonant atoms. The analysis of the temperature dependence reveals a change
of relative probabilities of the different proton configurations. They follow
the Arrhenius law, and the activation energies of polar and Slater
configurations are 18.6 and 7.3 meV, respectively
CYCD3 D-type cyclins regulate cambial cell proliferation and secondary growth inArabidopsis
A major proportion of plant biomass is derived from the activity of the cambium, a lateral meristem responsible for vascular tissue formation and radial organ enlargement in a process termed secondary growth. In contrast to our relatively good understanding of the regulation of primary meristems, remarkably little is known concerning the mechanisms controlling secondary growth, particularly how cambial cell divisions are regulated and integrated with vascular differentiation. A genetic loss-of-function approach was used here to reveal a rate-limiting role for the Arabidopsis CYCLIN D3 (CYCD3) subgroup of cell-cycle genes in the control of cambial cell proliferation and secondary growth, providing conclusive evidence of a direct link between the cell cycle and vascular development. It is shown that all three CYCD3 genes are specifically expressed in the cambium throughout vascular development. Analysis of a triple loss-of-function CYCD3 mutant revealed a requirement for CYCD3 in promoting the cambial cell cycle since mutant stems and hypocotyls showed a marked reduction in diameter linked to reduced mitotic activity in the cambium. Conversely, loss of CYCD3 provoked an increase in xylem cell size and the expression of differentiation markers, showing that CYCD3 is required to restrain the differentiation of xylem precursor cells. Together, our data show that tight control of cambial cell division through developmental- and cell type-specific regulation of CYCD3 is required for normal vascular development, constituting part of a novel mechanism controlling organ growth in higher plants
Asymptotic behavior of double parton distribution functions
The double parton distribution functions are investigated in the region of
small longitudinal momentum fractions in the leading logarithm approximation of
perturbative QCD. It is shown that these functions have the factorization
property in the case of one slow and one fast parton.Comment: 7 pages, revtex
Renormalization constants and beta functions for the gauge couplings of the Standard Model to three-loop order
We compute the beta functions for the three gauge couplings of the Standard
Model in the minimal subtraction scheme to three loops. We take into account
contributions from all sectors of the Standard Model. The calculation is
performed using both Lorenz gauge in the unbroken phase of the Standard Model
and background field gauge in the spontaneously broken phase. Furthermore, we
describe in detail the treatment of and present the automated setup
which we use for the calculation of the Feynman diagrams. It starts with the
generation of the Feynman rules and leads to the bare result for the Green's
function of a given process.Comment: 44 pages, 9 figures; v2: sign in eq.(29) corrected; final result
unchange
Fractional Systems and Fractional Bogoliubov Hierarchy Equations
We consider the fractional generalizations of the phase volume, volume
element and Poisson brackets. These generalizations lead us to the fractional
analog of the phase space. We consider systems on this fractional phase space
and fractional analogs of the Hamilton equations. The fractional generalization
of the average value is suggested. The fractional analogs of the Bogoliubov
hierarchy equations are derived from the fractional Liouville equation. We
define the fractional reduced distribution functions. The fractional analog of
the Vlasov equation and the Debye radius are considered.Comment: 12 page
Stirring Unmagnetized Plasma
A new concept for spinning unmagnetized plasma is demonstrated
experimentally. Plasma is confined by an axisymmetric multi-cusp magnetic field
and biased cathodes are used to drive currents and impart a torque in the
magnetized edge. Measurements show that flow viscously couples momentum from
the magnetized edge (where the plasma viscosity is small) into the unmagnetized
core (where the viscosity is large) and that the core rotates as a solid body.
To be effective, collisional viscosity must overcome the ion-neutral drag due
to charge exchange collisions
Correlation of the Havemeyer endoscopic laryngeal grading system with histopathological changes in equine Cricoarytenoideus dorsalis muscles
The establishment of a single validated endoscopic laryngeal grading system for assessing recurrent laryngeal neuropathy (RLN) is desirable to facilitate direct comparisons between the findings of different clinical and research groups worldwide. The objective of this study was to assess the relationship between the Havemeyer endoscopic laryngeal grading system and histopathological changes consistent with RLN in the left cricoarytenoideus dorsalis (CAD) muscle of horses of different breeds with a full range of clinical severities of RLN, i.e., from normal endoscopic laryngeal function to complete laryngeal hemiplegia. Endoscopic grading of laryngeal function of 22 horses was performed using the Havemeyer endoscopic laryngeal grading system. A biopsy sample of the left CAD muscle was obtained from each horse, either at post mortem examination (n = 16), or during routine laryngoplasty surgery (n = 6). A semi-quantitative histopathological scoring system was used to grade the severity of histopathological lesions consistent with RLN in the left CAD muscle of each horse. A significant positive correlation (rs = 0.705, p < 0.001) was found between the Havemeyer grades and sub-grades of laryngeal function and the semi-quantitative assessment of histopathological lesions consistent with RLN in the left CAD muscle. However, a wide spread of muscle histopathological scores was obtained, particularly from horses with Havemeyer sub-grades II.1, III.1 and III.2. In conclusion, the Havemeyer endoscopic laryngeal grading system was found to broadly correlate with histopathological changes consistent with RLN in equine cricoarytenoideus dorsalis muscle
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