1,524 research outputs found
The Overlap Representation of Skewed Quark and Gluon Distributions
Within the framework of light-cone quantisation we derive the complete and
exact overlap representation of skewed parton distributions for unpolarised and
polarised quarks and gluons. Symmetry properties and phenomenological
applications are discussed.Comment: LaTex, 36 pages. v2: incorrect paper attached originally. v3: erratum
adde
Soft-collinear effective theory and heavy-to-light currents beyond leading power
An important unresolved question in strong interaction physics concerns the
parameterization of power-suppressed long-distance effects to hard processes
that do not admit an operator product expansion (OPE). Recently Bauer et al.\
have developed an effective field theory framework that allows one to formulate
the problem of soft-collinear factorization in terms of fields and operators.
We extend the formulation of soft-collinear effective theory, previously worked
out to leading order, to second order in a power series in the inverse of the
hard scale. We give the effective Lagrangian and the expansion of ``currents''
that produce collinear particles in heavy quark decay. This is the first step
towards a theory of power corrections to hard processes where the OPE cannot be
used. We apply this framework to heavy-to-light meson transition form factors
at large recoil energy.Comment: 46 pages, LaTeX; v2: two references added, eq. (52) correcte
Proton mass effects in wide-angle Compton scattering
We investigate proton mass effects in the handbag approach to wide-angle
Compton scattering. We find that theoretical uncertainties due to the proton
mass are significant for photon energies presently studied at Jefferson Lab.
With the proposed energy upgrade such uncertainties will be clearly reduced.Comment: 4 pages, uses revtex, 3 figure
Generalized parton distributions from nucleon form factor data
We present a simple empirical parameterization of the x- and t-dependence of
generalized parton distributions at zero skewness, using forward parton
distributions as input. A fit to experimental data for the Dirac, Pauli and
axial form factors of the nucleon allows us to discuss quantitatively the
interplay between longitudinal and transverse partonic degrees of freedom in
the nucleon ("nucleon tomography"). In particular we obtain the transverse
distribution of valence quarks at given momentum fraction x. We calculate
various moments of the distributions, including the form factors that appear in
the handbag approximation to wide-angle Compton scattering. This allows us to
estimate the minimal momentum transfer required for reliable predictions in
that approach to be around |t|~3 GeV^2. We also evaluate the valence
contributions to the energy-momentum form factors entering Ji's sum rule.Comment: 69 pages, 36 figures. v2: small improvements in text and figures;
references adde
Hepatic fibrogenesis requires sympathetic neurotransmitters
Background and aims: Hepatic stellate cells (HSC) are activated by liver injury to become proliferative fibrogenic myofibroblasts. This process may be regulated by the sympathetic nervous system (SNS) but the mechanisms involved are unclear. Methods: We studied cultured HSC and intact mice with liver injury to test the hypothesis that HSC respond to and produce SNS neurotransmitters to promote fibrogenesis. Results: HSC expressed adrenoceptors, catecholamine biosynthetic enzymes, released norepinephrine (NE), and were growth inhibited by α- and β-adrenoceptor antagonists. HSC from dopamine β-hydroxylase deficient (Dbh(−/−)) mice, which cannot make NE, grew poorly in culture and were rescued by NE. Inhibitor studies demonstrated that this effect was mediated via G protein coupled adrenoceptors, mitogen activated kinases, and phosphatidylinositol 3-kinase. Injury related fibrogenic responses were inhibited in Dbh(−/−) mice, as evidenced by reduced hepatic accumulation of α-smooth muscle actin(+ve) HSC and decreased induction of transforming growth factor β1 (TGF-β1) and collagen. Treatment with isoprenaline rescued HSC activation. HSC were also reduced in leptin deficient ob/ob mice which have reduced NE levels and are resistant to hepatic fibrosis. Treating ob/ob mice with NE induced HSC proliferation, upregulated hepatic TGF-β1 and collagen, and increased liver fibrosis. Conclusions: HSC are hepatic neuroglia that produce and respond to SNS neurotransmitters to promote hepatic fibrosis
Aperiodic and modulated Pb thin films on fivefold icosahedral Al-Cu-Fe and Al(111): Tailoring the structure of Pb
We report on the growth of Pb thin films deposited either on the Al-rich fivefold surface of the icosahedral Al-Cu-Fe quasicrystal or on the (111) surface of fcc Al. On the quasicrystalline substrate, the diffusion length of Pb adatoms is short due to heterogeneous nucleation that enforces a quasiperiodic structure in the monolayer. On the Al(111) substrate, the mobility of Pb adatoms is high and the interaction with the substrate is flatter, leading to the formation of a (√31×√31)R8.95° higher-order commensurate structure. This moiré structure propagates up to the highest coverages investigated
Moments of nucleon spin-dependent generalized parton distributions
We present a lattice measurement of the first two moments of the
spin-dependent GPD H-tilde(x,xi,t). From these we obtain the axial coupling
constant and the second moment of the spin-dependent forward parton
distribution. The measurements are done in full QCD using Wilson fermions. In
addition, we also present results from a first exploratory study of full QCD
using Asqtad sea and domain-wall valence fermions.Comment: Lattice2003(Theory), 3 pages, 3 figures, to appear in the Proceedings
of Lattice 200
Moments of Nucleon Generalized Parton Distributions in Lattice QCD
Calculation of moments of generalized parton distributions in lattice QCD
requires more powerful techniques than those previously used to calculate
moments of structure functions. Hence, we present a novel approach that
exploits the full information content from a given lattice configuration by
measuring an overdetermined set of lattice observables to provide maximal
statistical constraints on the generalized form factors at a given virtuality,
t. In an exploratory investigation using unquenched QCD configurations at
intermediate sea quark masses, we demonstrate that our new technique is
superior to conventional methods and leads to reliable numerical signals for
the n=2 flavor singlet generalized form factors up to 3 GeV^2. The contribution
from connected diagrams in the flavor singlet sector to the total quark angular
momentum is measured to an accuracy of the order of one percent.Comment: 16 pages, 8 figures, LaTeX, minor elaboration of formalism and
singular value decomposition for non-specialists and addition of several
reference
Phenomenology of eta-gamma and eta'-gamma transition form factors at large momentum transfer
I discuss the progress in our theoretical understanding of the eta-gamma and
eta'-gamma transition form factors, including the recent data from CLEO and L3
at large momentum transfer, Q^2. The experimental data above Q^2=1 GeV^2 can be
well described by the hard scattering approach if the distribution amplitudes
for eta and eta' mesons are taken close to the asymptotic one. Particular
attention is paid to the interpretation of the data in terms of properly
defined eta-eta' mixing parameters. I also comment on the use and misuse of
interpolation formulas for P-gamma and P-gamma^* transition form factors.Comment: 6 pages + 4 figures (using espcrc2.sty, feynmp.sty); Talk presented
at conference PHOTON'99, Freiburg, May 199
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