839 research outputs found
Probing Quark Distribution Amplitudes Through Generalized Parton Distributions at Large Momentum Transfer
In the large momentum transfer limit, generalized parton distributions can be
calculated through a QCD factorization theorem which involves
perturbatively-calculable hard kernels and light-cone parton distribution
amplitudes of hadrons. We illustrate this through the
distribution for the pion and proton, presenting the hard kernels at leading
order. As a result, experimental data on the generalized parton distributions
in this regime can be used to determine the functional form of the parton
distribution amplitudes which has thus far been quite challenging to obtain.
Our result can also be used as a constraint in phenomenological GPD
parametrizations.Comment: 7 pages, 4 figures; new references and figure added, errors correcte
Twist-four Corrections to Parity-Violating Electron-Deuteron Scattering
Parity violating electron-deuteron scattering can potentially provide a clean
access to electroweak couplings that are sensitive to physics beyond the
Standard Model. However hadronic effects can contaminate their extraction from
high-precision measurements. Power-suppressed contributions are one of the main
sources of uncertainties along with charge-symmetry violating effects in
leading-twist parton densities. In this work we calculate the twist-four
correlation functions contributing to the left-right polarization asymmetry
making use of nucleon multiparton light-cone wave functions.Comment: 12 pages, 3 figure
FAPT: a Mathematica package for calculations in QCD Fractional Analytic Perturbation Theory
We provide here all the procedures in \texttt{Mathematica} which are needed
for the computation of the analytic images of the strong coupling constant
powers in Minkowski ( and ) and Euclidean (
and ) domains at arbitrary energy scales
( and , correspondingly) for both schemes --- with fixed number of
active flavours and the global one with taking into account
all heavy-quark thresholds. These singularity-free couplings are inevitable
elements of Analytic Perturbation Theory (APT) in QCD and its generalization
--- Fractional APT, needed to apply the APT imperative for
renormalization-group improved hadronic observables.Comment: 23 pages, 6 figures. Citations added. Now it matches version approved
for publication in Comp. Phys. Commu
Taming Landau singularities in QCD perturbation theory: The analytic approach 2.0
The aim of this topical article is to outline the fundamental ideas
underlying the recently developed Fractional Analytic Perturbation Theory
(FAPT) of QCD and present its main calculational tools together with key
applications. For this, it is first necessary to review previous methods to
apply QCD perturbation theory at low spacelike momentum scales, where the
influence of the Landau singularities becomes inevitable. Several concepts are
considered and their limitations are pointed out. The usefulness of FAPT is
discussed in terms of two characteristic hadronic quantities: the
perturbatively calculable part of the pion's electromagnetic form factor in the
spacelike region and the Higgs-boson decay into a pair in the
timelike region. In the first case, the focus is on the optimization of the
prediction with respect to the choice of the renormalization scheme and the
dependence on the renormalization and the factorization scales. The second case
serves to show that the application of FAPT to this reaction reaches already at
the four-loop level an accuracy of the order of 1%, avoiding difficulties
inherent in the standard perturbative expansion. The obtained results are
compared with estimates from fixed-order and contour-improved QCD perturbation
theory. Using the brand-new Higgs mass value of about 125 GeV, measured at the
Large Hadron Collider (CERN), a prediction for is extracted.Comment: v3: 23 pages, 7 figures, Invited topical article published in
Particles and Nuclei with update using the CERN Higgs discovery. Abridged
version presented as plenary talk at International Conference on
Renormalization Group and Related Topics (RG 2008), Dubna, Russia, September
1 - 5, 2008. v4 typo in Eq. (3) correcte
Nucleon distribution amplitudes from lattice QCD
We calculate low moments of the leading-twist and next-to-leading twist
nucleon distribution amplitudes on the lattice using two flavors of clover
fermions. The results are presented in the MSbar scheme at a scale of 2 GeV and
can be immediately applied in phenomenological studies. We find that the
deviation of the leading-twist nucleon distribution amplitude from its
asymptotic form is less pronounced than sometimes claimed in the literature.Comment: 5 pages, 3 figures, 2 tables. RevTeX style. Normalization for
\lambda_i corrected. Discussion of the results extended. To be published in
PR
Skewed parton distributions and the scale dependence of the transverse size parameter
We discuss the scale dependence of a skewed parton distribution of the pion
obtained from a generalized light-cone wave function overlap formula. Using a
simple ansatz for the transverse momentum dependence of the light-cone wave
function and restricting ourselves to the case of a zero skewedness parameter,
the skewed parton distribution can be expressed through an ordinary parton
distribution multiplied by an exponential function. Matching the generalized
and ordinary DGLAP evolution equations of the skewed and ordinary parton
distributions, respectively, we derive a constraint for the scale dependence of
the transverse size parameter, which describes the width of the pion wave
function in transverse momentum space. This constraint has implications for the
Fock state probability and valence distribution. We apply our results to the
pion form factor.Comment: 10 pages, 4 figures; version to appear in Phys. Rev. D; Refs. added,
new discussion of results for pion form factor in view of new dat
Baryon wave function in large-Nc QCD: Universality, nonlinear evolution equation and asymptotic limit
The 1/Nc expansion is formulated for the baryon wave function in terms of a
specially constructed generating functional. The leading order of this 1/Nc
expansion is universal for all low-lying baryons [including the O(1/Nc) and
O(Nc^0) excited resonances] and for baryon-meson scattering states. A nonlinear
evolution equation of Hamilton-Jacobi type is derived for the generating
functional describing the baryon distribution amplitude in the large-Nc limit.
In the asymptotic regime this nonlinear equation is solved analytically. The
anomalous dimensions of the leading-twist baryon operators diagonalizing the
evolution are computed analytically up to the next-to-leading order of the 1/Nc
expansion.Comment: 44 page
The neurogenic basic helix–loop–helix transcription factor NeuroD6 confers tolerance to oxidative stress by triggering an antioxidant response and sustaining the mitochondrial biomass
Preserving mitochondrial mass, bioenergetic functions and ROS (reactive oxygen species) homoeostasis is key to neuronal differentiation and survival, as mitochondria produce most of the energy in the form of ATP to execute and maintain these cellular processes. In view of our previous studies showing that NeuroD6 promotes neuronal differentiation and survival on trophic factor withdrawal, combined with its ability to stimulate the mitochondrial biomass and to trigger comprehensive antiapoptotic and molecular chaperone responses, we investigated whether NeuroD6 could concomitantly modulate the mitochondrial biomass and ROS homoeostasis on oxidative stress mediated by serum deprivation. In the present study, we report a novel role of NeuroD6 as a regulator of ROS homoeostasis, resulting in enhanced tolerance to oxidative stress. Using a combination of flow cytometry, confocal fluorescence microscopy and mitochondrial fractionation, we found that NeuroD6 sustains mitochondrial mass, intracellular ATP levels and expression of specific subunits of respiratory complexes upon oxidative stress triggered by withdrawal of trophic factors. NeuroD6 also maintains the expression of nuclear-encoded transcription factors, known to regulate mitochondrial biogenesis, such as PGC-1α (peroxisome-proliferator-activated receptor γ co-activator-1α), Tfam (transcription factor A, mitochondrial) and NRF-1 (nuclear respiratory factor-1). Finally, NeuroD6 triggers a comprehensive antioxidant response to endow PC12-ND6 cells with intracellular ROS scavenging capacity. The NeuroD6 effect is not limited to the classic induction of the ROS-scavenging enzymes, such as SOD2 (superoxide dismutase 2), GPx1 (glutathione peroxidase 1) and PRDX5 (peroxiredoxin 5), but also to the recently identified powerful ROS suppressors PGC-1α, PINK1 (phosphatase and tensin homologue-induced kinase 1) and SIRT1. Thus our collective results support the concept that the NeuroD6–PGC-1α–SIRT1 neuroprotective axis may be critical in co-ordinating the mitochondrial biomass with the antioxidant reserve to confer tolerance to oxidative stress
Gauge-invariant Green function in 3+1 dimensional QED (QCD) and 2+1 dimensional Abelian (Non-Abelian) Chern-Simon theory
By applying the simple and effective method developed to study the the
gauge-invariant fermion Green function in dimensional non-compact QED,
we study the gauge-invariant Green function in dimensional QED and dimensional non-compact Chern-Simon theory. We also extend our results to
the corresponding non-Abelian gauge theories. Implications for
Fractional Quantum Hall effect are briefly discussed.Comment: 8 pages, 4 figures, published versio
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