323 research outputs found
Viewing the Proton Through "Color"-Filters
While the form factors and parton distributions provide separately the shape
of the proton in coordinate and momentum spaces, a more powerful imaging of the
proton structure can be obtained through phase-space distributions. Here we
introduce the Wigner-type quark and gluon distributions which depict a full-3D
proton at every fixed light-cone momentum, like what seen through
momentum("color")-filters. After appropriate phase-space reductions, the Wigner
distributions are related to the generalized parton distributions (GPD's) and
transverse-momentum dependent parton distributions which are measurable in
high-energy experiments. The new interpretation of GPD's provides a classical
way to visualize the orbital motion of the quarks which is known to be the key
to the spin and magnetic moment of the proton.Comment: 4 page
Model study of generalized parton distributions with helicity flip
Generalized parton distributions with helicity flip are studied in the quark
sector, within a simple version of the MIT bag model, assuming an SU(6) wave
function for the proton target. In the framework under scrutiny it turns out
that only the generalized transversity distribution, H_T^q, is non vanishing.
For this quantity, the forward limit is properly recovered and numerical
results are found to underestimate recent lattice data for its first moment.
Positivity bounds recently proposed are fulfilled by the obtained distribution.
The relevance of the analysis for the planning of measurements of the quark
generalized transversity is addressed.Comment: 15 pages, 4 figures; shortened version accepted for publication in
Phys. Rev.
Magnetization screening from gluonic currents and scaling law violation in the ratio of magnetic form factors for neutron and proton
The ratio exhibits a decrease for four-momentum transfer
Q^2 increasing beyond 1 GeV^2 indicating different spatial distributions for
charge and for magnetization inside the proton. One-gluon exchange currents can
explain this behaviour. The SU(6) breaking induced by gluonic currents predicts
furthermore that the ratio of neutron to proton magnetic form factors
falls with increasing Q^2. We find that the
experimental data are consistent with our expectations of an almost linear
decrease of the ratio with increasing Q^2,
supporting the statement that the spatial distributions of magnetization are
different for protons and for neutrons.Comment: 10 pages, 3 figure
Heavy quarks in deeply virtual Compton scattering
A detailed study of the heavy quark h=c,b,... contributions to deeply virtual
Compton scattering is performed at both the amplitude and the cross section
level, and their phenomenological relevance is discussed. For this purpose I
calculate the lowest order off-forward photon-gluon scattering amplitude with a
massive quark loop and the corresponding hard scattering coefficients. In a
first numerical analysis these fixed order perturbation theory results are
compared with the conventional intrinsic "massless" parton approach considering
generalized parton distributions for the heavy quarks. The differences between
these two prescriptions can be quite significant, especially at small
skewedness where the massless approach largely overestimates the deeply virtual
Compton scattering cross section.Comment: 15 pages, 12 eps-figures, LaTeX2e; (V2) include correct figure 3b,
remove 'bottom' from figure caption
Superconformal operators in N=4 super-Yang-Mills theory
We construct, in the framework of the N=4 SYM theory, a supermultiplet of
twist-two conformal operators and study their renormalization properties. The
components of the supermultiplet have the same anomalous dimension and enter as
building blocks into multi-particle quasipartonic operators. The latter are
determined by the condition that their twist equals the number of elementary
constituent fields from which they are built. A unique feature of the N=4 SYM
is that all quasipartonic operators with different SU(4) quantum numbers fall
into a single supermultiplet. Among them there is a subsector of the operators
of maximal helicity, which has been known to be integrable in the multi-color
limit in QCD, independent of the presence of supersymmetry. In the N=4 SYM
theory, this symmetry is extended to the whole supermultiplet of quasipartonic
operators and the one-loop dilatation operator coincides with a Hamiltonian of
integrable SL(2|4) Heisenberg spin chain.Comment: 45 pages, Latex, 4 figure
Operator product expansion in QCD in off-forward kinematics: Separation of kinematic and dynamical contributions
We develop a general approach to the calculation of target mass and finite
t=(p'-p)^2 corrections in hard processes which can be studied in the framework
of the operator product expansion and involve momentum transfer from the
initial to the final hadron state. Such corrections, which are usually referred
to as kinematic, can be defined as contributions of operators of all twists
that can be reduced to total derivatives of the leading twist operators. As the
principal result, we provide a set of projection operators that pick up the
"kinematic" part of an arbitrary flavor-nonsinglet twist-four operator in QCD.
A complete expression is derived for the time-ordered product of two
electromagnetic currents that includes all kinematic corrections to twist-four
accuracy. The results are immediately applicable to the studies of
deeply-virtual Compton scattering, transition gamma^*-> M gamma form factors
and related processes. As a byproduct of this study, we find a series of
"genuine" twist-four flavor-nonsinglet quark-antiquark-gluon operators which
have the same anomalous dimensions as the leading twist quark-antiquark
operators.Comment: 68 pages, 2 figures. Misprints in Eq.(5.64) and several equations in
Sec.6 are corrected. For readers' convenience all corrections are marked in
re
Evolution Equation for Generalized Parton Distributions
The extension of the method [arXiv:hep-ph/0503109] for solving the leading
order evolution equation for Generalized Parton Distributions (GPDs) is
presented. We obtain the solution of the evolution equation both for the flavor
nonsinglet quark GPD and singlet quark and gluon GPDs. The properties of the
solution and, in particular, the asymptotic form of GPDs in the small x and \xi
region are discussed.Comment: REVTeX4, 34 pages, 3 figure
Non-local charges on AdS_5 x S^5 and PP-waves
We show the existence of an infinite set of non-local classically conserved
charges on the Green-Schwarz closed superstring in a pp-wave background. We
find that these charges agree with the Penrose limit of non-local classically
conserved charges recently found for the Green-Schwarz
superstring. The charges constructed in this paper could help to understand the
role played by these on the full background.Comment: 20 pages. JHEP. v2:references adde
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