226 research outputs found
Delta electromagnetic form factors and quark transverse charge densities from lattice QCD
We discuss the techniques to extract the electromagnetic Delta form factors
in Lattice QCD. We evaluate these form factors using dynamical fermions with
smallest pion mass of about 350 MeV. We pay particular attention to the
extraction of the electric quadrupole form factor that signals a deformation of
the Delta. The magnetic moment of the is extrapolated using a chiral
effective field theory. Using the form factors we evaluate the transverse
density distributions in the infinite momentum frame showing deformation in the
Delta.Comment: 9 pages, 6 figures, Invited Talk at the 6th International Workshop on
Chiral Dynamics, CD09, July 6-10, 2009, Bern, Switzerlan
Double deeply virtual Compton scattering off the nucleon
We study the double deeply virtual Compton scattering (DDVCS) process off the
nucleon, through the scattering of a spacelike virtual photon with large
virtuality resulting in the production of a timelike virtual photon, decaying
into an e^+ e^- pair. This process is expressed in the Bjorken regime in terms
of generalized parton distributions (GPDs) and it is shown that by varying the
invariant mass of the lepton pair, one can directly extract the GPDs from the
observables. We give predictions for the DDVCS cross section and beam helicity
asymmetry and discuss its experimental feasibility.Comment: 4 pages, 4 figure
Delta-baryon electromagnetic form factors in lattice QCD
We develop techniques to calculate the four Delta electromagnetic form
factors using lattice QCD, with particular emphasis on the sub-dominant
electric quadrupole form factor that probes deformation of the Delta. Results
are presented for pion masses down to approximately 350 MeV for three cases:
quenched QCD, two flavors of dynamical Wilson quarks, and three flavors of
quarks described by a mixed action combining domain wall valence quarks and
dynamical staggered sea quarks. The magnetic moment of the Delta is chirally
extrapolated to the physical point and the Delta charge density distributions
are discussed.Comment: 4 pages, 5 figure
Can we extract the pion electromagnetic form factor from a t-channel diagram only?
We show that we are able to extract the pion electromagnetic form factor by
using the recent charged pion electroproduction data from JLab and a simple
t-channel diagram. For this purpose we have performed the Q2-independent and
dependent analyses. The result of the first analysis is in good agreement with
those of previous works and fit the Maris and Tandy model as well as the
monopole parameterization which describes a pion radius of 0.672 fm. The result
of the second analysis corroborates the findings in the first analysis. Our
findings therefore provide a direct proof that at the given kinematics the
t-channel diagram really dominates the process. This could also set a new
constraint to the phenomenological models that try to describe the process.Comment: 13 pages, 6 figure
Generalized polarizabilities and the spin-averaged amplitude in virtual Compton scattering off the nucleon
We discuss the low-energy behavior of the spin-averaged amplitude of virtual
Compton scattering (VCS) off a nucleon.
Based on gauge invariance, Lorentz invariance and the discrete symmetries, it
is shown that to first order in the frequency of the final real photon only two
generalized polarizabilities appear.
Different low-energy expansion schemes are discussed and put into
perspective.Comment: 13 pages, 1 postscript figure, Revtex using eps
Real and Virtual Compton Scattering: the nucleon polarisabilities
We give an overview of low-energy Compton scattering (gamma^(*) p --> gamma
p) with a real or virtual incoming photon. These processes allow the
investigation of one of the fundamental properties of the nucleon, i.e. how its
internal structure deforms under an applied static electromagnetic field. Our
knowledge of nucleon polarisabilities and their generalization to non-zero
four-momentum transfer will be reviewed, including the presently ongoing
experiments and future perspectives.Comment: 20 pages, 12 figures. Minireview/Proceedings of "Many-Body Structure
of Strongly Interacting Systems", Mainz, Germany, Feb. 23-25 2011 . V2: typos
corrected. version to appear in EPJ Special Topic
Pion production in deeply virtual Compton scattering
Using a soft pion theorem based on chiral symmetry and a
resonance model we propose an estimate for the production cross section of low
energy pions in the deeply virtual Compton scattering (DVCS) process. In
particular, we express the processes in terms of
generalized parton distributions. We provide estimates of the contamination of
the DVCS observables due to this associated pion
production processes when the experimental data are not fully exclusive, for a
set of kinematical conditions representative of present or planned experiments
at JLab, HERMES and COMPASS.Comment: 50 pages, 22 figure
Deeply Virtual Neutrino Scattering (DVNS)
We introduce the study of neutrino scattering off protons in the deeply
virtual kinematics, which describes under a unified formalism elastic and deep
inelastic neutrino scattering. A real final state photon and a recoiling
nucleon are detected in the few GeV ( GeV) region of momentum
transfer. This is performed via an extension of the notion of deeply virtual
Compton scattering, or DVCS, to the case of a neutral current exchange. The
relevance of this process and of other similar exclusive processes for the
study of neutrino interactions in neutrino factories for GeV neutrinos is
pointed out.Comment: 28 pages, 12 figures, revised final version, to appear in JHE
Electroinduced two-nucleon knockout and correlations in nuclei
We present a model to calculate cross sections for electroinduced two-nucleon
emission from finite nuclei. Short-range correlations in the wave functions and
meson-exchange contributions to the photoabsorption process are implemented.
Effects of the short-range correlations are studied with the aid of a
perturbation expansion method with various choices of the Jastrow correlation
function. The model is used to investigate the relative importance of the
different reaction mechanisms contributing to the A(e,epn) and A(e,epp)
process. Representative examples for the target nuclei C and O
and for kinematical conditions accessible with contemporary high-duty cycle
electron accelerators are presented. A procedure is outlined to calculate the
two-nucleon knockout contribution to the semi-exclusive (e,ep) cross
section. Using this technique we investigate in how far semi-exclusive
(e,ep) reactions can be used to detect high-momentum components in the
nuclear spectral function.Comment: 51 pages, Latex, uses epsf.sty and elsart.sty, 17 figures (in eps
format
One-Loop Helicity Amplitudes for Parton Level Virtual Compton Scattering
We calculate the one-loop QCD virtual corrections to all helicity amplitudes
for parton level virtual Compton scattering processes. We include the
amplitudes both on quark target process and on gluon
target process . The infrared pole structure of the
amplitudes is in agreement with the prediction of Catani's general formalism
for the singularities of one-loop amplitudes, while expressions for the finite
remainder are given in terms of logarithms and dilogarithms that are real in
the physical region.Comment: 16 pages, 2 figures, detailed comparison with DVCS include
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