35 research outputs found
Projecting the Bethe-Salpeter Equation onto the Light-Front and back: A Short Review
The technique of projecting the four-dimensional two-body Bethe-Salpeter
equation onto the three-dimensional Light-Front hypersurface, combined with the
quasi-potential approach, is briefly illustrated, by placing a particular
emphasis on the relation between the projection method and the effective
dynamics of the valence component of the Light-Front wave function. Some
details on how to construct the Fock expansion of both i) the Light-Front
effective interaction and ii) the electromagnetic current operator, satisfying
the proper Ward-Takahashi identity, will be presented, addressing the relevance
of the Fock content in the operators living onto the Light-Front hypersurface.
Finally, the generalization of the formalism to the three-particle case will
be outlined.Comment: 16 pages, macros included. Mini-review to be printed in a regular
issue of Few-Body Systems devoted to the Workshop on "Relativistic
Description of Two- and Three-body Systems in Nuclear Physics" ECT* Trento,
19 - 23 October 200
Factorizing the hard and soft spectator scattering contributions for the nucleon form factor F_1 at large Q^2
We investigate the soft spectator scattering contribution for the FF .
We focus our attention on factorization of the hard-collinear scale corresponding to transition from SCET-I to SCET-II. We compute the
leading order jet functions and find that the convolution integrals over the
soft fractions are logarithmically divergent. This divergency is the
consequence of the boost invariance and does not depend on the model of the
soft correlation function describing the soft spectator quarks. Using as
example a two-loop diagram we demonstrated that such a divergency corresponds
to the overlap of the soft and collinear regions. As a result one obtains large
rapidity logarithm which must be included in the correct factorization
formalism. We conclude that a consistent description of the factorization for
implies the end-point collinear divergencies in the hard and soft
spectator contributions, i.e. convolution integrals with respect to collinear
fractions are not well-defined. Such scenario can only be realized when the
twist-3 nucleon distribution amplitude has specific end-point behavior which
differs from one expected from the evolution of the nucleon distribution
amplitude. Such behavior leads to the violation of the collinear factorization
for the hard spectator scattering contribution. We suggest that the soft
spectator scattering and chiral symmetry breaking provide the mechanism
responsible for the violation of collinear factorization in case of form factor
.Comment: 25 pages, 6 figures, text is improved, few typos corrected, one
figure added, statement about end-point behavior of the nucleon DA is
formulated more accuratel
Demonstration of a novel technique to measure two-photon exchange effects in elastic scattering
The discrepancy between proton electromagnetic form factors extracted using
unpolarized and polarized scattering data is believed to be a consequence of
two-photon exchange (TPE) effects. However, the calculations of TPE corrections
have significant model dependence, and there is limited direct experimental
evidence for such corrections. We present the results of a new experimental
technique for making direct comparisons, which has the potential to
make precise measurements over a broad range in and scattering angles. We
use the Jefferson Lab electron beam and the Hall B photon tagger to generate a
clean but untagged photon beam. The photon beam impinges on a converter foil to
generate a mixed beam of electrons, positrons, and photons. A chicane is used
to separate and recombine the electron and positron beams while the photon beam
is stopped by a photon blocker. This provides a combined electron and positron
beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen
target. The large acceptance CLAS detector is used to identify and reconstruct
elastic scattering events, determining both the initial lepton energy and the
sign of the scattered lepton. The data were collected in two days with a
primary electron beam energy of only 3.3 GeV, limiting the data from this run
to smaller values of and scattering angle. Nonetheless, this measurement
yields a data sample for with statistics comparable to those of the
best previous measurements. We have shown that we can cleanly identify elastic
scattering events and correct for the difference in acceptance for electron and
positron scattering. The final ratio of positron to electron scattering:
for GeV and
Determination of the Titanium Spectral Function From (e, e\u27p) Data
The E12-14-012 experiment, performed in Jefferson Lab Hall A, has measured the (e,e′p) cross section in parallel kinematics using a natural titanium target. In this paper, we report the analysis of the dataset obtained in different kinematics for our solid natural titanium target. Data were obtained in a range of missing momentum and missing energy between 15 ≲ pm ≲ 250 MeV/c and 12 ≲ Em ≲ 80 MeV, respectively, and using an electron beam energy of 2.2 GeV. We measured the reduced cross section with ∼7% accuracy as a function of both missing momentum and missing energy. Our Monte Carlo simulation, including both a model spectral function and the effects of final-state interactions, satisfactorily reproduces the data
Measurement of the single-spin asymmetry A y 0 in quasi-elastic 3He↑(e,e′n) scattering at 0.4 < Q 2 < 1.0 GeV/c 2
No abstract available
Comparing proton momentum distributions in and 3 nuclei via H H and He measurements
We report the first measurement of the reaction cross-section
ratios for Helium-3 (He), Tritium (H), and Deuterium (). The
measurement covered a missing momentum range of
MeV, at large momentum transfer (
(GeV)) and , which minimized contributions from non
quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave
impulse approximation (PWIA) calculations using realistic spectral functions
and momentum distributions. The measured and PWIA-calculated cross-section
ratios for He and H extend to just above the typical nucleon
Fermi-momentum ( MeV) and differ from each other by , while for He/H they agree within the measurement accuracy of
about 3\%. At momenta above , the measured He/H ratios differ from
the calculation by . Final state interaction (FSI) calculations
using the generalized Eikonal Approximation indicate that FSI should change the
He/H cross-section ratio for this measurement by less than 5\%. If
these calculations are correct, then the differences at large missing momenta
between the He/H experimental and calculated ratios could be due to the
underlying interaction, and thus could provide new constraints on the
previously loosely-constrained short-distance parts of the interaction.Comment: 8 pages, 3 figures (4 panels
Strangeness Suppression of q(q)over-bar Creation Observed in Exclusive Reactions
We measured the ratios of electroproduction cross-sections from a proton
target for three exclusive meson-baryon final states: , ,
and , with the CLAS detector at Jefferson Lab. Using a simple model of
quark hadronization we extract q-qbar creation probabilities for the first time
in exclusive two-body production, in which only a single q-qbar pair is
created. We observe a sizable suppression of strange quark-antiquark pairs
compared to non-strange pairs, similar to that seen in high-energy production.Comment: 5pages, 2figure
First measurement of the helicity asymmetry E in eta photoproduction on the proton
Results are presented for the first measurement of the double-polarization
helicity asymmetry E for the photoproduction reaction . Data were obtained using the FROzen Spin Target (FROST)
with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of
center-of-mass energy W from threshold to 2.15 GeV and a large range in
center-of-mass polar angle. As an initial application of these data, the
results have been incorporated into the J\"ulich model to examine the case for
the existence of a narrow resonance between 1.66 and 1.70 GeV. The
addition of these data to the world database results in marked changes in the
predictions for the E observable using that model. Further comparison with
several theoretical approaches indicates these data will significantly enhance
our understanding of nucleon resonances
First measurement of the polarization observable E in the p→(γ→,π<sup>+</sup>)n reaction up to 2.25 GeV
First results from the longitudinally polarized frozen-spin target (FROST)
program are reported. The double-polarization observable E, for the reaction
, has been measured using a circularly polarized
tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions
were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the
Thomas Jefferson National Accelerator Facility. These polarization data agree
fairly well with previous partial-wave analyses at low photon energies. Over
much of the covered energy range, however, significant deviations are observed,
particularly in the high-energy region where high-L multipoles contribute. The
data have been included in new multipole analyses resulting in updated nucleon
resonance parameters. We report updated fits from the Bonn-Gatchina, J\"ulich,
and SAID groups.Comment: 6 pages, 3 figure