Session CC02: V: Electromagnetic Interactions

Reduced nuclear helicity amplitudes: Deuteron form factors

We extend the reduced nuclear amplitude (RNA) approach of Brodsky and Chertok for exclusive nuclear processes to include helicity amplitudes.  As with the original RNA method, the soft QCD physics of nucleons is represented by model or empirical form factors, while the fundamental hard-scattering process is treated perturbatively, represented by an elemental tree diagram.  Our extension uses G_E for a nucleon that does not flip its helicity and G_M for one that does, with these form factors evaluated at the net momentum transfer to the nucleon.  The particular quantities discussed in this talk are those associated with elastic electron-deuteron scattering: form factors, structure functions, and tensor polarization observables.  These provide estimates of the momentum transfer required for scaling onset.   

Reduced nuclear helicity amplitudes: Deuteron disintegration

The reduced nuclear helicity amplitude (RNHA) method is applied to deuteron photodisintegration and electrodisintegration. This extends the original reduced nuclear amplitude idea of Brodsky and Chertok to helicity amplitudes and allows the construction of polarization observables. In particular, we find the Σ asymmetry to be approximately -0.06, much closer to experiment than the value of -1 originally obtained in perturbative QCD.   

Extraction of the 12C Longitudinal and Transverse Nuclear Electromagnetic Response Functions from all Electron Scattering Measurements on Carbon

We report on an extraction of the ¹²C Longitudinal  (R_L) and Transverse (R_T) nuclear electromagnetic response functions from  an analysis of all available electron scattering dats on carbon. The response functions are extracted for a large range of energy transfer ν  (spanning the nuclear excitation, quasielastic,  and △(1232 MeV) region over a large range of  the square of the four-momentum transfer Q²  (02<0.5 GeV²). We extract   R_L and R_T as a  function of ν for both  fixed values of Q² as well fixed values momentum transfer  q. The data sample consists of  more than  10,000 ¹²C  differential  electron scattering and photoproduction cross section measurements. Since  the extracted response functions cover a large range of  Q² and ν, they can be used to validate nuclear models as well  Monte Caro generators for electron and neutrino scattering experiments.   

Meuasurement of the N →Δ transition form factors at low momentum transfers

The first excited state of the nucleon dominates many nuclear phenomena at energies above the pion-production threshold and plays a prominent role in the physics of the strong interaction. The study of the N →Δ transition form factors (TFFs) allows to shed light on key aspects of the nucleonic structure that are essential for the complete understanding of the nucleon dynamics. An experimental program for the study of the TFFs is currently ongoing in Hall C at JLab.  A first phase of measurements has focused on intermediate momentum transfers, utilizing the SHMS and the HMS spectrometers, while upcoming measurements will extend the range of the kinematic reach to low four-momentum transfer squared. The experimental program focuses on a region where the mesonic cloud dynamics are dominant and rapidly changing. More specifically, it will provide high precision measurements of the quadrupole TFFs, that have emerged as the experimental signature for the presence of non-spherical components in the nucleon wavefunction and will allow to decode the underlying system dynamics responsible for their existence. The measurements will offer a test bed for chiral effective field theory calculations and benchmark data for the lattice QCD calculations, and will allow to test the theoretical prediction that the Electric and the Coulomb quadrupole amplitudes converge as Q2 →0.   

Analysis Progress of the GMN Experiment with Super BigBite Spectrometer at Jefferson Lab

The nucleon form factors are one of the most fundamental aspects of the nucleon structure.

At high Q2, they are linked to the generalized parton distributions which provide information on the proton spin.

The Super Bigbite Spectrometer (SBS) in Hall~A at Jefferson Lab is dedicated to measure the form factors at high Q2. This includes GEP (proton electric form Factor, recorded) up to Q2 = 12.5 GeV2, GMN (neutron magnetic form factor, upcoming) up to Q2 = 13.5 GeV2, GEN and GEN-RP (neutron electric form factor, recorded and upcoming, respectively) up to Q2 = 10 GeV2, and nTPE (two-photon exchange contribution in elastic electron-neutron scattering, recorded with GMN).

GMN/NTPE was the first SBS experiment to run, and its successful run has helped tremendeously to the successfull run of all following experiments. In this presentation we will focus on the GMN/NTPE experiment with an emphasis on the experimental analysis method and the progress of the data analysis.   

High luminosity optimized Jefferson Lab SBS setup for K_LL, GEn-RP and GEp-V experiments

Super BigBite Spectrometer programme at Jefferson Lab plans to run K_LL , GEn-RP and GEp-V experiments this year, which will improve the figure of merit of these measurements significantly due to the open geometry large acceptance spectrometer used for hadron detection. However, the open geometry and the direct line of sight from the target to the tracking detectors exposing them to an unprecedentedly high luminosity introduce us to a new set of challenges in getting the optimum performance out of the tracking detectors. SBS tracker consists of large area Gas Electron Multiplier (GEM) detectors designed to handle high background rates and to provide good position resolution. Even using GEM detectors, traditional high voltage distributions of the detectors need to be optimized to enhance the efficiency at high background rates. Furthermore, tracking algorithms are updated to avoid losses of tracking efficiency caused by high background rates. Presentation will include a brief description about the physics motivation for these experiments, tracking efficiency results comparing upgraded systems operating in beam with simulations. These upgrades will be beneficial for future experiments such as the Solenoidal Large Intensity Device (SoLID).