Elastic Electron Scattering off He and He at Large Momentum Transfers Elena Khrosinkova Kent State University Hall A Collaboration Meeting Jefferson Lab January 2007

Introduction He and He form factors are important for understanding the structure and dynamics of the three- and four-body systems Elastic scattering off few-body systems and the deuteron has been a major part of the experimental programs of nuclear physics laboratories Elastic electron scattering off He, He are the simplest experimentally measurable and theoretically calculable reactions of the electromagnetic probe with the few-body nuclei

Elastic Electron-Helium Scattering He charge and magnetic form factors: He charge form factor:

Beyond the Impulse Approximation IA alone cannot describe the two-body form factor data - Position of first diffraction minimum and height of secondary maximum either overestimated or underestimated Inclusion of Meson-Exchange Currents (MEC) brings theory in better agreement with data - Model-independent (pion-like exchanges) - Model-dependent (transition currents) Isobar Configurations calculated to have small effects Three-Body Force effects have been shown to be small at low

Selected diagrams contributing to elastic electron- He scattering:

Outgrowth of DIS Precocious Scaling / naive QM Dimensional scaling of high energy amplitudes using quark counting Prediction for Helium Form factor: Dominance of the constituent-interchange force between quarks of different nucleons to share Q/A A = 3 for He, A = 4 for He Multi-Quark Admixtures in nuclear wave function

The experiment He Form Factors - Forward angle scattering - Backward angle scattering - Coincidence between both HRSs He Form Factor - Forward angle scattering - Double-arm data - Adequate single-arm missing mass resolution Normalization check / Monitoring - e-p elastic scattering in coincidence - Match Jacobian of He kinematics - Full Monte Carlo simulation

Coincidence Time e – He elastic, = 40 fm, forward kinematics TDC chanel number Events

Right Momentum Versus Left Momentum Position Along Dispersive Axis, = 40 fm, e He Right, m Left, m

Summary Elastic form-factors allow us to test the nucleon-nucleon interaction and associated current operator Expect to be able to see the predicted second diffraction minimum of all three form factors Results will be pivotal for the establishment of a canonical theory of few-body systems New data are expected to initiate new theoretical calculations for the few-body systems

Elastic electron- He scattering in the dimensional-scaling quark model of Brodsky and Chertok:

Experimental Apparatus Helium / Hydrogen Cryotarget - 2 cells - Cells have been constructed and tested Spectrometers - Standard HRS configurations - Standard electron HRS detectors - Subset of hadron HRS detectors Electron-Helium Coincidences - Electron scintillator trigger - Recoil scintillator trigger - TOF between two triggers cut on electron detectors