Elastic Electron Scatteringoff He and He

at Large Momentum Transfers

Elena KhrosinkovaKent State University

Hall A Collaboration Meeting

Jefferson Lab

January 2007

3 4

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

3

3 4

• Elastic electron scattering off He, He are the simplest experimentally measurable and theoretically calculable reactions of the electromagnetic probe with the few-body nuclei

4

Elastic Electron-Helium Scattering

• He charge and magnetic form factors:

2

sin2

cos

2sin4

' 2222

43

22

QBQA

E

EZ

d

d

1

222222

QFQFQA MC 2222 2 QFQB M

2

sin4 22 EEQ

22 4/ MQ

22

43

222

2sin4

2cos'

QFE

EZ

d

dC

• He charge form factor:4

3

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 2Q

Selected diagrams contributing to elastic electron- He scattering:3

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

HeHe QQAQF31222

A = 3 for He3 , A = 4 for He4

• 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

3

4

Coincidence Time

e – He elastic, = 40 fm , forward kinematics

TDC chanel number

Eve

nts

23 2Q

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

Rig

ht,

m

Left, m

322Q

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:

3

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