Elastic Electron Scattering off He and He at Large Momentum Transfers Elena Khrosinkova Kent State...
-
Upload
nickolas-stewart -
Category
Documents
-
view
215 -
download
2
Transcript of Elastic Electron Scattering off He and He at Large Momentum Transfers Elena Khrosinkova Kent State...
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