Speaker
Description
Nucleon elastic form factors encode crucial information about its charge and
magnetization distributions. For many decades, nucleon form factors were studied by
using unpolarized electron-nucleon cross section measurements. The advent of electron
beams with higher luminosities and beam polarization coupled with large acceptance
detectors, polarized targets and recoil polarimeters enabled a wealth of information on
nucleon form factors over a broad range of momentum transfer (Q^2). While plenty of
information is available on the proton, no data above Q^2 = 3.5 GeV^2 is available on the
neutron electric form factor. Pushing the data to a higher Q^2 allows constraining spin
flip GPDs and serves as a bench mark for various theoretical models. Using quasi-elastic
scattering of a polarized electron beam on a polarized ^3He target, one can extract the
GEn term which is proportional to the measured asymmetry from opposite electron
beam helicity.
The GEn-II experiment at Jefferson lab utilizes a polarized He3 target for a high Q^2
measurement of the neutron electric form factor. The target consists of a pumping
chamber (where polarization of He3 takes place), a target chamber (where e- beam
interacts with the target material) and transfer tubes (which facilitate a convective flow
of the polarized material). ^3He gas at ~8 atm pressure is filled into the glass cells along
with Rb-K alkali mixture and narrow band diode lasers are used for polarizing the He3
using a SEOP (Spin Exchange Optical Pumping) technique. The target system used in
the experiment includes multiple Helmholtz coils to create a holding field that
determines the direction of polarization for the ^3He nuclei. Two polarimetry techniques
are used to determine the absolute (EPR) and relative (EPR) polarization during
production running. The GEn-II experiment finished taking data at three out of four
kinematic settings and is scheduled for completion in the Fall of this year. Overall target
performance and other instrumentation used in the GEn-II experiment (GEMs,
calorimeters and hodoscopes) will be briefly discussed in this talk.
| session | A. Facilities and Detectors |
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