Neutron Elastic Form Factors

Measurement of the neutron electric form factor was identified in the last LRP as one of our highest priority goals. Significant progress has been made in the first generation of double-polarization experiments attacking this problem. The technical success of these measurements augers well for the next generation of high precision experiments.

Two approaches have been utilized at MIT/Bates to date: quasi-elastic scattering of polarized electrons from deuterium with subsequent detection of the recoil neutron polarization [Ed94] and inclusive quasi- elastic scattering from a polarized He target [Th92][Jo93]. Although the statistical precision of the results was limited, they clearly indicate the viability of the techniques. In addition, higher data from SLAC [Lu93], employing the more traditional H(e,e) reaction, has ruled out the possibility that the Dirac form factor .

A second generation higher statistics polarized He experiment was performed at MIT/Bates in 1992. [Ga94][Ha95] Theoretical developments since the initial round of experiments indicate, however, that the proton contribution to the LT-asymmetry on He dominates at low so that this asymmetry is actually a sensitive probe of the He ground state. However, the coincidence (,en) reaction seems better able to select the neutron contributions and has recently been measured at Mainz. [Me94]

On the other hand, the transverse asymmetry in inclusive scattering, being proportional to , is relatively unaffected by proton contamination and has recently been used [Ga94] to measure the neutron magnetic form factor at (GeV/c). These data are in good agreement with conventional determinations (Figure ) and indicate that polarized He will play an important role in future determinations of .

Also shown in Figure is a determination of from the d(e,en) reaction measured at MIT/Bates. [Ma93]. Three values of were sampled and an absolute calibration performed utilizing the H) over-determined reaction. The new data indicate a low enhancement with respect to the dipole parameterization, in agreement with earlier studies.