Determination of the Electric Formfactor of the Neutron


The goal of the A3-collaboration at MAMI is to measure the electric formfactor of the neutron G_e^n in the region of q^2=-8 fm^{-2} up to an error of 10\%. In the ^3He(e,e'n)pp experiment the cross section asymmetry with respect to the electron polarisation for a target polarisation parallel and perpendicular to the momentum transfer q was measured.
The scattered electron is detected in a lead glas detector with a very good angular but a poor energy resolution, whereas the momentum of the knocked-out neutron is measured in an array of time-of-flight scintillators. The ratio of these two asymmetries is proportional to the ratio of the electric and magnetic neutron formfactors.

In order to verify the performance of the experiment a pilot-run with a reduced detector setup has taken place in spring 1994. For the determination of the formfactor it is vital to ensure quasielastic scattering of the electron off the neutron. The purpose of my diploma thesis was to investigate improved methods to separate inelastic and quasielastic processes. The basic idea was an event-wise reconstruction of the reaction kinematics. Assuming PWIA and neglecting FSI the energy of a quasielastic scattered electron may be derived from the electron angle and the final neutron momentum. It is also possible to reconstruct the initial neutron momentum.

A comparison of the reconstructed momentum distribution with the (e,e'p) data of E. Jans et al. served as a check of the method. A Monte Carlo simulation of the quasielastic process, pion production and delta resonance was in agreement with the experimental spectra and was used to investigate the potential of the separation method.
The separation based on the electron energy measured in the lead glas detector and the reconstructed energy improved the uncertainties substantially. This method will become very important in the datat production run with the full detector setup due to the enlarged phase space which was performed end of 1995.