A new proposal to the Jefferson Lab Program Advisory Committee (PAC33) Precision Measurement of the Neutron Magnetic Form Factor up to Q = 8.0 (GeV/c) by the Ratio Method

We propose to make a high-precision measurement of the neutron’s magnetic form factor, Gn M , at six kinematic points: Q2 = 3.5, 4.5, 5.25, 6.0, 7.0 and 8.0 (GeV/c)2. Little data on Gn M exists in this kinematic range and the existing data have large systematic uncertainties. In the proposed experiment, systematic errors are greatly reduced by the use of the “ratio” method in which Gn M is extracted from the ratio of neutron-coincident to proton-coincident quasi-elastic electron scattering from the deuteron. The experiment would be performed in Hall A using the BigBite spectrometer to detect the scattered electrons and the BigHAND to detect both neutrons and protons. A large aperture dipole magnet on the nucleon flight path will greatly enhance particle identification by slightly deflecting the protons. Calibration reactions will be used to produce tagged neutrons and protons at essentially the same momentum as the quasi-elastic nucleons. The efficiency of BigHAND is expected to be high and stable and to be well calibrated. Projected systematic errors are below 3% on the measured ratio of cross sections (1.5% on the extracted form factor) and we aim for statistical errors at a similar level. We request a total of 28 days divided among three beam energies.