A proposal to the Jefferson Lab Program Advisory Committee (PAC35) Precision Measurement of the Neutron Magnetic Form Factor at Q=16.0 and 18.0 (GeV/c) by the Ratio Method

We propose to make high-precision measurement of the neutron’s magnetic form factor, Gn M , at Q2 = 16.0 and 18.0 (GeV/c)2. This would extend the coverage beyond the seven kinematic points already approved in experiment E-09-019. A major improvement of experimental equipment from that originally planned in E-09-019 will permit the use of much higher luminosity for these high-Q2 points while reducing background rates and simplifying the experiment installation and analysis. The proposed experiment would run contiguously with E-09-019 to take advantage of calibrations and systematics studies made possible by the copious statistics available at lower Q2. The new data will push the Q2 range for Gn M measurements beyond any other existing or planned experiments. 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 HCal 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. The efficiency of HCal is expected to be very high and stable and to be well calibrated from, already approved, lower-Q2 measurement. Projected systematic errors on the measured ratio of cross sections are 4.8% and 5.4% (corresponding to 2.4% and 2.7% on the ratio of the magnetic form factor to that of the proton). Statistical errors are projected to be smaller, allowing adequate statistics for tests of systematic effects. This proposal significantly extends the kinematics range of the already approved 12 GeV proposals E-09-019 and E12-07-104. The form factors assumed here for rate predictions are more conservative, especially at high Q2 than the scaled dipole used in the earlier proposals. The high luminosity of the present proposal yields adequate statistics despite the reduction in cross sections. These kinematic points were originally included as part of E-09-019 but were not approved as part of that experiment. We request a total of 16 days at a beam energy of 11. GeV.