E99007, or GEp(II), is the extension of E93027, which measured for the first time the ratio of the electric to magnetic form factors G_E/G_M for the proton up to 3.5 GeV². E99007 will extend this measurement with one overlap point, at 3.5, and 3 new points: 4.0, 4.8, 5.6 GeV².

The technique used is the recoil polarization technique. In the elastic scattering of a polarized electron beam on an unpolarized hydrogen target, the incident electron transfers its polarization to the proton. In the approximation of single photon exchange, the normal component of this proton polarization is zero, while the ratio of the longitudinal to transverse components is directly proportional to the ratio G_E/G_M .  This ratio will then be measured with unprecedented accuracy at such a high Q² (0.05 absolute error at 5.6 GeV²), and along with the already known and precise measurements of G_Mp via Rosenbluth separation method and single-arm cross-section measurements, will give us more insight on the behavior of G_Ep in this intermediate region of momentum transfer.

The recoil proton polarization is measured with the Hall A Focal Plane Polarimeter (FPP), which can measure the normal and transverse components of the polarization at the focal plane (for scheme of FPP, click here, in .eps format: to download, push Shift and click on the link). These are related to the components at the target via the Spin Transfer Matrix (STM), which is calculated with a code such as SNAKE or COSY.
At high proton momentum (over 2.0 GeV/c), the analyzing power of the graphite (currently used in the FPP) dramatically drops down. Therefore we need to replace it by polyethylene (CH₂) (click here for layout of installation, .eps format: to download, push Shift and click on the link).

Critical to this experiment is solid angle matching. At the maximum Q² (5.6 GeV²) with this beam energy (5.734 GeV), the solid angle covered by the electron is greater than the one covered on the hadron side. Thus, to make full use of the Left HRS (in which the proton is detected), it is necessary to detect the electron in a detector with bigger solid angle than the Right HRS. The option that has been chosen and tested is a lead-glass calorimeter. 144 15*15 cm² blocks of lead-glass will be assembled in an array of 9*17, and connected to ADCs and TDCs. The blocks come from the pion rejector that was used in the current hadron arm, and from the shower counter used in the current electron arm.

As a minor detail, a fault in what is currently the hadron arm (Right HRS) dipole prevents us from detecting protons in it at more than 3.2 GeV/c. This means the detector stacks of the two HRSs have to be exchanged, so that the FPP and all hadron detectors are on the Left HRS. This swap will be done during the 2 months down period this August and September 2000, by the Hall A technical staff, under the coordination of Jack Segal.  We will take advantage of this long period to assemble the calorimeter, and the CH₂ analyzer (see the dedicated page).

The polarization of the electron beam will be measured using the Hall A Compton polarimeter, built by Saclay, and which has been proven to be very reliable during HAPPEX and N-Delta experiments in the last 2 years.