GEpIII and GEp2 Sept 29 2007 Safety Review

GEp(III) and GEp(2γ) Sept. 29 -2007 Safety Review, C. F. Perdrisat, W&M Both experiments measure the ratio of the electric and magnetic form factors of the proton. Form factors are connected to the spatial distributions of charge and current inside the proton. The reaction is elastic ep scattering, with longitudinally polarized beam electrons. We will detect the scattered electrons in Big. Cal, and the recoil proton in the HMS. The Focal Plane polarimeter (FPP) in the HMS will determine the polarization of the recoiling protons.

GEp(III) In GEp(III) we continue on path of 1998 and 2000 Hall A experiments which covered invariant-four momentum squared 0. 5<Q 2<5. 6 Ge. V 2. The proposal maximum Q 2 is 9 Ge. V 2; what we really achieve depends directly upon beam energy: with 5. 74 Ge. V we can get 8. 6 Ge. V 2. Why is largest possible Q 2 interesting? The larger the momentum transfer, the smaller the region of interaction, hence the better the localization, the more exquisite the details we see.

GEP(2γ) In GEp(2γ) we ask if GE/GM is truly a relativistic invariant; in which case its value is independent of kinematics (beam energy, scattering angle), but depends only upon Q 2. A test of the basic Born approximation on which all electron scattering work is based (elastic, quasielastic…) Does the next order diagram: 2 photon exchange, play a role, as some of our theorist colleagues suggest? Basic to the field of e. m. physics.

GEP(2γ)

Equipment required Both experiments use same equipment Big. Cal and HMS with FPP, electronic platform. The beam line and target are standard (20 cm LH 2 and max current 80 micro. A); Moeller polarimeter reconfigured to standard geometry. Big. Cal: 1744 bars of lead glass, 4 x 4 cm 2, 40 and 45 cm long; weight 4300 kg. Phototubes (FEU-84); all will be operated in 1100 -1400 V range. To cover 9 kinematics of 2 experiment combined will have to move Big. Cal 6 times from current 600; 1 move sofar, 30 to 600, in 90 minutes; no problem.

Equipment required, cont. Signals amplified, summed on Big. Cal platform to make trigger. Analog- and timing signal digitized on electronic platform. Focal Plane Polarimeter FPP located between S 1 scintillator plane and lead glass Cerenkov detector. Consists of two 55 cm thick CH 2 analyzer blocs, and 4 drift chambers; sits where gas Cerenkov used to; S 2 scintillator plane removed. New S 0 detector in front of HMS focal plane drift chambers added.

Appearance of experiment To Hall C Counting room

The real FPP

The real Big. Cal

And the cables

Readiness status v. All components of the experiments have been installed except target, & missing section of downstream beam pipe. v. Both FPP and Big. Cal are being extensively tested on cosmics and will continue to be til beam comes. v. Documentation: COO, ESAD and RSAD done or almost done. Big. Cal, under development for FPP. GEp wiki documents all system. v. Iime allocated for experiment adequate assuming 50% efficiency in first part, 60% in second part (large Q 2).

Last slide v Instrumentation documentation (howto’s) prepared for Big. Cal, under preparation for FPP v So far 31 external people registered for shifts. Local core group of 10; at least 20 more have indicated intention to sign up. Collaboration list has another 20 we will convince to sign: it will be a long experiment!