1. JLab, October 31, 2008 WACS in 12 GeV era 1 GPDs Wide-Angle Compton Scattering pi-0 photo-production in 12 GeV era B. Wojtsekhowski Outline WACS and other Hard Exclusive Reactions. WACS physics and GPDs. Theoretical framework. Experimental technique. How will we do these experiments?  p ->  p

2. JLab, October 31, 2008 WACS in 12 GeV era 2 GPDs WACS: Introduction Part of JLab program of Hard Exclusive Reactions Elastic Form Factors: GM P,GE P,GM N,GE N,FF  WACS: high t in two-photon reaction Deeply Virtual Compton Scattering (DVCS) Deeply Virtual Meson Production Common issues: Handbag diagram Interplay between hard and soft processes Threshold for onset of asymptotic regime Role of hadron helicity flip

3. JLab, October 31, 2008 WACS in 12 GeV era 3 GPDs DISForm factors DVCS WACS tx N -> N*DVMP Unification of nucleon structure within GPDs 

4. JLab, October 31, 2008 WACS in 12 GeV era 4 GPDs GPDs and form factors of WACS G A at  w    

5. JLab, October 31, 2008 WACS in 12 GeV era 5 GPDs Cross section of Wide-Angle Compton Scattering Three-quark mechanism dominates at “asymptopia” 2 hard gluon exchanges. Constituent counting rules: d  /dt = f(  CM )/s 6 “complicated” polarization observables Single-quark mechanism “handbag” diagram dominates. Form factors: “simple” polarization observables

6. JLab, October 31, 2008 WACS in 12 GeV era 6 GPDs Polarization observables of WACS in GPDs handbag calculations LO + R T : photon helicity and P L of the recoil proton hard soft LO:

7. Mixed e/  beam –> productivity 1300 higher than “clean”  ep events RCS events “pion” events Two body kinematics 10 13 photons/sec

8. JLab, October 31, 2008 WACS in 12 GeV era 8 GPDs Results of 6 GeV experiment PRL 94, 242001 (2005)

9. JLab, October 31, 2008 WACS in 12 GeV era 9 GPDs Form factors of RCS and partonic structure of the nucleon

10. JLab, October 31, 2008 WACS in 12 GeV era 10 GPDs Polarization transfer in Wide Angle Compton Scattering ep Very clean data Calibration to elastic e- p polarization data taken parallel to WACS Expected small systematic uncertainty Result is favor to CQM

11. JLab, October 31, 2008 WACS in 12 GeV era 11 GPDs A LL - initial state polarization correlation parameter A LL - K LL diff. as a measure of quark spin flip ~ m q /E  A LL =K LL in QCD D.Day, BW

12. JLab, October 31, 2008 WACS in 12 GeV era 12 GPDs RCS perspective with 12 GeV Jlab beam

13. JLab, October 31, 2008 WACS in 12 GeV era 13 GPDs Neutron Compton Scattering Need to know incident photon energy -> Limited luminosity Post-target tagging! For s = 7, t = 4 GeV 2 120 hours beam time per data point

14. JLab, October 31, 2008 WACS in 12 GeV era 14 GPDs Joint Hall A + C project consists of 1744 blocks front face - 1.2 m x 2.2 m resolution ∆E ~ 6%/√E σ position ~ 1 cm Ready for experiments BIGCAL Calorimeter

15. JLab, October 31, 2008 WACS in 12 GeV era 15 GPDs Summary of the physics plan Proton WACS – cross section with 11 GeV beam, ALL&KLL Neutron WACS – cross section Pi-0 from the proton – cross section, KLL, ALL Pi-0 from the neutron – cross section Strange form-factor at large Q2 ( ep->e’p, both in calorimeters)

16. JLab, October 31, 2008 WACS in 12 GeV era 16 GPDs Calorimeter parameters Calorimeter size – match of Super Bigbite Spectrometer – 20 cm long target, 70 msr ( horizontal +/- 5 degree, vertical +/- 12 degree), energy resolution <10%, angular/position resolution of 1-2 mrad, two cluster resolution < 10 mrad, need at least 2-3 m between magnet and the calorimeter. Radiation hardness!! For example: at distance 5-10 meters from the target: BigCal 1.2x 2.2 m, 6% energy resolution, 1 cm coordinate resolution, R Mol ~ 3.7 cm, limit for radiation dose of 1 krad. What need to be done: a) Study of radiation hardness and resolution at high luminosity b) Budget and used equipment: PMT, DAQ, HV, crystals