1. Envisioned PbWO4 detector Wide-Angle Compton Scattering at JLab-12 GeV with a neutral-particle detector With much input from B. Wojtsekhowski and P. Kroll and thanks to M. Fowler

2. Part of JLab program of Hard Exclusive Reactions Elastic Form Factors 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 DVCS pp e e WACS pp   Wide-Angle Compton Scattering: Introduction

3. GPDs DISForm factors DVCS WACS tx N  N*DVMP  WACS was the first process in which predictive power of GPD calculations was demonstrated WACS deals with nucleon GPDs - the same GPDs as in Form factors WACS allow to obtain information on the axial GPD - unique information at high -t Unification of Nucleon Structure with GPDs

4. WACS and GPDs WACS pp   GPD For large Mandelstam variables s, -t, -u : CM helicity amplitudes for WACS factorize in a hard sub-process  q   q and in form factors that represent the 1/x moments of GPDs. This factorization is a generalization of the handbag factorization for deeply virtual exclusive processes, achieved in a frame where skewedness  is zero.

5. GPDs: form factors and WACS

6. WACS and GPDs WACS pp   GPD For large Mandelstam variables s, -t, -u : CM helicity amplitudes for WACS factorize in a hard sub-process  q   q and in form factors that represent the 1/x moments of GPDs. This factorization is a generalization of the handbag factorization for deeply virtual exclusive processes, achieved in a frame where skewedness  is zero. Compton form factors evaluated from nucleon form factors exploiting sum rules Kroll

7. Cross section of WACS 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

8. Cross section of WACS – cont. Compton cross section at several values of s Green bands depict uncertainty in FFs Yellow bands include uncertainty due to target-mass corrections Kroll

9. JLab experiments used mixed e/  beam  productivity 1300 higher than with “clean”  beam ep events RCS events “pion” events Two body kinematics 10 13 photons/sec Exp. Details: 0.3 Tm magnet 72 x 100 cm 2 calorimeter veto not needed separation o.k. between RCS and ep events for E  ~ 2-3 GeV

10. Results of 6-GeV RCS experiment in Hall A PRL 94, 242001 (2005) Miller Kroll w. GPD adjusted to elastic FFs Fit d  /dt = f(  CM )/s n

11. Cross sections of WACS: partonic structure Recall connection between GPDs accessed in elastic e-p and WACS Promising: similar –t behavior!

12. Polarization observables of WACS LO + R T : photon helicity and P L of the recoil proton hard soft LO: GPD handbag calculation Neglect quark masses  A LL = K LL in handbag A LL = correlation between helicity of incoming photon and incoming proton K LL = correlation between helicity of incoming photon and outgoing proton

13. ep Calibration to elastic e-p polarization data taken in parallel to WACS  expect small systematic uncertainty CQM closest Polarization observables of WACS – cont. New: Hall C E07-002 experiment clean data Green band reflects uncertainty in FFs  q   q subprocess P. Kroll

14. WACS perspective with 12-GeV JLab beam 12 GeV provides all new data, no Compton scattering data exist in this region!

15. WACS perspective with 12-GeV JLab beam Overlap with existing Lessons: Requires large photon angles Spectrometer momentum above 4 GeV/c  Hall C

16. WACS perspective with 12-GeV JLab beam Consider HMS first, gain of solid angle of ~ factor of two

17. WACS perspective with 12-GeV JLab beam 1) Consider HMS first, gain of solid angle of ~ factor of two  Loose  cm > 120 o region  cos(  ) < -0.5 2) Need photon detection system on SHMS side covering up to ~ 55 o

18. Angle range = 5 – 30 degrees

19. Minimum angle = 25 degree (comfortably) Move neutral-particle channel to other side ~20 Deg.Sweeper magnet Beamline Target chamber Deck supports as designed Detector

20. Move neutral-particle channel to other side Fits on platform, but need to evade/add/move post

21. Structure Sweeper magnet Target chamber Deck Supports Detector Looking downstream

22. Target chamber Sweeper magnet Deck Support Detector Looking Upstream

23. Structure Utility platform Sweeper magnet Target chamber Deck Supports Detector Side View Fits!

24. Special features of the WACS experiment: 1)Radiator and Luminosity: 40  A x {8% Cu + 15 cm LH2} [1 st guess: 50 days]  this needs further study: background rates at higher photon angles? 2)Large scattering angle of the photon up to 50-60 degrees  YES, can have neutral particle detector covering 5-30 degrees AND 25 degrees and higher Common features of the WACS and Pion experiments: 1) Good photon detector: coordinate and energy resolutions, large solid angle  PbWO4 detector with good position resolution helps  solid angle similar as for 6-GeV experiments, well matched to proton arm 2)Magnet between the target and the photon arm  0.3 Tm appropriate 3)Magnetic spectrometer for the second arm  HMS appropriate WACS perspective - summary