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Polarization transfer in Wide-Angle Compton Scattering Proposal 08-017

Polarization transfer in Wide-Angle Compton Scattering Proposal 08-017. D. Hamilton, R. Gilman, A. Nathan, and B. Wojtsekhowski, co-spokespersons. Outline Mechanism of the reaction is a key question Compton  e - >  e process and polarization observables

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Polarization transfer in Wide-Angle Compton Scattering Proposal 08-017

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  1. Polarization transfer in Wide-Angle Compton ScatteringProposal 08-017 D. Hamilton, R. Gilman, A. Nathan, and B. Wojtsekhowski, co-spokespersons Outline • Mechanism of the reaction is a key question • Compton e -> e process and polarization observables • WACS present results and experimental program • Proposed experiment and expected results

  2. Mechanism of the process Two basic options for mechanism: Collective response - several partons involved in high momentum interaction with the photons Individual response - one quark absorbs incident photon and the same quark emits scattered photon

  3. Studies of the RCS process • Regge poles - VMD - since 1960’s …, Laget • pQCD - two-gluon - Brodsky, …, Guichon&Vanderhaeghen, Brooks&Dixon, Thomson et al. • Diquark model - Guichon&Kroll, 1996 • Leading quark - Brodsky et al., 1972 • GPDs (handbag) - Radyushkin, Kroll et al. • CQM - G.Miller Main issues: • Competing mechanisms • Interplay between hard and soft processes • Threshold for onset of asymptotic regime • Role of hadron helicity flip

  4. Exact formalism for CS off electron

  5. Compton in 1925 photo electron Directed Quanta of Scattered X-Rays Arthur Compton and Alfred Simon, PR 26, 3 (1925) recoil electron

  6. Cross section of WACSvs pQCD calculations Cornell data JLab data Calculation by Brooks & Dixon Calculation by Thomson et al normalization problems, calculations are 3-10 times below data

  7. WACS in GPD approach • Single-quark mechanism “handbag” diagram accounts for scattering from quark and introduces the FFs for ++,+-, .. Form factors: RV, RT, RA KN-like polarization observables quark coupling is small compare t Form factors allows to fit cross section

  8. The plan of proposed experiment p  proposed -u = 2.3 -u = 3.6 p

  9. Result: test of reaction mechanism Proposed for E ~ 4.3 GeV for cm angles 90o and 110o ( a 70o cm data point will be obtained in E07-002)

  10. Result: test of reaction mechanism Proposed for E ~ 4.3 GeV for cm angles 90o and 110o ( a 70o cm data point will be obtained in E07-002)

  11. Experiment concept: similar to E99-114 Beam 80% polarized, 80 μA Ee= 4.8 , <Ēγ>~ 4.3 GeV Photon flux ~ 1013 eqv. /s; in this experiment it will be boosted by an additional factor 5 with larger calorimeter

  12. Selection of the WACS events Magnet separates e'/γ, no veto needed, allows higher luminosity

  13. Focal Plane Polarimeter FPP used in many Hall A experiments FOM = εAy2 ~ 0.02, so we need ≈1-2 M RCS events Calibrated with ep elastic scattering

  14. E99-114 Results vs new plan s = 6.9 GeV2 -t = 4.0 -u = 1.13 PRL 94, 242001 PRL 98, 152001 Present proposal (KLL,LT) has photon energy 4.3 GeV, large s, -t, -u

  15. WACS research program CS research has significant chance to discover the basic reaction mechanism CS research would be extended for cross sections up to to s = 20 GeV2 with the 12 GeV upgrade, could possibly find onset of pQCD in exclusive reactions

  16. Proposed Kinematics Eγ= 4.3 GeV, s = 8.9 GeV2 Large -t,-u Precision data for KLL, and KLT

  17. Beam Time Request 21 days beam on target in Hall A

  18. Summary • Mechanism of Real Compton Scattering could be found from proposed experiment - need two data points • WACS has largely the same physics as FFs and DVCS • Experimental technique is well understood and tested • This polarization experiment belongs to 6-GeV program • Beam time request: 21 days beam on target in Hall A

  19. Backup slides

  20. Beam Time Request - by hours 21 days of beam in Hall A

  21. Reply to TAC comments • Installation time ~ < 1 month in Hall A. • As suggested by RadCon group the photon radiator will be made from Pb, for which case rad-budget is twice lower than for the standard Cu radiator.

  22. Polarization observables in QED

  23. GPD-based prediction for KLS

  24. NLO GPD-based calculations NLO corrections to the cross section are 10-20% Corrections to polarization observables ~ 1-2% at cm=90o ~ 10-15% at 120o

  25. GPDs and form factors of WACS GA at w 

  26. Systematic Uncertainties • By far the largest contribution to systematic uncertainty from dilution factor/background asymmetry stability. • In previous analysis, background was fitted with polynomial distribution. • In future analysis, a dedicated Monte Carlo (already developed) will be used to better understand background. • Expected improvement in systematic uncertainty by factor two or more- dKLLsyst ~ 0.01

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