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Real Compton Scattering from the Proton in the Hard Scattering Regime

Real Compton Scattering from the Proton in the Hard Scattering Regime. Alan M. Nathan SLAC Experimental Seminar December 2, 2003. I. Compton Scattering from Nucleon at Large p  Reaction Mechanisms Nucleon Structure II. JLab E99-114 the experiment preliminary results

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Real Compton Scattering from the Proton in the Hard Scattering Regime

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  1. Real Compton Scattering from the Proton in the Hard Scattering Regime Alan M. Nathan SLAC Experimental Seminar December 2, 2003 • I. Compton Scattering from Nucleon at Large p • Reaction Mechanisms • Nucleon Structure • II. JLab E99-114 • the experiment • preliminary results • III. Summary & Outlook Ph.D. Students: A. Danagoulian, D. Hamilton, V. Mamyan, M. Roedelbronn

  2. Hard Scattering Regime (s, -t, -u >> m2) (plarge) Factorization: calculable in pQCD nonperturbative structure process-independent

  3. Real Compton Scattering on the Nucleon in the Hard Scattering Limit Some Possible Factorization Schemes • hard gluon exchange: • 3 active quarks • 2 hard gluons • 3-body “form factor” • handbag: • 1 active quark • 0 hard gluons • 1-body form factor • The physics issues: • Which, if either, dominates at few GeV? • What does RCS teach us about nucleon structure?

  4. I. Asymptotic (pQCD) Mechanism • Brodsky/LePage, Kronfeld, Vanderhaeghen, Dixon ... • momentum shared by hard gluon exchange • 3 active quarks • valence configuration dominates • soft physics in distribution amplitude f(x1, x2, x3) • constituent scaling: d/dt = f(CM)/s6 • dominates at “sufficiently high energy”

  5. Constituent Scaling KS COZ Cornell 1977 data Asymptotic(symmetric)DA x1 x2 x3 s6d/dt Cornell data approximately support scaling but... When is this the dominant mechamism?

  6. pQCD Generalized Parton Distribution II. “Handbag” Mechanism (Radyushkin; Diehl, Kroll, et al.) • One active parton—rest are spectators • Momentum shared by soft overlap: GPD’s • Central assumptions: • -- s,-t,-u >> m2 • -- struck quark nearly real and ~co-linear with proton • Formally power correction to leading-twist • -- asymptotically subdominant but ...

  7. KN Rv,RA,RT Handbag Description of RCS See Kroll, hep-ph/0110208 Various approximations improve as s,-t,-u  m2 • Factorization is simple product • Hard scattering: Klein-Nishina (KN) from nearly on-shell parton • Soft physics: form factors RV(t), RA(t) • |RV +/- RA|2 : • active quark spin parallel/antiparallel to proton spin • Important feature: /KN ~ s-independent at fixed t

  8. RCS and Form Factors: GPD’s GeneralizedParton Distributions (GPD’s) links among diverse processes RCS sensitive to unskewed (=0) GPD’s at high –t, moderate x

  9. Scaling of Cross Sections at fixed CM: d/dt ~ (1/s)2 R2(t) • R ~ 1/t2 for -t = 3-10 GeV2 n  6 scaling (accidental!) • d/dt ~ s-2t-4 • Asymptotically R ~ 1/t4 n 10 scaling •  ultimately subdominant (when?) • Handbag gives right order of magnitude for Cornell data

  10.    p Polarization of Recoil Proton: Longitudinal Robust prediction: depends only on ratio of form factors

  11. -KLT pQCD Polarization of Recoil Proton: Transverse • RT : hadron helicity flip • pQCD: -t F2/F1 ~ constant • JLab GEp expt: -t½F2/F1 ~ constant • Does RT/RV behave similarly?

  12. Goals of JLab E99-114 • Measure cross sections to 5% + 5% over broad kinematic range of s, t • --/KN vs. s @ fixed t • --1/sn @ fixed qcm • --detailed comparison with handbag • Measure KLL and KLT at s=7, -t=4

  13. Proton Spectrometer Polarimeter Radiator LH2 e- e- γ Lead-Glass Calorimeter Deflectionmagnet  e- Experimental Setup Kinematic Range: • mixed e- beam • e-p/RCS discrimination needed • background & calibrations • good angular resolution • FPP Experiment ran in Hall A in 2002

  14. Some Experimental Details • e- beam: 5-40 A, 2.5-5.75 GeV, 75% polarization • radiator: 6% Cu, 10 cm from target • target: 15 cm LH2 •  beam: 1013 sec-1 equivalent photons • deflection magnet: 10 cm, 1 T • calorimeter: 704 blocks, 4 cm x 4 cm x 40 cm lead glass • proton spectrometer: HRS-L, =6 msr, Pp4.3 GeV/c • proton polarimeter: 6 MWDC’s, 60 cm CH2, 60 cmC

  15. Event Identification RCS: p(,)p ep: p(e,e)p 0: p(,0)p p   RCS deflected ep e or  1/2 = m/E e' ' 0

  16. RCS Event Identification x y ep RCS+ep RCS 0 0 x vs E t

  17. Analyzing the Recoil Proton Polarization: Proton Spin Precession FPP • =270o • max sensitivity to L,T • no sensitivity to N

  18. Analyzing the Recoil Proton Polarization: Focal Plane Polarimeter spin-orbit interaction • FPP calibration from ep • Transformation from Lab to CM L and T get mixed • Dilution due to 0 background • easily measured

  19. Longitudinal Polarization Transfer: Preliminary Result from E99-114…final results by Fall 2003 KLL Kroll Vanderhaeghen Dixon KLL consistent with single-quark mechanism with active quark carrying proton spin (RA Rv)

  20. KLL  KRCSLL  KKNLL • predicted by handbag • five more points measured but not yet analyzed (75o-130o)

  21. pQCD Transverse Polarization Transfer: Preliminary Result from E99-114 Calculation by Kroll Conclusion: -KLT  RT/RV (0.50.4) F2/F1 Hard to draw any conclusions with this precision

  22. this and the next few slides show preliminary cross sections • (good to about 20%) • s-6 scaling at fixed CMworks only approximately • Leading twist still badly underestimates cross section

  23. d/dKN Preliminary Cross Sections Prediction of s-independence at fixed t not well followed But...

  24. Preliminary Cross Sections • Prediction of s-independence at fixed t not well followed • But...not so bad for data with s,-t,-u > 2.5 GeV2 • Higher energy data would be desirable

  25. Preliminary Cross Sections Calculations by Kroll et al. • For handbag, t4d/dKN nearly independent of s,t • Data for s,-t,-u > 2.5 GeV2 in rough agreement

  26. A New Experiment: JLab E03-003: • Measure KLL,KLT,PN : confirm single-quark dominance • s = 7.0 ; -t: 1.3 - 5.1 ; -u: 3.0 - 0.1 [GeV]2 • Determine form factors: • RA/RV: Δu(x)/u(x) • RT/RV:F2/F1 • Observe NLO effects • nonzero PN

  27. Summary of Goals of E03-003 s = 7 GeV2

  28. Summary and Outlook • E99-114 successfully completed • KLL and KLT at s=7, -t=4 • KLL is “large” and positive • Suggestive of single-quark mechanism • Similar result for 0 photoproduction • Consistent with handbag prediction • KLT/KLL consistent with F2/F1, with poor statistics • d/dt over broad kinematic range • t4/KN roughly constantfor s,-t,-u > 2.5 GeV2 • s-6 scaling at fixed CM works only approximately • Lots of p(,0) to be mined • Planned extension • JLab E03-003: angular distribution of KLL ,KLT , PN at s=7 • Higher energy desirable

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