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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 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
Hard Scattering Regime (s, -t, -u >> m2) (plarge) Factorization: calculable in pQCD nonperturbative structure process-independent
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?
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”
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?
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 ...
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
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
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
p Polarization of Recoil Proton: Longitudinal Robust prediction: depends only on ratio of form factors
-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?
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
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
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, Pp4.3 GeV/c • proton polarimeter: 6 MWDC’s, 60 cm CH2, 60 cmC
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
RCS Event Identification x y ep RCS+ep RCS 0 0 x vs E t
Analyzing the Recoil Proton Polarization: Proton Spin Precession FPP • =270o • max sensitivity to L,T • no sensitivity to N
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
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)
KLL KRCSLL KKNLL • predicted by handbag • five more points measured but not yet analyzed (75o-130o)
pQCD Transverse Polarization Transfer: Preliminary Result from E99-114 Calculation by Kroll Conclusion: -KLT RT/RV (0.50.4) F2/F1 Hard to draw any conclusions with this precision
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
d/dKN Preliminary Cross Sections Prediction of s-independence at fixed t not well followed But...
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
Preliminary Cross Sections Calculations by Kroll et al. • For handbag, t4d/dKN nearly independent of s,t • Data for s,-t,-u > 2.5 GeV2 in rough agreement
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
Summary of Goals of E03-003 s = 7 GeV2
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