Real Compton Scattering from the Proton

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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
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Hard Scattering Regime (s, -t, -u gtgt m2) (p?
large)
Factorization
calculable in pQCD
nonperturbative structure process-independent
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Real Compton Scattering on the Nucleon in the
Hard Scattering Limit Some Possible Factorization
Schemes

• 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

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Constituent Scaling
s6d?/dt
Cornell data approximately support scaling
but... When is this the dominant mechamism?
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II. Handbag Mechanism
(Radyushkin Diehl, Kroll, et al.)

• One active partonrest are spectators
• Momentum shared by soft overlap GPDs
• Central assumptions
• -- s,-t,-u gtgt m2
• -- struck quark nearly real and co-linear with
  proton
• Formally power correction to leading-twist
• -- asymptotically subdominant but ...

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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 /- RA2
• active quark spin parallel/antiparallel to
  proton spin
• Important feature ?/?KN s-independent at
  fixed t

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• RCS and Form Factors GPDs

Generalized Parton Distributions (GPDs) links
among diverse processes
GPD x-1 moment x0 moment t0 limit
Rv(t) F1(t) q(x)
RA(t) GA(t) ?q(x)
RT(t) F2(t) 2J(x)/x - q(x)
RCS sensitive to unskewed (?0) GPDs at high t,
moderate x
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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

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Polarization of Recoil Proton Longitudinal
Robust prediction depends only on ratio of form
factors
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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?

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Goals of JLab E99-114

• Measure cross sections to 5 5 over broad
  kinematic range of s, t
• --?/?KN vs. s _at_ fixed t
• --1/sn _at_ fixed qcm
• --detailed comparison with handbag
• Measure KLL and KLT at s7, -t4

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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
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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 MWDCs, 60 cm CH2, 60 cm C

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Event Identification
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RCS Event Identification
?x
?y
ep
RCSep
RCS
?0
?0
?x vs E
?t
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Analyzing the Recoil Proton Polarization Proton
Spin Precession

• ?270o ?
• max sensitivity to L,T
• no sensitivity to N

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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

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Longitudinal Polarization Transfer Preliminary
Result from E99-114final results by Fall 2003
Kroll
Vanderhaeghen Dixon

• KLL consistent with single-quark mechanism with
  active quark carrying proton spin (RA ? Rv)

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

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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
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this and the next few slides show preliminary
cross sections (good to about 20)

• s-6 scaling at fixed ?CM works only
  approximately
• Leading twist still badly underestimates cross
  section

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Preliminary Cross Sections
Prediction of s-independence at fixed t not well
followed But...
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Preliminary Cross Sections

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

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Preliminary Cross Sections
Calculations by Kroll et al.

• For handbag, t4d?/d?KN nearly independent of s,t
• Data for s,-t,-u gt 2.5 GeV2 in rough agreement

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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
  GeV2
• Determine form factors
• RA/RV ?u(x)/u(x)
• RT/RV F2/F1
• Observe NLO effects
• nonzero PN

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Summary of Goals of E03-003
s 7 GeV2

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Summary and Outlook

• E99-114 successfully completed
• KLL and KLT at s7, -t4
• 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 constant for s,-t,-u gt 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 s7
• Higher energy desirable