Timelike Compton Scattering

1
Timelike Compton Scattering
Tanja Horn, Y. Ilieva, F.J. Klein, P.
Nadel-Turonski, S. Stepanyan
Deep Exclusive Processes Working Group 30 May 2008
2
Exclusive Processes and GPDs

• Increasing the virtuality of the photon (Q2)
  allows one to probe short distances
• Sensitivity to partonic degrees of freedom
• At sufficiently high Q2, the process should be
  understandable in terms of the handbag diagram
• Incoming virtual photon scatters off one quark
• interaction can be calculated in perturbative QCD
• The non-perturbative (soft) physics is
  represented by the GPDs
• Shown to factorize from QCD perturbative
  processes for longitudinal photons Collins,
  Frankfurt, Strikman, 1997

3
Experimental Access to GPDs DVCS

• Using a polarized beam on an unpolarized target,
  two observables can be measured

• ? has a large spacelike virtuality
• -t is small

4
Experimental Access to GPDs TCS
Bethe-Heitler (BH)
TCS
2




s(ep?ep?)

• The observable for TCS is the angular
  distribution of a photoproduced heavy lepton pair
  
• The hard scale of the process is given by the
  invariant mass of the lepton pair
• To leading order, TCS and DVCS amplitudes are
  equivalent
• Not true at O(as) and for finite Q2
• In principle carry the same information about
  GPDs
• Combining the results provides the most complete
  information

• ? has a large timelike virtuality
• -t is small

5
TCS Kinematics

• p,p momentum of the incoming and scattered
  proton
• q,qmomentum of the incoming and scattered
  photon
• k,kmomentum of e-, e
• ? angle between the scattered proton and the
  electron
• f angle between lepton scattering and reaction
  plane

6
TCS and BH Cross Sections
E. Berger et al., hep-ph/0110062
A. Psaker, hep-ph/0511283
Relevant for 12 GeV experiments
E? 5 GeV


Q2 3 GeV2
Relevant for 6 GeV experiments
BH
TCS
E? 13 GeV
Q2 5 GeV2
T?? (deg)

• BH dominates in all kinematics and prevents
  direct access to the TCS cross section

• Interference term is large and can be isolated
• odd under exchange of lepton momenta
• observables that change sign project out only
  interference terms
• The real part of the amplitudes can be obtained
  with unpolarized, and the imaginary part with
  circularly polarized photon beams

7
TCS Interference Signal
E. Berger et al., hep-ph/0110062

• Strong BH provides access to TCS
• Interference terms through the angular
  distribution of the lepton pair
• A corresponding asymmetry in DVCS would require
• precise and consistent cross section measurements
  
• both electron and positron beams

flat
Weighted by ratio of lepton propagators
8
TCS Asymmetries
E. Berger et al., hep-ph/0110062

• Construct an asymmetry, R, from the weighted
  angular distribution
• Different GPD model assumptions predict different
  asymmetries
• 5-15 for Q25 GeV2

E?13 GeV
-t0.2 GeV2
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Additional TCS Challenges

• Resonance effects (f, J/?) limit the Q2 range to
  1-9 GeV2 at Jlab energies
• Branching ratios for leptonic decays of heavy
  vector mesons in that range are small, but
  photoproduction cross sections are not well known
• Relevant backgrounds can be studied through
  hadronic decay modes in the same experiment
• Higher order corrections could be large and may
  limit Q2 range further
• Higher order corrections can be studied by
  comparing TCS and DVCS, but should they be too
  large at low Q2 interpretability may be difficult

10
Electroproduction of ee- in CLAS

• Untagged photon beam
• Q20
• Beam electron not detected
• Exclusive ee-p final state
• 6 GeV data available
• e1-6, e1-DVCS
• Polarized beams
• CLAS12 proposal may be possible if the analysis
  is promising

e-1-6 data
7000 events above f peak
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Photoproduction the g12 experiment

• DOE funded Cerenkov gas
• New trigger system allows tagger, STTOF, ECCC
  coincidences
• Statistics
• 25 billion events (gt60 with good tracks)
• 5 hadron/lepton triggers with a 4.4-5.4 GeV
  photon
• Large contribution from misidentified pions
• Maybe 100,000 good lepton/hadron events
• Expected TCS-BH interference events
• At most a few hundred
• CAA planned

12
g12 Acceptance

• g12 target very far upstream
• Massive lepton pairs will decay with large
  opening angles
• One may miss Cerenkov
• Several event topologies possible
• ee-, e-p, ep, ee-p
• Kinematics shown for phase space events generated
  flat in Q2 and E?
• No GSIM, but cuts on inner limits of Cerenkovs
  and minimum proton momentum

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Summary

• Timelike Compton Scattering may become an
  important complement to the Hall B DVCS program
• First look possible at 6 GeV
• E1-6 and e1-DVCS provide data with untagged
  polarized photon
• First photoproduction data will soon be available
  from g12
• New trigger system will provide understanding of
  hadron/lepton trigger efficiency