[ee] TCS proposal -- skeleton for scientific motivation section

[Christian Weiss]

Dear Friends,

Vadim and I produced a draft skeleton of the scientific motivation
section of the TCS proposal; see attachment. Please have a look
and send us your comments or suggestions, if any. Best if we 
fix the logic and structure at this level; Vadim will then take
the lead in fleshing out the actual text.

The skeleton assumes a top-level organization into two main
sections: "Scientific motivation" and "Proposed experiment."

As we discussed this morning, the 6 GeV results will be discussed
in a subsection of "Scientific motivation," with the aims of
demonstrating the feasibility, bringing out whatever conclusions
can be drawn, and quantifying the improvements expected with 12 GeV 
kinematics. The more technical aspects of the 6 GeV experiment
can be discussed in the experimental section.

I hope we can move quickly with the main text.

Cheers,

   Ch.


Christian Weiss
Theory Center
Jefferson Lab
12000 Jefferson Ave
Newport News, VA 23606, USA
Phone: +1.757.269.7013
Fax:   +1.757.269.7002
E-mail: weiss at jlab.org
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I) SCIENTIFIC MOTIVATION


1) INTRODUCTION

  Spacelike-timelike correspondence: Basic property of hadronic physics,
  rooted in fundamental principles of relativity and causality;
  examples are spacelike/timelike FFs and DIS/Drell-Yan


  Hard exclusive processes: New class of processes, QCD factorization,
  GPDs, combine aspects of elastic FFs and PDFs


  DVCS: Well-developed leading-twist formalism, access to GPDs at 
  amplitude level, promising early data, extensive program


  TCS: Cross-channel version of DVCS, QCD factorization and handbag 
  mechanism, growing interest


  Main motivation for TCS:

      1) Test universality of GPDs/CFFs implied by factorization;
         analog of DIS -- DY correspondence

      2) Special sensitivity to real part of CFFs; impacts on GPD models

      3) Prospect of global GPD analysis: Amplitude analysis
         Dispersion relations, other developments


  Feasibility demonstrated by CLAS 6 GeV experiment



2) TIMELIKE VIRTUAL COMPTON SCATTERING AND GPDs

  Kinematic range for leading-twist appoximation: 
  1 GeV^2 < M^2 < M_psi^2, high W, low t
  [Figure with e+e- -> hadrons data]


  Present/discuss leading-twist expressions
  [Figure with handbag diagrams]


  GPD integrals/CFFs same as in DVCS
  Discuss sensitivity to real part
  [Figure with GPD model predictions for Re(CFF)]


  Comment on NLO corrections: Timelike expected to be sensitive to NLO;
  experience with elastic FFs (Radyushkin & Stefanis); work in progress
  [Figure for illustration -- schematic]


  Comment on amplitude analysis (Guidal et al.): Status, issues,
  prospect of future joint DVCS-TCS analysis
  [Figure for illustration -- projected results]


  Comment on dispersion analysis: Experimentally accessible information 
  on GPDs; model-independent analysis (Polyakov & Vanderhaeghen); 
  importance of data on real part; prospect of future joint DVCS-TCS 
  analysis


  Comment on higher-twist corrections: Phenomenological estimates;
  alternative models; duality of GVMD and quark description --
  Bjorken scaling through off-diagonal terms



3) TIMELIKE COMPTON SCATTERING RESULTS FROM CLAS 6 GEV

  Discussion of data & analysis prospects

  Feasibility demonstrated

  Limitations of 6 GeV kinematics -- why 12 GeV is needed.



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II) PROPOSED EXPERIMENT


   [Description of 12 GeV experiment]
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