1. Pion Photoproduction from the Nucleon and Scaling
Dipangkar Dutta : Duke University & TUNL
Spokespersons: Haiyan Gao, Dipangkar Dutta, Patrizia Rossi vs
Pion Electroproduction from the Nucleon and Scaling
Kijun Park
University of South Carolina

2. Challenge Ana. proposal
Outline
Introduction
Constituent quark counting
JLab experiment E on photo-pion production from the nucleon [Hall-A]
New proposal (can run with Super-G [E04-005] )
Challenge Ana. proposal
New Window :
Observe the dσ/dt in single pion electro-production with existed data [E99-107]
γp→π+n vs. γ*p→π+n
Original Exp. proposal

3. Introduction
Mapping the transition from the nucleon-meson degrees of freedom to the quark-gluon degrees of freedom of QCD.
In order to describe nucleons and nuclei in terms of quarks and gluons one needs to reconcile these two pictures. In trying to do that a natural questions is …
Nucleon-meson degrees of freedom effective at low energies
Quark-gluon degrees of freedom at high energies

4. Constituent Counting Rule
Exclusive two body reactions (A+B C+D) at large momentum transfers should scale as:
s = c.m. energy sq.
n = # of constituent fields
First derived based on dimensional analysis (Matveev et al., Brodsky, Farrar,….)
Confirmed within short distance pQCD framework (Brodsky, LePage)
Recently, derived from anti-de Sitter/Conformal Field Theory correspondence or string/gauge duality ( Polchinski, Strassler …..)
Many exclusive processes exhibit global scaling behavior
elastic pp 90 deg CM angle (dσ/dt S-10)
deuteron photo-disintegration at large angles (dσ/dt S-11)
C. Bochna et al., PRL 81, 4576 (1998), E.C. Schulte, et al., PRL 87, (2001),
M. Mirazita et al., PRC 70, (2004), P. Rossi, PRL 94, (2005).
Photopion production from nucleon at large angles
L.Y. Zhu et al., PRL 91, (2003)

5. p-p elastic scattering at 900
Deuteron Photodisintegration
Scaling onset:
Pt~1.2 GeV/c
From data compilation of Landshoff and Polkinghorn
E. Schulte et al. PRL 87, (2001)
Bochna et al., PRL 81,
4576 (1998)

6. p-p elastic scattering at 900
What is the origin for such oscillatory scaling behavior?
R = s10ds/dt
Global scaling in good agreement with constituent quark counting
Detailed investigation reveals oscillatory scaling behavior
Oscillatory Scaling Behavior
The oscillations in the scaled cross-section explained as:
Resonance state production near the charm threshold (Brodsky, Schmidt, etc).
Interference between short distance (Born) and long distance (Landshoff) amplitudes, (Ralston & Pire and Carlson, Myhrer, …)

7. Exciting New Developments
``restricted locality'' of quark-hadron duality results in oscillations (Zhao & Close).
a generalized counting rule based on pQCD analysis, by systematically enumerating the Fock components of a hadronic light-cone wave function. (Ji, Ma & Yuan and also Brodsky et al.)
The generalized counting rule includes parton orbital angular momentum and hadron helicity flip.
They provide the scaling behavior of the helicity flipping amplitudes
If = 0;
reduces to constituent quark
counting rule of Brodsky-Farrar.
number of partons
orbital angular momentum projection

8. Generalized Counting Rule & pp Elastic Scattering
Using their Generalized Counting Rule Ji et al. predict the s dependence of the helicity flipping amplitudes in pp elastic scattering as ~S-4.5 and ~S-5
no hel. flip
with hel. flip
D.Dutta and H. Gao, hep-ph/ (accepted for publication in PRC as rapid comm. )

9. Photopion Production
(n=9)

10. Transition observed (90 degree)?
NN n-2=9
L.Y. Zhu, PRL 91, (2003)
Scale factor of 5.4!

11. Limited angular distribution
L.Y. Zhu et al., Nucl-exp/ , PRC 71, (2005)

12. Scaling at 90 degree between 2.0 -2.5 GeV
Very narrow energy window for transition?
Scaled data show resonance structure around 2.2 GeV center-of-mass energy
Missing nucleon resonances from QM?
Threshold associated with strange quark?
Brodsky, de Teramond (1988)
N bound state?
Brodsky, Schmidt, de Teramond (1990)
De Teramond et al. (1998)
Gao, Lee and Marinov (2001)
What can CLAS do?
Very fine energy scan of the scaled differential cross-section in this interesting center-of-mass energy region
Angular dependence of the scaled differential cross-section, important to identify the enhancement around 2.0 GeV
It can also be used to investigate contributions from helicity non-conserving amplitudes.
Ideally one would like to study all nucleon photopion production channels, but new proposal is to measure p+ production.

13. We have looked at single p+ events.
During g11 running period, one run taken with one charge particle trigger.- Patrizia Rossi (priv. comm.)
single p+ events at 900 C.M. angle, used to estimate projected results for new proposal.

14. Kinematic limitaiton : Absence of models
Angular Coverage
Kinematic Coverage E Experiments
W & Q2 Ranges

15. New Proposal Projected Results
Assuming same running
conditions as g11a, and
same target.
For 1000 hrs of running
as approved for superG
1. Collect pre-scaled singles (one charge particle trigger) events during super-G (E04-005) running period.
2. Analyze γp→nπ+ events for a fine energy scan of the scaled cross-section at 900 C.M.angle to verify scaling behavior seen in E94104.
3. Also investigate the angular dependence of the scaled cross-section, specially around √s= 2.0 GeV where an enhancement is seen.
Collaboration
Duke University / George Washington University / INFN, Laboratori Nazionali di Frascati / Carnegie Mellon University / Rensselaer Polytechnic Institute / SLAC, Stanford University / University de Costa Rica / University of Illinois, Urbana, IL

16. Transition observed (90 degree) in pion electroproduction

17. Q2 dependence

18. CLAS data
Rossi et al,PRL 94, (2005)