1. Resononace electrocouplings from 2p electroproduction
V.I.Mokeev
analysis of recent CLAS data on double charge pion electroproduction off protons at W<1.6 GeV and 0.25<Q2<0.6 GeV2 ;
various degrees of freedom surfaced in Q2 evolution of P11(1440) and D13(1520) electrocouplings;
prospects for the studies of full N*’ spectrum at photon virtualities less then 10 GeV2.

2. Physics motivation for N* studies at low W and Q2
First data on Q2 evolution of P11(1440) electrocouplings from 2p electroproduction at Q2<0.6 GeV2.
Considerably improved data on electrocouplings of D13(1520), obtained in Q2-bins of minimal sizes ever achieved; access to poorly known 2p hadronic couplings.
Preferable kinematics area to study meson-baryon dressing of N*’s.
Most suitable for the initial studies of N*’s in combined analysis of 1p/2p channels within the framework of advanced coupled channel approach, developing by EBAC.

3. Key importance of the data on 1p/2p electroproduction mechanisms for the N* studies
CLAS data on meson electroproduction
at Q2<4.0 GeV2
Exclusive channels in pN
scattering responsible for
FSI in meson electroproduction
W (GeV)
1p/2p channels are strongly coupled by FSI.
1p/2p channels are major contributors.

4. Comprehensive data on 2p electroproduction, for the first time available CLAS.
The measurements with unpolarized e- beam, p target, offer nine independent differential cross-sections in each (W,Q2) bin, which are various 1-d projections of 5-d cross-section
Contributing mechanisms may be established from their manifestations in a variety of observables JM approach .

5. JLAB-MSU model (JM) for 2-p electroproduction.
Isobar channels included:
3-body processes:
p-D++
All well established N* with pD decays and 3/2+(1720) candidate, seen in CLAS 2p data.
Reggetized Born terms & effective FSI&ISI treatment .
Extra pD contact term. rp
All well established N* with rp decays
and 3/2+(1720) candidate.
Diffractive ansatz for non-resonant part & r-line shrinkage in N* region.

6. con’d 3-body processes: Direct 2p production Isobar channels included:
p+D013(1520), p+F015(1685), p-P++33(1640) isobar channels, observed for the first time in the CLAS data at W>1.65 GeV.
(P++33(1640))
(p+)
F015(1685)
Direct 2p production
V.Mokeev, V.Burkert, J. Phys. 69, (2007).

7. Description of the CLAS data within the framework of JM model
JM full
p-D++
p+D0
2p direct
All significant mechanisms, contributing to ep→e’p-p+p channel at W<1.6 GeV and Q2<0.6 GeV2 were established from the CLAS data analysis.

8. Fit of the CLAS 2p data, varying N
Fit of the CLAS 2p data, varying N* electrocouplings and parameters of non-resonant mechanisms.
CLAS data
fit within the framework of JM model
c2/d.p. for selected in fit cross-sections:
Q2-interval, GeV2
2 /d.p.
<2.8
<1.9
<1.8

9. Isobar channel contributions
CLAS data
the contributions from all isobar channels
CLAS data fit
p-D++ channel
p+D0 channel
new opportunities to understand the mechanisms of 2p production.
valuable information for coupled channel analysis, under development at EBAC
(T-S.H.Lee, L.C.Smith, J.Phys, G34, S83 (2007)).

10. Partial waves of total spin J for non-resonant helicity amplitudes in p-D++ isobar channel
1/2
Born terms
3/2
5/2
Extra contact terms
Will be used for N* studies in coupled channel approach developing by EBAC.

11. P11(1440) and D13(1520) electrocouplings at Q2<0.6 GeV2.
from analysis of 1p CLAS data
from analysis of CLAS 2p data
within the framework of JM06
combined analysis of 1p/2p CLAS data
CLAS 2p data provided compelling evidence for sign flip of P11(1440) A1/2 electrocoupling.
Electrocouplings obtained in analyses of major 1p and 2p channels are in reasonable agreement.

12. Robustness of the data on P11(1440) electrocouplings
P11 on
P11 is substituted by non-resonant mechanisms
Q2 independent fit
Q2 dependent fit
2.6<c2/d.p.<2.8
3.6<c2/d.p.<4.0

13. Ground state and P11(1440) electrocouplings & quark model expectations
Ground p state
P11(1440)
S.Capstick
light cone (LC)
model
B.Metsch
Bethe-Salpeter
model
I.Aznauryan
LC model
3q core
3q core +??
M.Giannini/
E.Santopinto
hyper-centric
CQM
P11(1440) electrocouplings at high Q2 are consistent with major contribution from 3q core in radial excitation, while at Q2<0.6 GeV2 additional contributions become evident.

14. Evidence for meson-baryon dressing
bare electrocouplings from I.Aznauryan LC model
P11(1440)
dressed electrocouplings (accounting for only pN interactions in dressing)
meson-baryon cloud accounts for difference between curves
Q2 evolution of meson-baryon cloud was estimated within the framework of model independent coupled channel approach
(B.Julia-Diaz, T-S.H.Lee, et.al, Phys. Rev. C77, (2008)).
Much better data description at Q2<0.6 GeV2 offer an evidence for sizable contribution from meson-baryon cloud

15. D13(1520) electrocouplings & quark model expectations
data on electromagnetic form factors for ground and all well established excited nucleon states offer complementary information on baryon structure
various degrees of freedom in the structure of P11(1440), D13(1520) are surfaced at low and high Q2
3q core
How significant meson-baryon dressing for high lying N*’s?
3q core +meson –baryon cloud +??

16. Electrocouplings of high lying N*’s.
First consistent mapping of Q2-dependence for D33(1700), P13(1720) electrocouplings
D33(1700)
D33(1700)
Extension of analysis, fitting
9 differential cross-section
is in progress.
First data on S31(1620), D13(1700), D33(1700), P13(1720) electrocouplings are expected by the end of 08’.
P13(1720)
P13(1720)
No?Yes

17. New regime in N* excitation at Q2>4.0 GeV2
EBAC estimates for evolution of meson-baryon dressing with Q2
dressing effects decrease with Q2, as highly virtual photons penetrate meson-baryon cloud
data on N* electrocouplings at Q2 > 4.0 GeV2 will allow us to access internal quark core structure, getting rid of meson-baryon degrees of freedom
feasible after 12 GeV Upgrade with CLAS12
B.Julia-Diaz, T-S.H.Lee, et.al, Phys. Rev. C77, (2008).

18. very preliminary Resonance signals in 2p electroproduction at high Q2.
Fully integrated 2p cross section at
Q2 from 4.5 to 5.2 GeV2
After 12 GeV Upgrade CLAS12 will be only facility foreseen worldwide, capable to study electrocouplings for full spectrum of N*’s at Q2 from 5.0 to 10 GeV2.
very preliminary
D33(1700),P13(1720)
3/2+(1720),F15(1685)
D13(1520)
S11(1535)
Access to constituent quark structure and interactions through quark core excitation in N*’s for the first time.
Evidence for substantial N* contributions

19. Conclusions and outlook
Analysis of the CLAS data on 2p electroproduction, allowed us for the first time to determine isobar channel cross-sections and partial wave amplitudes. This information will be used by EBAC for N* studies in advanced coupled channel analysis.
P11(1440) and D13(1520) electrocouplings were obtained at Q2 from 0.2 to 0.6 GeV2 from 2p data for the first time. Analysis extension is in progress, with the goal to determine electrocouplings for almost all N*’s with M < 2.5 GeV at 0.2<Q2<4.5 GeV2.
Various degrees of freedom were surfaced in Q2 evolution of P11(1440) and D13(1520) electrocouplings : sizable meson-baryon dressing at Q2<0.6 GeV2; major contribution from 3-quark core at Q2>1.7 GeV2 .
N* studies with CLAS12 will allow us to explore new regime corresponded to excitations of bare quark core by highly virtual photons. The proposal is in preparation.
Comprehensive data on N* electrocouplings at wide Q2 range offer new challenging opportunities for baryon structure theory to access fundamental mechanisms responsible for baryon formation.

20. Back-up

21. CLAS data M.Ripani, V.D.Burkert, et. al.
Fit of CLAS 2p data at 1.4<W<1.9 GeV and 0.5<Q2<1.5 GeV2 within the framework of JM05.
s, mcbn
CLAS data M.Ripani, V.D.Burkert, et. al.
Q2=0.65 Gev2
Q2=0.95 Gev2
Q2=1.30 Gev2
Reasonable description of invariant mass and p- angular distributions was achieved in entire kinematics area covered by CLAS data.
W, GeV

22. Resonant and non-resonant contributions
fit within the framework of JM06 model
resonant part
non-resonant part
differences in the shapes of resonant/non-resonant cross-sections make possible to isolate N* contribution.