1. Nucleon resonance studies in 2p photo and electroproduction with CLAS.
presented by V.I.Mokeev
N* studies represents an important part of efforts, aimed to understand fundamental mechanisms, responsible for baryon formation from quarks and gluons
Baryons are only known 3q systems, where the non-abelian character of
QCD is manifest through the shape of binding potential. Nucleon resonance
studies open a way to map-out this potential based on N* electropcoupling data.
gluon self coupling in
pQCD
Lattice QCD calculation of gluon flux distribution in a system of 3 heavy quarks.

2. Nucleon resonance in inclusive electroproduction.
Q2 evolution of non-resonant parts is well described by QCD-based approaches. Q2 evolution of the N* peaks is completely different and depends from properties of contributing N*’s.
N*’s offer complementary information on the nucleon structure with respect to those, obtained from the studies of ground state.

3. Electromagnetic Excitation of N*’s.
Transition form factors of N*’s at various Q2’s probe their internal structure and confining mechanism e’
p, h, pp,.. γv e
lgp=1/2
N*,△ gv N N N’
lgp=3/2
A3/2, A1/2, S1/2
Gm, Ge, Gc
The experimental program has two major components, the measurement of trasnition formfactors which encode the internal structure and confining mechanism, and the spectroscopy of excited states and the search for new states predicted in widely accepted models. As excited states at higher masses can couple to a variety of final states it is essential to search in different channels.
Successful description of measured observables in major meson electroproduction channels, using common set of N* electrocouplings, offers compelling evidence for credible extraction of N* parameters.

4. Various exclusive channels in meson electroproduction and pN scattering.
Exclusive channels in pN
scattering responsible for
FSI in meson electro-
production
CLAS data on meson electroproduction
at Q2<4.0 GeV2
p(e,e’)X (trigger)
p(e,e’p)p0
p(e,e’p+)n
p(e,e’pp+)p-
p(e,e’pp+)X

5. CLAS experiment data on p-p+p photo and electroproduction.
Publication
Comments on data available
Number of citations
S.Strauch, et.al., Phys.Rev.Lett., C (2006).
Real photons, beam asymmetries at W from 1.40 to 2.30 GeV 3
M.Ripani, et.al., Phys.Rev.Lett, (2003).
Virt. photons, fully integrated and 9 1-diff. cross-sections in each (W&Q2),1.4<W<2.1 GeV, 0.5 <Q2<1.5 GeV2. 26
M.Battaglieri, et.al., Phys.Rev.Lett, (2001).
Real photons, 3 invar. Mass distributions sliced over t, defined for the initial and final protons, 2.5<W<3.0 GeV
G.Fedotov, L.Elouadrhiri, et.al.,
intended for PRC
Virt. photons, fully integrated and 9 1-diff. cross-sections in each (W&Q2),1.3<W<1.6 GeV, 0.25 <Q2<0.6 GeV2.
Under PWG review
100% of world electroproduction data comes from CLAS.

6. Complete set of unpolarized 1-differential cross-sections in gr,vp→p-p+p reactions.
For unpolarized beam/target, the final state, gr,vp→p-p+p reaction offers 9 independent single-differential cross-sections in each (W&Q2) bin. All these cross-sections are available from CLAS for the first time.

7. JLAB-MSU model (JM05) for 2-p electroproduction.
3-body processes:
Quasi-2-body mechanisms included:
All well established N* with pD decays +P33(1600)&P11(1710)&P13(1720)&P33(1920) 3-star states and 3/2+(1720) candidate.
Reggetiezed Born terms & effective FSI&ISI treatment .
extra pD contact term
All well established N* with rp decays
+P11(1710)&P13(1720) 3-star states and 3/2+(1720) candidate.
Diffractive ansatz for non-resonant part & r-line shrinkage in N* region.

8. con’d 3-body processes: Direct 2p production.
Quasi-2-body mechanisms included:
(p-)
for p+D013(1520) set of Born terms similar
to those was used for pD isobar channels.
for p+F015(1685), p-P++33(1640) 3-body contact term with exponential propagators of isobars in quasi-two- body intermediate states.
(P++33(1640))
(p+)
F015(1685)
Direct 2p production.
Exchange processes,
parameterized as:
V.Mokeev, V.Burkert, et al., Proc. of NSTAR2005 Workshop p. 47

9. Fit of E94-005 CLAS 2p data collected at low W&Q2 within the framework of JM05 model.
full
p-D++
p+D0
2p direct
Preliminary
Failure in description of p+ and
pf angular distributions revealed necessity to modify direct 2p production mechanisms.

10. + Modification of parameterization for direct 2 amplitudes JM05 JM06

11. Global fit of complete set of unpolarized 1-differential cross-sections within the framework of JM06 model.
JM06 full
p-D++
p+D0
2p direct
ai-angular distributions were
calculated with parameters of
JM06 fitted to 6 other 1-diff.
cross-sections.
Preliminary
Successful description of all E94-005
data was achieved.
Complete data on unpolarized
1-differential cross-sections
allowed us to establish all
significant mechanisms, contributing
to the 2p electroproduction at W<1.6 GeV and Q2<0.6 GeV2.

12. CLAS N* candidate at 1720 MeV in pπ+π- ? photoproduction
no 3/2+ (1720)
full
photoproduction
electroproduction
Background
Resonances
Interference
full calculation
no 3/2+
W(GeV)
W(GeV)
M. Ripani et al, Phys.Rev.Lett. 91, 2003

13. Structure at 1.7 GeV from 2p electroproduction.
Two alternative ways to describe the structure at 1.7 GeV:
Modification of hadronic couplings for P13(1720) PDG state
with respect to established values
Implementation of candidate baryon state.
M, MeV
G,MeV
GpD/G, %
Grp/G,
modified
P13(1720)
1725±20
114±19
60±12
19±9
PDG
absent
70-85
candidate
3/2+(1720)
1720±20
88±17
41±13
17±10

14. High lying N* electrocouplings.
D33(1700), P13(1720)
resonances mostly
decay with 2p
emission. 1p data do
not have enough
sensitivity to these
states.
D33(1700)
D33(1700)
First preliminary data on
Q2 -evolution of
S31(1620), D13(1700),
D33(1700), P13(1720)
electromagnetic form
factors were obtained
from analysis of CLAS
E experiment on
2p electroproduction.
P13(1720)
P13(1720)
No?Yes

15. Total p-p+p cross-sections calculated within the framework of JM06, most closed to the preliminary CLAS data.
preliminary CLAS data,
exper. E94-005,
G.Fedotov, L.Elouadrhiri, et.al.
Q2-interv., GeV2
2 thresh.
2.80
1.80
1.70
Preliminary

16. P11(1440) and D13(1520) electrocouplings.
Preliminary
from analysis of E CLAS 2p
data within the framework of JM06
from analysis of 1p CLAS data
1p/2p CLAS data

17. Further studies of N* electrocoupling from the CLAS data on 2p electroptoduction.
Short term objective: evaluation of electrocouplings for all well established N*’s with M<2.0 GeV at 0.5<Q2<1.5 GeV2 from E data within the framework of JM06.
Intermediate term objective: evaluation of electrocouplings for all well established N*, search for new N* in mass range W<3.0 GeV and 1.5<Q2<5.0 GeV2 from analysis of e1-6 CLAS data. Update of JM06 is expected, implementing partonic degrees of freedom in combination with meson-baryon content.
Final goal: comprehensive information on electrocouplings for all established N*’s as well as for possible new baryon states in entire N* excitation region and at photon virtualities from 0.2 to 5.0 GeV2.

18. γ*pΔ+ - Magnetic Transition Form Factor G*M
T.-S. H. Lee
T. Sato
Pion cloud
contribution
N* with CLAS12
Q2>5.0 GeV2 11 GeV beam era e
Quark core
contribution
Large pion contribution needed
to explain NΔ transition.
The impact of the pion cloud is dramatically visible in the magnetic transition form factor, here shown as the ratio to the dipole form. Quark models using “bare” quarks are unable to reproduce the full magnitude.
Pion contribution predicted to drop more rapidly with Q2 than the quark core. Probe core at sufficiently high Q2.

19. N* electrocouplings & expectations from quark model without accounting for meson-baryon cloud.
Ground p state
P11(1440)
S.Capstick
light cone (LC)
model
Meson-baryon cloud?
B.Metsch
Bethe-Salpeter
model
I.Aznauryan
LC model
M.Giannini/
E.Santopinto
hyper-centric
CQM
The data on electromagnetic form factors for ground and excited nucleon states offer complementary information on the nucleon structure.
The studies of ground and all well established excited states are equally important.

20. cont’d
The quark models, which describe reasonably well the data at Q2>1.5 GeV2, fail to reproduce the data at Q2<1.0 GeV2. This is an indication for possible sizeable contributions from meson-baryon cloud.
D13(1520)
EBAC is developing an approach, aimed to obtain “bare” quark core N* electrocouplings from the measured N* e.m. form factors.
Meson-baryon cloud?

21. Direct access to bare quark core in N* studies at high Q2>5.0 GeV2.
Scaling behavior, observed for A1/2 couplings at Q2>3.0 GeV2 offers an evidence for penetration of meson-baryon cloud and photon interaction to bare quark core.
V.D.Burkert, Presentation at LRP Meeting
S11
Q3A1/2
Data on N* electrocouplings at Q2 from 5.0 to 10 GeV2, feasible after 12 GeV Upgrade, will allow us to study structure of quark core in details.
P11
F15
D13
CLAS12 is only facility planned worldwide, which will be capable to study partonic content in N* structure.

22. The quark core structure in N*’s & fundamental QCD.
Structure of N* quark core may be described by Light Front Wave Function (LFWF) (S.J.Brodsky, Light-Front QCD, hep-ph/ ). There are several approaches, which are capable to relate LFWF to fundamental QCD:
a) discretized light front quantization
b) the transverse lattice
c) lattice gauge theory moments
Bethe-Salpeter approach relates “bare” N* electrocouplings to the dressed quark propagator and qq scattering amplitudes, which can be estimated from fundamental QCD within the framework of model independent Dyson-Schwinger technique (A.Holl, C.D.Roberts, S.V.Wright, Hadron Physics and Dyson-Schwinger equations, nucl-th/ , B.Metsch, et.al., Electromagnetic form factors of non-strange baryons, hep-ph/ ).
QCD

23. Conclusions and outlook.
Available and foreseen CLAS data on 2p photo and electroproduction allow us to determine the transition electromagnetic form factors for almost all excited proton states with masses less than 2.0 GeV and at photon virtualities from 0.2 to 5.0 GeV2 within the framework of JM phenomenological approach (Q2 from 0.2 to 1.5 GeV2 short term plan; Q2 from 1.5 to 5.0 GeV2 intermediate term plan).
Combined analysis of two major 1p/2p exclusive channels within the framework of most advanced coupled channel analysis, developed by EBAC, is underway in collaboration between Hall B and EBAC. It will create most reliable data on Q2 evolution of N* electrocouplings in the transition area from soft to parrtonic degrees of freedom .
CLAS12 is only facility planned worldwide, which will be capable to study N*’s at high photon virtualities, providing comprehensive information on their quark core structure.
Several approaches: lattice simulations, LFWF, Dyson-Schwinger & Bethe-Salpeter could relate data on N* quark core structure to QCD. N* studies with CLAS12 will open up most promising opportunity to relate N* phenomenology to QCD.

24. Back - up

25. Search for 3/2+(1720) candidate state in combined analysis of 2p photo & electro production.
Fit with 3/2+(1720)
3/2+(1720) off; variation of
electromagnetic, pD rP
hadronic couplings and masses
of P13(1720), P33(1600) states
P13(1720) BF(rP), extracted from
the fit within the framework of JM05
model (no new state):
Q2, GeV
BF,%
63±25
20±10
discrepancy!
Preliminary real (M.Bellis) and published (M.Ripani) virtual photon data combined
could be fitted only if both candidate 3/2+(1720) and P13(1720) states contribute

26. Origin of the structure at 1.7 GeV
Definitive conclusion
on the structure at 1.7 GeV may be made after 2p real photon
data will be available, based on combined fit
of photo- & electro-
production data within
the framework of JM06.