Results from the analysis of the CLAS data on double charge pion electroproduction.
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Presented by v i mokeev

Results from the analysis of the CLAS data on double charge pion electroproduction.

presented by V.I.Mokeev

The studies of N* electrocouplings at various Q2 elucidate relevant degrees of freedom in their structure and allow us to understand quark interactions, which are responsible for baryon formation from fundamental constituents.

Promising way to study non-Abelian origin of strong interactions in non-perturbative domain, predictedin lattice simulations for quark binding in baryons.

gluon self coupling in


Lattice QCD calculation of gluon flux distribution in a system of 3 heavy quarks.

Presented by v i mokeev

How to obtain reliable information on N* electrocouplings. pion electroproduction.

Non-resonant amplitudes and N* hadronic couplings in various meson electroproduction channels are entirely different, while N* electrocouplings are expected to be the same.


p, h, pp,..










A3/2, A1/2, S1/2

Gm, Ge, Gc

Credible evaluation of N* electrocouplings should be achieved in successful description of all available observables in various electroproduction channels combined with common resonance electrocouplings.

Presented by v i mokeev
Why the data on 1 pion electroproduction.p/2p electroproduction mechanisms are vital for N* electrocoupling evaluation in any exclusive channel.

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.

New opportunities for phenomenological studies of 2p electroproduction offered by the clas data
New opportunities for phenomenological studies of pion electroproduction.2p electroproduction, offered by the CLAS data.

Simplest measurements with unpolarized e- beam and p target, no use of polarization observables in the final state offer 9 independent single differential cross-sections in each (W,Q2) bin.

All these cross-sections are available from CLAS for the first time.

Contributing mechanisms may be established from theirs manifestations in variety of observables JM approach.


Jlab msu model jm for 2 p electroproduction
JLAB-MSU model (JM) for 2- pion electroproduction.p electroproduction.

Quasi-2-body mechanisms included:

3-body processes:

  • All well established N*’s with pD decays and 3/2+(1720) candidate/P13(1720) with major decays to pD, seen in CLAS 2p data.

  • Reggetized Born terms & effective FSI&ISI treatment .

  • Extra pD contact term.

  • All well established N*’s with rp decays and 3/2+(1720) candidate

  • /modified P13(1720).

  • Diffractive ansatz for non-resonant part & r-line shrinkage in N* region.

Con d
con’d pion electroproduction.

3-body processes:

Quasi-2-body mechanisms included:


  • p+D013(1520), p+F015(1685), p-P++33(1640) isobar channels, observed for the first time in CLAS data at W>1.65 GeV

  • the amplitudes were described at phenomenological level (see ref. below) .




Direct 2p production in JM05.

Exchange processes,

parameterized as:

V.Mokeev, V.Burkert, et al., Proc. of NSTAR2005 Workshop p. 47

Presented by v i mokeev

Direct 2 pion electroproduction.p production mechanisms, established from analysis of the CLAS data: M.Ripani, V.Burkert,, Phys.Rev. Lett. 91, 022002 (2003).

CLAS data fit under various assumptions on direct 2p production mechanisms:

W=1.49 GeV

Q2=0.65 GeV2

Q2=0.95 GeV2

exchange processes

3-body phase space

the contributions from

direct 2p mechanisms,

described on previous


Fit of clas 2 p data on 3 invariant masses and p angular distributions within the framework of jm05
Fit of CLAS 2 pion electroproduction.p data on 3 invariant masses and p- angular distributions within the frameworkof 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

Presented by v i mokeev

Fit of single differential cross-sections from recent CLAS 2p data (1.3<W<1.6 GeV, 0.25<Q2<0.6 GeV2) within the framework of JM05 model.




2p direct

Reasonable description of 4 differential cross-sections, studied previously, was achieved.


Failure in description of p+ and

p angular distributions is related to the shortcomings in treatment of direct 2p production (dashed magenta lines).

Presented by v i mokeev

Modification of direct 2 2p production mechanisms. From JM05 to JM06.




Fit of clas data at low w and q 2 g fedotov l elouadrhiri et al within the framework of jm06 model
Fit of CLAS data at low W and Q 22 (G.Fedotov, L.Elouadrhiri, within the framework of JM06 model.

JM06 full



2p direct

resonant part

non-resonant part

All significant mechanisms, contributing to ep→p-p+p channel at W<1.6 GeV and Q2<0.6 GeV2 were established from the CLAS data analysis.


Reasonable description of

ai-angular distributions with model parameters fitted to 6 other diff. cross-sections offers compelling evidence for reliability of JM06 model.

Evaluation n electrocouplings isobar channel contributions from the clas 2 p data fit
Evaluation N* electrocouplings & isobar channel contributions from the CLAS 2p data fit.

preliminary CLAS data

ofE94-005 experiment

N* electrocouplings and parameters for non-resonant mechanisms were varied. The c2/d.p. were estimated from comparison between measured and calculated differential cross-sections.


c2/d.p. for selected in fit cross-sections:

Isobar channel contributions from clas data fit within the framework of jm06
Isobar channel contributions from CLAS data fit within the framework of JM06.

CLAS data

the contributions from all isobar channels

CLAS data fit

The data on isobar channel contributions are available for N* studies in the coupled channel analysis, under development at EBAC.

Presented by v i mokeev
P11(1440) and D13(1520) electrocouplings determined from the CLAS data on 1p and 2p electroproduction.



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 two independent analyses of 1p and 2p channels are in reasonable agreement.

Presented by v i mokeev
N CLAS data on 1* electrocouplings from exp. data & expectations from the models, accounting for quark degrees of freedom only.

Ground p state



light cone (LC)






LC model

3q core

3q core +meson -baryon cloud +??





  • data on electromagnetic form factors for ground and 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

Cont d
cont’d CLAS data on 1

EBAC is developing coupled channel approach to disentangle meson-baryon dressing and “bare” quark core contributions to N* electrocouplings.


The contribution from quark core increases with Q2. Data on N* electrocouplings at high Q2 are needed to study quark interactions in baryons .

3q core

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.

3q core +meson –baryon cloud +??

N electrocouplings mechanisms responsible for baryon formation
N* electrocouplings & mechanisms responsible for baryon formation.

  • The contributions from quark models of baryon structure are needed. In particular:

  • contributions from potential quark models, allowing us to relate N* electrocouplings to the binding potential and the interactions, responsible for 3q configuration mixing;

  • contributions from the models, based on Bethe-Saltpeter formalism, which allow us, using the data on N* electrocouplings, to constrain/establish quark scattering amplitudes, dressed quark propagators and elucidate their connection to QCD within the framework Dyson-Schwinger equations;

  • establish constraints, imposed by the data on N* electrocouplings to light cone wave functions (LCWF) for N*, and relate LCWF to QCD. (S.Brodsky hep-ph/0412101)

  • it will be important to have LQCD predictions for understanding of the results from N* phenomenology.

Conclusions and outlook
Conclusions and outlook. formation.

  • Analysis of the most comprehensive worldwide CLAS data on ep→p-p+p cross-sections, carried out with the framework of JM approach, allowed us to establish from successful data fit the mechanisms with significant contributions to this exclusive channel in N* excitation region at Q2 from 0.2 to 1.5 GeV2.

  • The amplitudes/cross-sections of various isobar channels were determined from the fit of the CLAS 2p data. They will be used by EBAC for evaluation of N* electrocouplings in advanced coupled channel analysis of various exclusive channels combined. The resonant amplitudes in 2p electroproduction were isolated in the data fit.

  • P11(1440) and D13(1520) electrocouplings at Q2 from 0.2 to 0.6 GeV2 were obtained for the first time from analysis of 2p electroproduction data. Extension of this analysis is in progress with a goal to obtained electrocuplings for almost all N*’s with M < 3.0 GeV at 0.3<Q2<5.0 GeV2.

  • Comparison of the CLAS data on P11(1440), D13(1520) electrocouplings with quark model expectations showed, that various degrees of freedom contribute to the structure of N*’s at low and high Q2. Focused effort of baryon structure theory is needed to relate N* phenomenology to fundamental strong interaction mechanisms, responsible for baryon formation.

Presented by v i mokeev

Further studies of n electrocoupling from the clas data on 2 p electroptoduction
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 0.25<Q2<1.5GeV2from E93-006,E94-005 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 0.<Q2<5.0 GeV2 from analysis of e1-6,e1e,g1c,g11 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. to 5.0 GeV2.

Presented by v i mokeev

Very preliminary results on 2 2p electroproduction at high Q2.

Fully integrated 2p cross section at Q2 from 4.5 to 5.2 GeV2

Evidence for substantial N* contributions