Nucleon resonance studies in π+ π- electroproduction off protons at high

photon virtualities

E. Isupov, EMIN-2009

Plan of the talk

– Major objectives for N* studies in Nelectroproduction at high Q2

– Recent CLAS data on the π+ π- p electroproduction

– Evaluation of vNN* electrocouplings

– Preliminary data at high Q2

– Conclusions and outlook

Primary objectives in the studies of N* structure in 2 electroproduction• Our experimental program seeks to determine

– N-N* transition helicity amplitudes (electrocouplings) at photon virtualities 0.2< Q2<5.0 GeV2 for almost all excited proton states from analyzing double-pion electroproduction.

– provide input for advanced coupled-channel approach developing by EBAC.

• This comprehensive information on Q2 evolution of the N-N* electrocouplings will allow us to: – determine the active degrees of freedom in N* structure at various

distances;– study the nonperturbative strong interactions which are responsible for

• the ground and excited nucleon state formation, • and how they emerge from QCD.

How N* electrocouplings can be accessed

v Np

p

e

e’

γv

N N’

N*,△

A3/2, A1/2, S1/2

GM, GE, GC

Consistent results on N* electrocouplings obtained in analyses of various meson channels (e.g. πN, ηp, ππN) with entirely different non-resonant amplitudes will show that they are determined reliably

Advanced coupled-channel analysis methods are being developing at EBAC: B.Julia-Diaz, T-S.H.Lee et al., PRC76, 065201 (2007);B.Julia-Diaz, et al., arXiv:0904.1918[nucl-th]

• Isolate the resonant part of production amplitudes by fitting the measured observables within the framework of reaction models, which are rigorously tested against data.

• N* electrocouplings can then be determined from resonant amplitudes under minimal model assumptions.

N

γv

N’+Non-resonant amplitudes.

*

P11(1440) electrocouplings from the CLAS data on N/N electroproduction

Npreliminary

N

Light front models:

I. Aznauryan

S. Capstick

hybrid P11(1440)

• Good agreement between the electrocouplings obtained from the N and N channels: Reliable measure of the electrocouplings.

• The electrocouplings for Q2 > 2.0 GeV2 are consistent with P11(1440)

structure as a 3-quark radial excitation of the nucleon.

• Zero crossing for the A1/2 amplitude has been observed for the first time, indicating the importance of light-front dynamics.

High lying resonance electrocouplings from N CLAS data analysis

NCLASpreliminary

Nworld

NCLAS Q2=0

Δ(1700)D33

N(1720)P13

CLAS

Event Selection

• Electron ID– Calorimeter cuts– Cherenkov cut– Fiducial cuts– Zvertex cut– Momentum corrections– Zvertex corrections

EC sampling fraction before and after electron ID cuts

Charged hadrons ID

• Beta vs Momentum cuts

• Fiducial cuts

• Momentum corrections for positive pion

• Energy loss corrections for proton

• Zvertex corrections

• Zvertex cut

Delta beta vs Momentum for charged hadrons

Missing Mass of negative pion

3-body final state kinematics variables:M+- , Mp+ are invariant masses of the +- and p+ systems respectively;d- =d(cos-)d- is solid angle for emitted -;p+ is the angle between two planes on the plot.

3-body final state kinematics variables

Cross-section extraction

22

7 1

QWeff

N

LddWdQ

d 7-fold differential cross-section

L – luminosity, N – number of events inside multidimensional cell, eff-efficiency determined from monte-carlo simulation. Then we obtain virtual photon cross-section

cos( )p p

d dM dM d d d

ddWdQ

d

d

d

v2

75 1

Preliminary differential cross-sections at W=1.934 GeV2 2 24.2 5.0GeV Q GeV

Prel

imin

ary

Cross-sections at higher Q2

Fully integrated 2 cross section

Q2=2.2 GeV2

Q2=0.95 GeV2

preliminary

Cross-sections at higher Q2

Fully integrated 2 cross section

Q2=3.9 GeV2

preliminary

Q2=4.6 GeV2

Q2=2.7 GeV2

Q2=3.3 GeV2

18

JLAB-MSU isobar model (JM) for N electroproduction.

3-body processes:Isobar channels included:

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

• Reggeized Born terms with effective FSI & ISI treatment .

• Extra contact term.

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

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

-++

p

continued3-body processes: Isobar channels included:

• +D013(1520), +F0

15(1685), -P++33(1640)

isobar channels; observed for the first time in the CLAS data at W > 1.5 GeV.

Direct 2 production

F015(1685)

(P++33(1640))

(-)

(+)

V. Mokeev, V .Burkert, J. Phys. 69, 012019 (2007); V. Mokeev et al., arXiv:0809:4158[hep-ph]

Description of the CLAS N differential cross sections within the framework of JM model

full JM calc.

-++

+0

2 direct

p

+D013(1520)

+F015(1685)

Resonant & non-resonant parts of N cross sections as determined from the CLAS data fit within the framework of JM

model

full cross sections resonant part non-resonant part

Conclusions and outlook

• For the first time differential cross-sections of double pion

electroproduction were extracted in 2.0<Q2<5.0 GeV2

• It makes possible to use phenomenological model JM in order to establish all essential mechanisms, contributing to double pion electroproduction at this still unexplored kinematic area

• In near term prospect we expect to evaluate electrocouplings for prominent resonances in 2.0<Q2<5.0 GeV2 region.