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 N  electroproduction at high Q 2 –Recent CLAS data on the π + π - p electroproduction –Evaluation of  v NN* electrocouplings –Preliminary data at high Q 2 –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v N  p   p  e e’ γvγv N N’N’ N*, △ A 3/2, A 1/2, S 1/2 G M, G E, G C  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, (2007);B.Julia-Diaz, et al., arXiv: [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 γvγv  N’N’ + Non-resonant amplitudes. *

P 11 (1440) electrocouplings from the CLAS data on N  /N  electroproduction N  preliminary NN Light front models: I. Aznauryan S. Capstick hybrid P 11 (1440)‏ Good agreement between the electrocouplings obtained from the N  and N  channels: Reliable measure of the electrocouplings. The electrocouplings for Q 2 > 2.0 GeV 2 are consistent with P 11 (1440) structure as a 3-quark radial excitation of the nucleon. Zero crossing for the A 1/2 amplitude has been observed for the first time, indicating the importance of light-front dynamics.

High lying resonance electrocouplings from N  CLAS data analysis N  CLAS preliminary N  world N  CLAS Q 2 =0 Δ(1700)D 33 N(1720)P 13

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  +  -, M p  + 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 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

Preliminary differential cross-sections at W=1.934 GeV Preliminary

Cross-sections at higher Q 2 Fully integrated 2  cross section Q 2 =2.2 GeV 2 Q 2 =0.95 GeV 2 preliminary

Cross-sections at higher Q 2 Fully integrated 2  cross section Q 2 =3.9 GeV 2 preliminary Q 2 =4.6 GeV 2 Q 2 =2.7 GeV 2 Q 2 =3.3 GeV 2

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p

continued 3-body processes: Isobar channels included:  + D 0 13 (1520),  + F 0 15 (1685),  - P (1640) isobar channels; observed for the first time in the CLAS data at W > 1.5 GeV. Direct 2  production F 0 15(1685)‏ (P ++ 33(1640))‏ (-)‏(-)‏ (+)‏(+)‏ V. Mokeev, V.Burkert, J. Phys. 69, (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+0 2  direct pp  + D 0 13 (1520)‏  + F 0 15 (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<Q 2 <5.0 GeV 2 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<Q 2 <5.0 GeV 2 region.