1. Yerevan Physics Institute
Electroexcitation of P11(1440), D13(1520), and S11(1535) from CLAS data and quark model predictions. On the definitions of the g*p N* helicity amplitudes
I. G. Aznauryan
Jefferson Lab
Yerevan Physics Institute
October 13, 2008, Jlab
Electromagnetic N-N* Transition
Form Factors Workshop

2. Q2 dependence of the g* NN* amplitudes extracted in wide region
Outline
Results on the g * pP11(1440), D13(1520), S11(1535) helicity
amplitudes extracted from CLAS p and 2p electroproduction data,
comparison with earlier data
Correct definition of the amplitudes : very important as
Q2 dependence of the g* NN* amplitudes extracted in wide region
of Q2 is highly sensitive to different description of N and N* :
- 3q picture
- additional qq components
- hybrid q3G states
- resonances dynamically generated in p N interaction
- results of lattice QCD
Comparison with quark model predicions
Summary

3. CLAS: the eNeNp data Q2 = 0.4, 065 GeV2
s(epeNp+,0 )  data points:
K. Joo et al., PRL 88 (2002)
PR C68 (2003)
PR C70 (2004)
H.Egiyan et al., PR C73(2006)
Analysis: DR, UIM
I.Aznauryan et al.,
PR C71 (2005)
PR C72 (2005)
Q2 = 1.72, 2.05, 2.44, 2.91, 3.48, 4.16 GeV2
s(epeNp+ )  data points:
K. Park et al., PR C77 (2008)
Analysis: DR, UIM
I.Aznauryan et al.,
nucl-exp/ ,
will appear in PRC

4. CLAS: the epepp+p- data
Q2 = GeV2
Combined analysis of epepp , epepp+p- data:
I.Aznauryan, V.Burkert, V.Mokeev et al.,
PR C72 (2005)
Q2 = 0.275, 0.325, 0.375, 0.425, 0.475, 0.525, GeV2
Data:
G.Fedotov,V.Mokeev, V.Burkert,…
nucl-ex/
Analysis:
V.Mokeev, V.Burkert,
J.Phys. Conf.Ser. 69 (2007) ;
Proc. of NSTAR2007, p. 76

5. Helicity amplitudes of the g*p P11 (1440) transition
CLAS data : Np
Np, Npp, combined
Npp (preliminary)
gppp0
M.Dugger et al.,
PR C ,2007
PDG
First measurements of A1/2 at Q2 > 0
First measurements of S1/2

6. Helicity amplitudes of the g*p S11 (1535) transition
CLAS data : Np Nh
gppp0
M.Dugger
PDG
First measurements of S1/2 :
it is difficult to extract S1/2 in h electroproduction
Results for A 1/2 obtained in p and h production agree with each other with bp N = 0.45, b hN = 
PDG: bp N = , b hN =
Slow falloff of A1/2 observed in h production is confirmed by p data

7. Helicity amplitudes of the g*p D13(1520) transition
CLAS data : Np
Np, Npp, combined
Npp (preliminary)
gppp0 , M. Dugger
Old data:
Bonn, DESY, NINA
PDG
First definite results for A 1/2 , A 3/2
in wide range of Q2
First measurements of S1/2

8. Definitions: common sign of the g*p N* amplitudes
In the analyses of g*N N p data, the g*p N* helicity amplitudes are defined through reaction multipole amplitudes. For example, for g*p P11(1440) in g*p p p0 we have: N* N g* p
This definition contains
information on signs of
two vertices
g*NN* and N*Np :
g(N*Np ) 
{G (N*Np)}1/2

9. Common sign of the g*p N* amplitudes (con-d)
Definition of A1/2 in theoretical approaches :
Depends on the phase of FN*
Contains information on the g*N N* vertex only g* p N* N
FN*
(FN*)*

10. Common sign of the g*p N* amplitudes (con-d)
Commonly used definition of A1/2 in quark model is :
A1/2
sign pNN*
R.L.Walker, Proc. of IV Int. Symp. on Electron-Photon Inter.
at High Energies, Liverpool (1969), p. 21.
In QM, traditionally, the sign pNN* was chosen to describe
the sign of the experimental A1/2 amplitude for Q2=0;
sometimes this can bring to confusing and wrong results
Possibly, it will be right to make some changes in conventions to avoid this confusion, for example, to reflect in the amplitude extracted from experiment the final state: A ApN, …?

11. Common sign of the g*p N* amplitudes (con-d)
We need explicit formulas, how to account for the relative
sign of the contributions :
Res.:
Born
terms:
I.Aznauryan, V.Burkert, H.Lee, nucl-th/
Through covariant calculations, we have obtained the relations:
For example, for P11 (1440) :
, if

12. Definition of the g*p N* amplitudes (con-d)
In this way, we have also checked, which definition of
gives the sign consistent with the relative sign of the amplitudes
extracted from experiment, i.e. S1/2 relatively to A1/2, A3/2
We have presented different definitions of A1/2, A3/2, S1/2 :
Through the g*N N p multipole amplitudes
Through the g*N N* electromagnetic current
Through the g*N N* form factors
In nonrelativistic quark model
These definitions are consistent with each other,
and may be useful in theoretical calculations

13. Common sign of the g*p N* amplitudes (con-d)
For the resonances of [70,1- ] –plet, common signs of the g*p N* amplitudes in quark model were found
(using PCAC for the pN N* vertex) by
Aznauryan, Bagdasaryan, Sov.J.Nucl.Phys. 41 (1985) 158
For all resonances, except D13(1700), traditionally used sign is right
For P11(1440), sign of the g*p N* amplitudes was found
using 3Po model for the pN N* vertex by
Capstick, Keister, PR D51 (1995) 3598
using PCAC for the pN N* vertex by
Aznauryan, PR C76 (2007)

14. Signs for g*p  P11 (1440) Light-front RQM 1. strong model dependence
2. for some models
strong disagreement
with experiment
Corrected signs
1. less model dependence
2. better agreement with exp.
Signs taken in ‘traditional way’
Light-front RQM
Capstick, Keister (1995)
Weber, PR C41 (1990)2783
Simula… PL B397 (1997)13
NRQM
Warns… Z.Phys. C45 (1990)627
Giannini… J.Phys. G24 (1998)753

15. g*p  P11 (1440): 3q picture with P11 (1440) as [56,0+]r
All LF RQM describe
sign change of A1/2
the amplitude S1/2
Strong evidence
in favor of
P11 (1440) as a first
radial excitation of
3q ground state
LF RQM:
Weber, PR C41 (2783) 1990
Capstick, Keister, PR D51 (1995) 3598
Pace, Simula et.al., PR D51 (1995) 3598
Aznauryan, PR C76 (2007)
All LF RQM fail to describe the amplitude A1/2 at Q2 < 1 GeV2

16. P11 (1440): Additional components and contributions
Pion cloud
EBAC (preliminary)
Julia-Diaz et.al.,
PR C77(2008)045205
30% admixture of
qqqqq components in
the Roper resonance
G(theory) = G (exp) :
Li, Riska, PR C74(2006)015202
Pion cloud contributions and additional qqqqq components in the Roper resonance
can improve the description
at small Q2

17. P11 (1440) as a q3G hybrid state P11 (1440) as q3G hybrid state is
ruled out !!!
Supression of S1/2 has its origin
in the form of the vertex g*q  qG;
it is practically independent of
relativistic effects
P11 (1440) as q3G:
Li, Burkert,Li,
PR D46 (1992) 70

18. g*p D13 (1520): 3q picture + pion cloud
In 3q picture, the signs
of all amplitudes are
described; however, this
picture fails to describe
A3/2 at small Q2
Pion cloud:
EBAC (preliminary)
Significant contribution
at small Q2 for A3/2
Nonrelativistic approaches:
Warns et al., Z.Phys.C45(1990)627
Aiello et.al., J.Phys.G24 (1998)753
Merten…, Eur.Phys.J.A14 (2002)477

19. g*p S11 (1535): 3q picture Capstick, Keister, PR D51 (1995) 3598
Opposite sign of S1/2!!!
Impossible to change
in quark model !!!
Combined with the difficulties
in the description of large width
of S11(1535)  h N and large
S11(1535)  fN,LK couplings,
this shows that 3q picture for
S11(1535) should be complemented
LF RQM:
Capstick, Keister,
PR D51 (1995) 3598
Pace, Simula et.al.,

20. S11 (1535): Additional components and contributions
Pion cloud:
EBAC (preliminary), MAINZ
qq (mostly ss) :
An,Zou , nucl-th/
sign should be consistent with the
interference of (uu,dd) and ss
components in G(S11 (1535)hp)
It is possible that agreement of 3q picture with experimental
data will be achieved by taking into account pion cloud contribution and additional qqqqq components in S11(1535)

21. Summary
For the first time transverse and longitudinal amplitudes of the g* p P11(1440) transition are extracted from experiment for
Q2 > 0 in wide range of Q2
For the first time longitudinal amplitudes of the
g* p D13(1520), S11(1535) transitions are extracted from
experiment, and in wide range of Q2
For the first time definite results are obtained for the transverse amplitudes of the g* p D13(1520) transition in wide range
of Q2
The results for the g* p S11(1535) transverse amplitude extracted from p and h electroproduction data are consistent with each other

22. Summary: P11(1440)
The results for g* p P11(1440) available in wide region of Q2 allow us to make conclusions on the nature of P11(1440):
Comparison with quark model predictions provides strong evidence in favor of P11(1440) as a first radial excitation of the
3q ground state
Presentation of P11(1440) as a q3G hybrid state is ruled out
Quark model predictions underestimate the value of A1/2
at small Q2
Pion cloud contributions and additional qq components in the Roper resonance can improve description of A1/2 at small Q2

23. Summary: D13(1520), S11(1535)
Quark models describe the signs of all amplitudes for
the g* p D13(1520) transition
There is significant underestimation at small Q2 for A3/2 which apparently is related to the pion cloud contribution
Quark models predict opposite sign for the S1/2 amplitude of the
g* p S11(1535) transition !!! Combined with the difficulties
in the description of couplings to hadronic channels, this shows that 3q picture for S11(1535) should be complemented
Apparently, agreement of 3q picture with experimental data can be achieved by taking into account pion cloud contribution,
and additional qqqqq components in S11(1535)

24. Summary: definitions of the g* N N* amplitudes
The g* N N* amplitudes extracted from the experimental data
on the g* N Np reaction are related to
the g* N N* amplitudes calculated in theoretical approaches
through the sign of the pNN* vertex
Possibly, it makes sense to introduce new conventions in order
to avoid confusion caused by this fact

25. S11 (1535) as a dynamically generated resonance
generated S11 (1535):
Oset… , nucl-th/
sign should be checked via
calculation of the vertex
S11 (1535)pN in addition
to g*p S11 (1535)
For both signs, presentation
of S11 (1535) as only dynamically
generated resonance is ruled out.
However, it is interesting to
investigate the possibility of the
dynamically generated resonance as
a component additional to 3q state.