1. Electroexcitation of Nucleon Resonances
Volker D. Burkert
Jefferson Lab
BARYONS ‘02
9th International Conference on the Structure of Baryons
March 3 - 8, 2002
My talk will be about the electroexcitation of nucleon resonances.
This conferences is all about nucleons and baryons.
Nathan Isgur has been a strong supporter of a nucleon research program at this laboratory. He formulated the importance of studies of the nucleon in 3 remarks he made at the N*2000 conferences in this auditorium:

2. Why N*’s are important (Nathan Isgur, N*2000 Conference, Jlab)
Nucleons represent the real world, they must be at the center of any discussion on
“why the world is the way it is”
Perhaps the most intuitive one is that
Nucleons represent ....

3. Why Excitations of the Nucleon. (Nathan Isgur, N
Why Excitations of the Nucleon? (Nathan Isgur, N*2000 Conference, Jlab)
Nucleons represent the real world, they must be at the center of any discussion on
Nucleons represent the simplest system where
“why the world is the way it is”
“the non-abelian character of QCD is manifest”
...QCD is our main tool to understand the nucleon structure ... , and the nucleon is the simplest system where the "the non-abilian ...is manifest", as repesented in the similarity between the cartoon of a nucleon with flux tubes, and the 3-gluon vertex.
(next slide) ..nucleons are complex enough to

4. Why Excitations of the Nucleon. (Nathan Isgur, N
Why Excitations of the Nucleon? (Nathan Isgur, N*2000 Conference, Jlab)
Nucleons represent the real world, they must be at the center of any discussion on
Nucleons represent the simplest system where
Nucleons/baryons are complex enough to
“why the world is the way it is”
“the non-abelian character of QCD is manifest”
... baryons were at the foundation of the development of the
quark model by Gell-Mann and Zweig. And a hidden quantum number, later called "color", was introduced by Greenberg to
explain why the Delta++ resonance, which is represents the
largest cross section in pi+-proton scattering, could even exist.
The colorless quark model would not allow its excitation.
Here is the outline of my talk:
“reveal physics hidden from us in mesons”
Gell-Mann & Zweig - Quark Model
O. Greenberg - The D++ problem/color

5. OUTLINE Experimental results Summary/Outlook Why electroproduction?
Quadrupole deformation in the N-D transition
The Roper resonance - N’(1440)1/2+
Eta production and the N*(1535)1/2-
SQTM and higher mass states
Resonances in multi-pion, and KY* channels?
Summary/Outlook
30 years later we study more details of the inner workings of the nucleon, and this is the subject of my talk.
This iss the outline of my talk:
What can we learn from the excitation of nucleon resonances?,
specifically in electroexcitation.

6. Why N* Electroproduction?
Light quark baryon spectrum for N* Np
Internal structure of baryons
Meson production mechanism
Helicity amplitudes vs Q2 => Relevant
degrees of freedom vs distance scale
We still need to learn a lot more about the light quark baryon spectrum. It has been studied mostly with pion probes. However, there may be many states that do not couple
to pions, especially higher mass state, while possibly coupling to photons. The spectrum can tell us about the underlying symmetry properties due to different quark-gluon configurations, or interaction mechanism such as oge, gbe, instantons
At different photon virtualities we probe how the relevant dof
change as a function of the distance scale. The quantities of
interest are the helicity amplitudes A1/2,... as a function of Q2.
Since both photon and nucleon carry spin, we probe the
spin structure in the regime of strong QCD. This will be
discussed in another plenary talk.
Our main tool in resonance studies is meson production. However, mesons are not only produced via resonance decays. In order to understand the full production mechanism in terms of resonant and non-resonant amplitudes we need to further develop, what one may call the Standard Model of em meson production.
Finally, N* electroexcitation allows us to better understand the observed connections between the nucleon resonance region and the deeply inelastic regime. This will be covered in another talk.

7. The SU(6)xO(3) symmetry scheme is a representation of the states in the symmetric constituent quark model, with L being the orbital angular momentum of the 3-quark system and energy levels in the harmonic oscillator potential. The states appear in supermultipets clustered in h-bar omega and orbital angular momentum.
The states are split in masses according to different quark-spin combinations. Empty boxes indicate predicted but unobserved or very uncertain states.
These arrows indicate the states I will be discussing in the following.

8. CLAS: ep epX, E=4GeV missing states 2 1.5 1 0. 0.5 1.0 1.5
pi0, eta, or omega in the
final state. The missing mass vs invariant mass scatter plot shows clear correlations of resonance excitations and specific final states.
Strong excitation near 1500 is correlated with the p-eta channel, and there is a lot of strenght in p-omega throughout the higher mass region.
The Delta sticks out in the p-pi0 channel, and can best be studied in this channel. 1 0.
0.5
1.0
1.5

9. N-D(1232) Quadrupole Transition
SU(6): E1+=S1+=0
The Delta excitation has been studied experimentally for 3 decades, and the dominant magnetic dipole transition is well established, mediated by a simple spin flip. Electric or scalar quadrupole transitions, which are identical 0 in a symmetry SU(6) model, would indicate a deformation of the Delta and the nucleon.
Dynamically, such contributions may arise through interaction with the pion cloud, or through the one gluon exchange.
helicity conservation requires that the E/M asymptotically approaches +1. Qualitatively one expects such a behavior. The sign of the this ratio is related to the shape of the deformation of the Delta
in such a way.

10. Multipole Ratios REM, RSM - before 2001
This shows the data on the electric quadrupole of
magnetic dipole ratio and the scalar quadrupole to magnetic dipole ratio. The colored points are mostly from the
seventies and exhibit large systematic discrepancies.
The more recent data indicate that REM is small and negative,
-2% to -3%, and has a weak Q2 dependence, while RSM may
have a stronger Q2 dependence if one ignores this point.
Today, the emphasis is on control of systematics.
While in spectrometer experiments the kinematics needs to be adjusted many times during a measurement CLAS measures all kinematics simultaneously.

11. Extracting the multipoles requires first to separate the relevant response function, by measureing the azimuthal and polar angle dependence of the diff. cross section, and fitting it to the expected phi dependence, (next slide)
as shown in this graph.....

12. Multipole Analysis for g*p ppo
Q2 = 0.9 GeV2
CLAS
|M1+|2
Re(E1+M1+*)
|M1+|2
and with some approximation analysed in terms of multipoles.
The first terms is dominated by the transverse component
and strongly dependent on the dominant magnetic dipole transition, while the TT interference is most sensitive to the
electric quadrupole, and the LT interference to the scalar quadrupole.
(new graph)
Overlaying the new results on the old data....
Re(S1+M1+*)
L.C. Smith

13. Multipole Ratios REM, RSM - before 2001
(next graph)
shows now a consistent Q2 evolution....

14. Multipole Ratios REM, RSM - 2002
bridging the photon point for REM and the high Q2 results from earlier JLAB measurements. The REM ratio remains small and negative around -2% with little Q2 dependence.
The RSM ratio shows a strong Q2 dependence and becomes increasingly negative at higher Q2.
(next graph)
For comparison with some selected models I eliminate
old data and blow up the vertical scale....

15. Multipole Ratios REM(Q2), RSM (Q2)
Bonn(2002)
Data are compared with two relatized CQMs, a chiral quark soliton model, and a dynamical model with hadronic degrees of freedom. Non of the pure quark models is successful in describing both ratios, simultaneously.
The comparison strengthens the claim that most of the quadrupole strength is due to non-resonant meson exchange, implemented in the dynamical models.
A dynamical model by Kamalov and Yang, using a different unitarization procedure, gives similiarly good agreement with the data.
One should note that these models have been fitted to the photon point and the high Q2 data.
(next slide)
Ultimately, we want to come to a QCD description of these quantities.
Currently there is only one calculation in quenched QCD
for REM with a large error bar....
Sato
Ernst

16. Multipole Ratios REM(Q2), RSM (Q2)
Bonn(2002)
LQCD 1993
Extrapolating the compute power from 1993 to 2002 using Moore's law, one might expect a much reduced uncertainty..
(next slide)

17. Multipole Ratios REM(Q2), RSM (Q2)
Bonn(2002)
LQCD 2002?
(Moore’s law)
..at which point Lattice QCD may provide real input to this discussion.
(next slide)
The recent results establlish a new level of precision. However, the expected improvement in statistics will need to be complemented by a reduction of model dependencies in the analysis. The largest uncertainties are due to the non-resonant terms which grow increasingly more important as Q2 increases.
The RTL' response function, which is a longitudinal/transverse interference of resonant and background terms, provides a better handle on this.
This slide shows latest results from Mami and CLAS.....

18. Polarized Beam Observable ep eppo
slt’ response function / CLAS
Beam spin asymmetry
The Mami beam asymmetry data at large cms angles show a strong sensitivity to model descriptions is. The dotted line is a rescaled MAID model.
The CLAS data on the sigma_LT' response function cover the full angle range and show similar sensitivity to model descriptions.
Since the resonance contributions are nearly the same in either model, the discrepancies are largely due to the different description of non-resonant terms.
Mami/A2
Joo
Botto
Kuhn

19. The 2nd Resonance Region
The Roper N’(1440)P11
In CQM assigned as a N=2 radial
excitation of the nucleon
Poor description of properties such as
mass, photocouplings, Q2 evolution
Strong gluonic component?
Quark core with meson cloud?
Ns molecule?
which confirms the rapid fall drop in magnitude of the earlier analysis.
Clearly, the data needs to be extended to smaller and to larger larger Q2 values.
(next slide)
A lot more progress than on the Roper has been made recently on another state in that mass range, the s11(1535)...

20. The 2nd Resonance Region
CLAS
ep enp+
UnitaryIsobar fit
allows ectraction of resonant amplitudes in the 2nd resonance region.
(next slide)
the next graph shows the preliminary results for one Q2 point on the Roper resonance....

21. The 2nd Resonance Region
CLAS (preliminary)
s(po,p+), Ae(po,p+), unitary isobar fit
The Roper N’(1440)P11
In CQM assigned as a N=2 radial
excitation of the nucleon
Poor description of properties such as
mass, photocouplings, Q2 evolution
Strong gluonic component?
Quark core with meson cloud?
Ns molecule?
which confirms the rapid fall drop in magnitude of the earlier analysis.
Clearly, the data needs to be extended to smaller and to larger larger Q2 values.
(next slide)
A lot more progress than on the Roper has been made recently on another state in that mass range, the s11(1535)...
H. Egiyan

22. The 2nd Resonance Region
N*(1535)S11
CQM assigns state to
the [70,1-] multiplet
Speculation if it is
not a |q3> state but
a |KS> molecule
the CQM ....
however, experimentally one finds a very hard transition formfactor..

23. The 2nd Resonance Region
N*(1535)S11
CQM assigns state to
the [70,1-] multiplet
Speculation if it is
not a |q3> state but
a |KS> molecule
Hard e.m. formfactor
which maybe more indicative of a small quark core than of a molecule type state ...
Moreover, as we may hear in the next talk, lattice calculations show a clear 3-quark bevavior for that state
experimentally, the resonance is often studied in eta production, due to the strong coupling of the state to this channel, which also acts as an isospin filter...
LQCD indicates clear
|q3> behavior

24. The 2nd Resonance Region
N*(1535)S11
CQM assigns state to
the [70,1-] multiplet
Speculation if it is
not a |q3> state but
a |KS> molecule
Hard e.m. formfactor
(next slide)
new eta electropoduction data from CLAS have been analysed,
this graph shows the total cross section for various Q2 values,
LQCD indicates clear
|q3> behavior
Strong coupling to ph

25. The 2nd Resonance Region
ep eph
CLAS
N*(1535)S11
CQM assigns state to
the [70,1-] multiplet
Speculation if it is
not a |q3> state but
a |KS> molecule
Hard e.m. formfactor
showing the strong resonance and the slow falloff with Q2...
(next slide)
The resonance analysis gives now a consistent behavior over
a large Q2 range, including earlier data from JLab Hall-C.
LQCD indicates clear
|q3> behavior
Strong coupling to ph

26. The 2nd Resonance Region
Photocoupling amplitude A1/2
N*(1535)S11
Consistent Q2 evolution
from h production
Model descriptions failed to reproduce the main features of the data, the nearly flat behavior at small Q2, and the slow falloff at high Q2. For comparison, a dipole form is included.
A recent calculation by the Genova group, using a hypercentral potential ....
H. Denizli

27. Giannini and Santopinto
The 2nd Resonance Region
Photocoupling amplitude A1/2
N*(1535)S11
Consistent Q2 evolution
from h production
Giannini and Santopinto
gives a considerably improved agreement ...
There will be a talk on this model in on of the parallel sessions...
(next slide)
there has been a longstanding discrepancy between the photoproduction analysis of pion data and of eta data

28. The 2nd Resonance Region
Photocoupling amplitude A1/2
N*(1535)S11
Consistent Q2 evolution
from h production
Discrepancy with Np
analysis
as shown here. The low point is from the pion data analysis..
(next slide)
using the already mentioned new data sets on pi+ and pi0 production, including the spin-dependent response function ....

29. The 2nd Resonance Region
Photocoupling amplitude A1/2
N*(1535)S11
Consistent Q2 evolution
from h production
Discrepancy with Np
ppo,np+
analysis
CLAS
(preliminary)
...good agreement is obtained with the eta data at Q2=0.4GeV2.
Uncertainties in our knowledge of the p-eta and N-pi branching ratios allow shifts of the data about 10%....
(next slide)
CLAS ph and Np data
consistent

30. Single Quark Transition Model
described by 3 amplitudes, e.g.
determined from S11, D13
In the SQTM, knowledge of 3 amplitudes of states belonging to the same supermultiplet allow predictions For example,

31. Single Quark Transition Model
described by 3 amplitudes, e.g.
determined from S11, D13
Predicts all other amplitudes
in same supermultiplet
of all other 16 amplitudes for states in the same supermultiplet.
One of the interesting aspects is, as my theorist friends tell me,
is that one tests aspects of resonance transitions

32. Test of the Single Quark Transition Model
described by 3 amplitudes, e.g.
determined from S11, D13
Predicts all other amplitudes
in same supermultiplet
Tests model in the large Nc limit
in the large Nc limits...
a good description is achieved at the photon point, while electroproduction data show agreement where there is good
data available, e.g in the case of the neutral D13(1520)'s...
other data is too poor to draw any conclusion...
(next slide) the reason is mostly due to the lack of data in the
2-pion channel....
Good description of Q2=0
Insufficient Q2 = 0 data

33. Higher mass and “missing states”
Higher mass states tend to
couple strongly to Npp
as many higher mass states couple strongly to that channel, e.g in the case of those states highlighted in red...
(next slide)
Predictions for so-called missing states also favor strong oupling to 2-pion channels ...

34. “Missing” Resonances? |q3> |q2q>
Symmetric CQM predicts many more states than
observed in elastic pN scattering analysis
|q3>
=> predicted to couple to
Npp (Dp, Nr), Nw, KY
which model is closer to reality?
In recent years a number of alternatives to the "standard" constituent quark model have been proposed, in large measures to fix some of the real or perceived shortcomings of that model, especially to explain the absence of many states predicted in that model. Several talks at this conference are dedicated to these new approaches. One of the oldest alternative models is the quark-diquark model, where two of the quarks form an elementary object. This model has the advantage that due to the fewer excitation degrees of freedom a much smaller number of states are predicted which corresponds more to what we see in the data...
The various alternatives correspond to quite different underlying quark symmetries and interactions, search for at least some of the states predicted in the standard model, but not in the alternative models is imperative.
A series of P13 states with coupling to the omega channel are predicted in the standard model but not in the quark-diquark model
Electromagnetic production of omegas suffers from strong diffractive or t-channel contributions, leading to strong forward peaking which complicates the analysis. However, diffractive production is reduced with increasing Q2.
While the diffractive or t-channel contribution gives the forward peaking, s-channel resonance production produces a more symmetric angular dependence with significant large angle production.
(next) The data exhibit ..
|q2q>
=> fewer excitation degrees of freedom
fewer states
Klempt, Vijande

35. Resonances in g*p pp+p-
Total cross section
CLAS
Genova-Moscow
Isobar model fit
GNpp PDG
GNg AO/SQTM
These are the first high statistics electroproduction data in the 2-pi channel.
The data can be best fitted by a new P13 state with properties that are very different from the properties of the known
P13 located in the same mass range.
missing resonance strength

36. Isobar fit to D13(1700) and new P13
Total cross section
CLAS
Genova-Moscow
Isobar model fit
GNpp PDG
GNg AO/SQTM
alternate possibilities, e.g. an increase in D13(1700) strength give poorer fits especially to the hadronic distributions, ...
P13
D13(1700)
W(GeV)

38. Isobar fit - hadronic decays
CLAS
W = 1.74GeV
P13
Mp+p
D13(1700)
Data described best by new P13
Mp+p-
M = / GeV
GT = 88 +/- 17 MeV
Dp : /- 0.13
e.g. the pi+pi- mass which shows a rho paek for the D13, not seen inthe data, and in the pi- angular distribution. The fit
gives a fairly narrow resonance with the quantum numbers of a P13, strong Delta-pi coupling and small N-rho coupling.....
(next slide)
opposite of what .....
Nr : /- 0.10
qp- (deg)

39. Isobar fit - A new state? CLAS P13 D13(1700) W = 1.74GeV Mp+p
Data described best by new P13
Mp+p-
M = / GeV
GT = 88 +/- 17 MeV
Dp : /- 0.13
~ 0
has been seen in hadronic analyses ...
A new P13 would be consistent with a "missing state" by Capstick & Roberts, except for a 130MeV lower mass....
(next slide, omega)
While many resonances are known to decay into the 2-pion channel not a single resonance listed in the RPP is known to couple to the N-omega channel. Therefore, any state seen in that channel would be a discovery, either of a new decay, or of a missing state.
(next) Why is the search for at least some of the missing states so important for the understanding of baryon structure? ...
Nr : /- 0.10
consistent with “missing”
qp- (deg)
P13 state, but mass low
known P13
F. Klein

40. Search for resonances in hyperon production
CLAS
g*p K+Y
forward hemisphere
backward hemishere
preliminary N* N*
Niculescu/Feuerbach

41. Resonances in g*p pw? CLAS above resonance region s in resonanceregion
cosqpw +1 -1 s p g w N*
CLAS in
resonanceregion
F. Klein

42. Resonances in Virtual Compton Scattering
Hall A - E93-50
ep epg
First measurement through
entire resonance region
advantage over mesons, the
lack of final state interaction
N*(1520)
The process most recently employed in studies of resonance
excitations is Virtual Compton Scattering. This graph shows the first measurement throughout the resonance region of this process, exhibiting 3 prominent peaks related to the Delta, and two higher mass states.
The advantage of photons over mesons is the lack of final state interactions...
D(1232)
strong resonance excitations
N*(1650)
Fonvieille
Todor

43. Summary Accurate results on transition amplitudes for several states
give a consistent picture, and allow stringent test of theory
D(1232), N*(1535), (Roper)
Searches in various final states suggest excitations of states
not seen before
pp+p-, pw, K+L , ....
N* electroexcitation has become a major tool in studying
the complex regime of strong QCD and confinement

44. Outlook Transition amplitudes for several states under study
CLAS, Hall A/C, OOPS
New instrumentation/facilities - BLAST, MAMI upgrade

45. Outlook Transition amplitudes for several states under study
CLAS, Hall A/C, OOPS
New instrumentation/facilities - BLAST, MAMI upgrade
The D(1232) is the only resonance so far seen first
in electron scattering experiments.
Perhaps, this long drought is over soon.
The potential is there!

46. It is an exciting time to work in this field !
Outlook
Transition amplitudes for several states under study
CLAS, Hall A/C, OOPS
New instrumentation/facilities - BLAST, MAMI upgrade
The D(1232) is the only resonance so far seen first
in electron scattering experiments.
Perhaps, this long drought is over soon.
The potential is there!
It is an exciting time to work in this field !