1. CLAS: A Wide-angle Lens for Hadronic Physics
Nucleon Resonance Production with CLAS
Elton S. Smith
Jefferson Lab
for the CLAS Collaboration

2. Why Electromagnetic Excitation?
Light quark baryon spectrum for N* → Np and underlying symmetries
Internal structure of baryons
Meson production mechanism
Connections between resonance regime and deep inelastic region - Duality
Helicity amplitudes vs Q2 => Relevant
degrees of freedom vs distance scale

3. Program Requirements Experiment Analysis
large high-quality data set for N* excitation covering
- a broad kinematical range in Q2, W, decay angles
- multiple decay modes (p, pp, h, r, w, K)
- polarization information (sensitive to interference terms)
Analysis
D(1232): full Partial Wave Analysis possible
(isolated resonance)
higher resonances
- need to incorporate Born terms, unitarity, channel coupling
- full PWA presently not possible due to lack of data (polarization)
(substitute by assuming energy dependence of resonance)
- skills required at the boundary between experiment and theory

4. Quark Model Classification of N*
“Missing”
P13(1870)
Capstick and Roberts
D13(1895)
Mart and Bennhold ?
D13(1520)
S11(1535)
Roper P11(1440)
D(1232)
S11(1790)
Non-quark Model State

5. CLAS Detector

6. CLAS Coverage for e p e’ X
2.0
1.0
1.5
2.5
4.0 3
5.0
For electron scattering experiments CLAS is usually triggered on electrons only.
The inclusive electron spectrum at 4 GeV beam energy,
shown here in the Q2 versus invariant mass W kinematics, exhibits the elastic scattering band, the 3 resonance bands, and the transition to the deep inelastic regime.
These lines indicate the region where many of the missing states are expected.
(next slide)
The projection onto the W axis shows the elastic peak and the
3 resonance bumps

7. CLAS Coverage for e p e’ p X, E=4 GeV0.
0.5
1.0
1.5
2.0
missing states
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.

8. Kinematics and Cross Sections
example:
e p → e’ p po
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.....

9. need broad coverage in pion decay angles cos(q*) and F

10. Multipole Analysis for g*p → p po
CLAS
Q2 = 0.9 GeV2
|M1+|2
Re(E1+M1+*)
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+*)

11. “Missing” Resonances?
Problem: symmetric CQM predicts many more states than have
been observed (in pN scattering)
Two possible solutions:
1. di-quark model
|q2q>
fewer degrees-of-freedom
open question: mechanism for q2 formation?
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 ..
2. not all states have been found
possible reason: decouple from pN-channel
|q3>
model calculations: missing states couple to Npp (Dp, Nr), Nh, Nw, KY
g coupling not suppressed electromagnetic excitation is ideal

12. Use of Selective Filters
Isospin
p+/p0 h L/S
Mass
Strange particle (K, h)
Multi-pion (2p, w, etc)
Polarization
sTT sLT sLT’ and hyperon polarization
(no target polarization included here)
Photoproduction and electroproduction

13. Missing mass gp → pX
0.85<Eg<0.9 GeV
0< cosq <0.2

14. Differential cross section gp → ph
Good agreement with other data
W < 1.73 GeV
Extends world data sample
to W = 2.1 GeV
M. Dugger, ASU

15. Total cross section gp → ph
REM: re-fit h MAID
including CLAS data
Additional
S11(1790)
improves
cQM fit
Non-Quark Model State
Added to cQM
Models underestimate
Data ~ 1.8 GeV
PhD Student
M. Dugger, ASU

16. Resonances in g*p → pp+p-
Analysis performed by Genova-Moscow collaboration
step #1:
use the best information presently available
GNpp from 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.
preliminary
extra strength
W(GeV)

17. Attempts to fit observed extra strength
Analysis step #2:
- vary parameters
of known D13
vary parameters
of known P13
introduce new P13
alternate possibilities, e.g. an increase in D13(1700) strength give poorer fits especially to the hadronic distributions, ...
preliminary
P13
D13(1700)
W(GeV)

18. consistent with “missing”
Isobar fit - A new state?
CLAS
W = 1.74GeV
D13(1700) Poor Fit
Mp+p
P13
Poor Fit
consistent with “missing”
P13(1870) state, but mass low
Good Fit with
New State or
Strongly Modified Old
1.72  0.02 GeV
88  17 MeV
0.41  0.13
0.19  0.10
preliminary
~ 0
known P13
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? ...
Mp+p-
M =
GT =
Dp :
Nr :
qp- (deg)

19. Photoproduction of K Y States on the Proton
L and S differential cross sections for 1.6 GeV < W < 2.3 GeV
Kaons identified by time-of-flight and Hyperons via missing mass g K+ L N* N p p0
L polarization measured via self-analyzing weak decay and
detection of protons.

20. Photoproduction of K Y States on the Proton
Dominant resonances
S11(1650)
P11(1710)
P13(1720)
Bump at 1.9 GeV
D13(1895) ?
Carnegie Mellon

21. Photoproduction of K Y States on the Proton
Model-t (Regge) has K and K* interference
- Misses at back angles
Model-s (Hadrodynamic)
Resonances, plus
K and K* exchange
Carnegie Mellon

22. Search for resonances in hyperon production
CLAS
ep eK+Y response functions
Strangeness production has become yet another tool in resonance studies. They present excellent conditions for obtaining detailed information on polarization observables due to the weak hyperon decay.
However, resonances are not contributing as obviously to the
production cross section as in pion production. It is thus important to get a better understanding of the production mechanism. This graph shows total cross section for Lambda and Sigma production.
The difference between the Lambda and Sigma production mechanism becomes obvious in the angular distribution of the
response functions. t-channel production seems dominant as seen in the strong forward peaking, while Sigma production seems more like s-channel dominated. The Lambda channel also has a strong longitudinal component as evidenced in the large RLT interference term.
preliminary

23. Resonances in hyperon Production?
CLAS
g*p K+Y
forward hemisphere
backward hemisphere
preliminary
N* ?

24. Summary
Understanding the structure of bound qqq systems is a central problem for the study of QCD in the strong coupling regime
Probing details of quark wave functions
consistent data set for N → D transition up to Q2 = 4 GeV2
E1+/M1+ small and negative
data emphasize the importance of pion degrees-of-freedom and relativity
Identify relevant degrees-of-freedom
finally getting electromagnetic data of sufficient quality to study missing resonance problem
initial data strongly suggest resonance contributions that cannot be explained by known baryon states