1. Volker D. Burkert Jefferson Lab
Excited Hadrons
Volker D. Burkert
Jefferson Lab
Contributions: T. Barnes, A. Dzierba, C. Meyer, A. Szczepaniak
Excited baryons
Classification
NΔ transition form factors
Low mass N* excitations, Roper
A near-threshold resonance S11(1535)
Search for new baryon states
Coupled channels analysis
Exotic hybrid mesons
What are they, what do we know?
Near term plans
Plans for JLab Upgrade
Summary
Workshop on Physics of Nucleons and Nuclei, October 16-17, 2006

2. Why excited baryons are important
Baryons (nucleons) make up most of the mass of the visible universe.
They represent the simplest system where the non-abelian character of QCD is manifest.
Study of the excitation spectrum is necessary to understand the ground state and explore origin quark confinement.
gluon self coupling
LQCD calculation of gluon flux
distribution in a 3-quark system.
... 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:

3. SU(6)xO(3) Classification of Baryons
3-Quark Shell Model
Quark orbital angular momentum
HO-Model Principal Energy Levels

4. Electromagnetic Excitation of N*’s
The experimental N* Program has two major components: 1) Transition form factors of known resonances to study their internal structure and confining potential 2) Spectroscopy of excited baryon states, search for new states. Both parts of the program are being pursuit in various decay channels, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using cross sections and polarization observables.

5. Reach of Current Accelerators
Spring-8
JLAB

6. SU(6)xO(3) Classification of Baryons
Quark orbital angular momentum
S11(1535)
P33(1232)
P11(1440)

7. Examples of Exclusive Processes in N* Studies
p(e,e’)X
Hadronic mass

8. Electromagnetic Excitation of N*’s
p, h, pp γv e gv N
lgp=1/2
lgp=3/2
N*,△ N N’
A3/2, A1/2, S1/2
Ml+/-, El+/-, Sl+/-
DOE Milestone 2012
Measure the electromagnetic excitations of low-lying baryon states (<2 GeV) and their transition form factors over the range Q2 = 0.1 – 7 GeV2 and measure the electro- and photo-production of final states with one and two pseudo-scalar mesons.

9. Resonance Analysis Tools
Nucleon resonances are broad and overlapping, careful analyses of angular distributions for differential cross sections and polarization observables are needed.
Amplitude & multipole analysis (GWU, SAID)
Phenomenological analysis procedures have been developed, e.g. unitary isobar models (UIM), dispersion relations (DR), that separate non-resonant and resonant amplitudes in single channels.
Dynamical coupled channel approaches for single and double pion analysis are being developed within the Exited Baryon Analysis Center (EBAC) effort. They are most important in the extraction of transition form factors for higher mass baryon states.
Event-based partial wave analyses with maximum-likelihood fit, developed in the search for new mesons states are now being utilized for baryon resonance studies. They fully utilize correlations in the final state (CMU). (Comments by Curtis Meyer).

10. The γNΔ(1232) Quadrupole Transition
pQCD
limit
Shape at low Q2
SU(6): E1+=S1+=0

11. NΔ Multipole Ratios REM, RSM
REM remains small and negative at -2% to -4% from 0 ≤ Q2 ≤ 6 GeV2.
No trend towards sign change or asymptotic behavior. REM → +100%, RSM→constant. Higher energies needed.
Dynamical models need to include pion contributions to explain magnitude and Q2 dependence.
Quenched LQCD agrees with E/M, deviates at low Q2 in S/M. Effect of pion cloud?
6.0
1.0
2.0
3.0
4.0
5.0

12. The Roper resonance N1/2+(1440)P11
RQM: P11(1440) = [56,0+]r
P11(1440) = Q3G
P11(1440) = (Q3)r(QQ)
Photocoupling amplitudes carry information on the the internal structure of the state.
The Roper resonance is not a gluonic excitation Q3G.
At large distances meson couplings may be important.
At short distances the Roper is best described as a radial excitation of the nucleon.
First observation of a sign change for
any nucleon resonance.

13. S11(1535): A near-threshold resonances
S11(1535) in the CQM is a L3Q=1, P=-1 state. It has also been described as a bound (KΣ) molecule with a large coupling to pη.
The very slow falloff of the A1/2(S11) form factor with Q2 suggests a Q3 system rather than a meson-baryon (QQQ-QQ) molecule (no form factor calculations exist for the molecular case).
S11(1535)

14. Cross section for Nπ and η-cusp
γp->π0p, Θ*=180o,
mostly resonant
0.6
0.7
0.8
0.9
Eγ(GeV)
γp->π+n, Θ*=180o,
mostly non-resonant.
0.6
0.7
0.8
0.9
Eγ(GeV)

15. SU(6)xO(3) Classification of Baryons
Quark orbital angular momentum
Missing
states

16. Search for undiscovered baryon states
Symmetric CQM |Q3> predicts many more states
than are observed in elastic πN → πN scattering analysis.
|Q3>
|Q2Q>
The diquark-quark model |Q2Q> has frozen degrees of freedom → fewer states. It accommodates all observed **** states.
Discovery of any new state could have significant impact on our understanding of the relevant degrees of freedom in baryonic matter.
Example: An additional P13 below 1900MeV would effectively rule out the |Q2Q> model.
Search for new states in different final states, e.g. Nππ, KΛ, KΣ, pω, pη’. Analyses are more complex and channel couplings become important.

17. New N* states in KS production?

18. New N* states in KL/KS production?
PWA of data on gp → K+L, K+S, K0S+
CLAS
K+L
K+S0
Partial wave analyses yield tantalizing hints of several new states. However solutions are not unique due to insufficient polarization data.

19. New N* candidate at 1710 MeV in pπ+π- ?
no 3/2+ (1720)
full
photoproduction
electroproduction
Background
Resonances
Interference
full calculation
no 3/2+
W(GeV)
W(GeV)
M. Ripani et al, Phys.Rev.Lett. 91, 2003

20. Search for Exited Baryon States
CLAS
Experiment reactions beam pol. target pol. recoil status
===================================================================================
G1/G10 γp→Nπ, pη, pππ, KΛ/Σ Λ,Σ complete
G8 γp→p(ρ,φ,ω) linear - - complete
G9-FROST γp→Nπ, pη, pππ, KΛ lin./circ. long./trans. Λ,Σ 2007
G13 γD→KΛ, KΣ circ./lin. unpol. Λ,Σ 2006/2008
G14-HD γ(HD)→KΛ, KΣ, Nπ lin./circ. long./trans. Λ,Σ 2009/2010
This program will, for the first time, provide complete amplitude information on the KΛ final state (more than 7 independent polarization measurements at each kinematics), and nearly complete information on the Nπ final states.

21. γp→KY (K+Λ, K+Σ0, K0Σ+)

22. γp→KY (K+Λ, K+Σ0, K0Σ+)

23. γn→K0Λ
DE=100MeV

24. Coupled Channel Analysis (EBAC)

25. Coupled Channel Analysis (EBAC)
Pion-nucleon and 2-pion-nucleon contributions to the non-resonant T matrix.

26. Hybrid mesons Flux Tube Model
Provides a framework to understand gluonic excitations.
The quarks in mesons are sources of color electric flux which is trapped in a flux tube connecting the quarks.
Conventional mesons have the flux tube in the ground state. When the flux tube is excited hybrid mesons emerge. For static quarks the excitation level above the ground state is ~1 GeV.
The excitation of the flux tube, when combined with the quarks, can lead to spin-parity quantum numbers that cannot be obtained in the quark model
(JPC - exotics).
The decay of hybrid mesons leads to complex final states.
Comments by Ted Barnes, Alex Dzierba, Adam Szczepaniak
1GeV
qqG qq
JPC = 0+-, 1-+, 2+-

27. LQCD supports the idea of flux tubes.
Flux distribution between
static quarks.
Flux tubes lead to a linear confining
potential.

28. Exotic Hybrid Mesons Masses
With 3 light quarks the conventional and hybrid mesons form flavor nonets for each JPC.

29. Evidence for Exotic Meson with JPC = 1-+
E852 - Evidence for the π1(1600) in π-p → η′π-p
M(η’π-) (GeV)
1.5
2.0
2.5

30. Evidence for Exotic Mesons
JPC = 1-+
E852 BNL
There is controversy about some of these channels.
experimental issues.
truncation of partial wave included in analysis.
interpretation of line shape and phase motion.
(A. Dzierba et al., Phys.Rev.D73:072001,2006)

31. Photons may be more suited to excite exotics
In the flux tube model, using photon beams, the production rate of hybrid mesons is not suppressed compared to conventional mesons.
N. Isgur, PRD (1999); A. Afanasev & A. Szczepaniak, PRD (2000); F. Close & J. Dudek (2004)

32. A first search for exotic meson with photons
Clarify evidence for exotic meson states, e.g. at 1600 MeV with high statistics.
Prepare for full study with GlueX.
Experiment planned to
run in 2007/2008.
Events from previous CLAS experiment. 45 35 25 15 5 a2 a1 p2
102 Events/ 20 MeV
Gluonic Meson?
p1(1600)
Expect 1-2 million 3-pion events,
3 orders more than any previously
published meson photoproduction
results, allowing a partial wave analysis.

33. GlueX – Exotic meson program at 12GeV
To meet these goals GlueX will:

34. GlueX – Mass reach Other facilities: PANDA (GSI/Darmstadt).
cc and cc-hybrid production in pp annihilation.

35. Summary
Transition form factors of the NΔ(1232) measured in large Q2 range.
no sign of approaching asymptotic QCD limit, needs 12 GeV upgrade
pion dressing of vertex needed to describe form factors
Roper P11 transition form factor determined for the first time.
zero-crossing of magnetic form factor
behaves like a Q3 radial excitation at short distances
Tantalizing hints of new baryon states in KY and Nππ channels
require polarization data to resolve ambiguities in analysis
Measurement of multiple polarization observables in Nπ, pη, and KY production needed to resolve ambiguities in baryon resonance analysis. EBAC essential to support the baryon resonance program with coupled channel calculations.
Experiment to clarify status of some exotic meson candidates with photon beams in preparation.
Full program with GlueX at the JLab 12GeV Upgrade will map out exotic hybrid meson mass spectrum with high precision.

36. JLAB-RIA Workshop Wash. DC
16-17 Oct 2006
Ted Barnes
Physics Div. ORNL
Dept. of Physics and
Astronomy, U.Tenn.
Structure of Exotic Mesons …in 2-3 slides, 5 mins, concentrate on future developments.
1. Definition(s) of exotic mesons
2. Current theoretical expectations (< 2.5 GeV)
3. Future developments
1. Exotic meson defn. (Wikipedia):
“…quantum numbers not possible for mesons in the quark model.” (hence .ne. qq )
FLAVOR EXOTICS: e.g. isospin=2, m++: requires higher Fock states, e.g. q2q2. Controversial! (recall q4q)
SPIN-PARITY EXOTICS: J PC forbidden to qq, e.g Hybrid mesons “qqg”. Not controversial!

37. 2. Current theoretical expectations for exotic mesons: (< 2.5 GeV)
SPIN-PARITY (J PC) exotics in this mass range are expected only from hybrids “qqg”.
All JPC can be formed from qqg.
____________________________________________________________________________________________________________________________________________________________
The lightest hybrid multiplet is predicted to contain
JPC = , 1- + , 2- + , (bag model)
all this and , 1+ - , 2+ - , (flux tube model)
9 flavor states (qq -> flavor nonets). Hence 36 new states (bag) or 72 new states (f.t.)
At what mass? MH = 1.9 GeV (famous flux-tube estimate, Isgur and Paton),
ca. 1½ GeV (bag model), 2.0 GeV (LGT).
Decaying to what? Famous flux-tube prediction: H -> S+P modes, e.g. f1p, b1p.
This may be wrong. Simple S+S modes (hp, rp, h’p) should also be studied.
The best experimental exotic candidate, p1(1600), is seen in h’p !

38. 3. Future developments: Experiment - Theory - JLAB:
_____________________________________________________________
Experiment -
JLAB:
High statistics investigation of meson spectroscopy using photoproduction
(Gluex/HallD/Jlab).
Other facilities:
PANDA (GSI/Darmstadt). cc and cc-hybrid production in pp annihilation.
“Super-LEAR” Start date ca collaborators.
____________________________________________________________
Theory -
Improved LGT studies of hybrid spectroscopy. LGT H strong decays.
Understanding photoproduction of meson resonances (CEX vs diffractive).

39. Adam Szczepaniak, University of Indiana
BNL (E852)
confirmed by Crystal Barrel
BNL (E852)

40. What is the QCD nature of exotic signals
What is the QCD nature of exotic signals? For reactions involving ground state pseudo-scalars there exists the possibility of effective theory formulation (chiral GB’s have derivative couplings and the U(1) GB couples via (heavy) glueball). What follows is a coupled channel analysis. For example to order p4 (schematically)
(other approaches use dispersion relations - Roy eqs. - yield similar amplitudes)

41. P-wave Meson spectrum up to √s ~ 1.2 GeV
(Gasser, et al., Pelaez et al., Oset et al., Lesniak et al.)
Includes the ρ,K*; σ,κ, f0, a0 ; f2
Effective Lagrangian projected onto the exotic channel (Marco,Bass) reproduces the E852
ηπ and η’π signals (AS et al.)
P-wave
P -wave ηπ, η’π
2 coupled channels
S, D -wave KK, ηπ, η’π
3 coupled channels _

42. deep into the complex plane
...but can we say something about the what is the quark content ...
Yes, study the large-Nc dependence of S-matrix poles
resonance pole moves
deep into the complex plane
resonance pole moves
towards the real axis

43. like a quark model state
complicated !
The Nc dependence of the E852 exotic is currently under study (J.R.Pelaez, AS)

44. Partial Wave Analysis at CMU
Curtis A. Meyer, Carnegie Mellon University
Photoproduction of hadrons
Now:
In the CLAS experiment at Jefferson Lab, there significant photoproduction
data that can be used to provide information on baryon resonances.
Future:
The GlueX experiment will study photoproduction of mesons to search for
exotic-quantum-number hybrids. States that involve the confining gluonic
field in their quantum numbers.
A partial wave analysis combines information from the initial state with angular and energy distributions of the final state particles to reconstruct the quantum numbers of intermediate states.
s-channel process in CLAS
t-channel process in GlueX

45. Missing Baryons? Baryons are made from three quarks:
proton = uud neutron = ddu
Predict a spectrum of baryons of spin (J) and parity (P)
p,n Positive Parity States (Observed)
Negative Parity States (Observed)
Positive Parity States (About 50% Missing)
What are the effective degrees of freedom in a baryon?
If “quarks”, then we are missing states.
If “quarks” and “small di-quarks”, then we might have all states.

46. Analysis Procedure
For two-body final states, the analysis can be simplified. For more than two bodies, the correlations between particles provides significant. This information is most effectively retained by keeping the events themselves, and not trying to bin the data.
Using 11TB of CLAS data from a recent run period,
simultaneously analyzing reactions:
 p ! p 
 p ! p 0
 p ! p 
 p !  K+
 p ! p+-
700,000 Events
250,000 Events
8,000,000 Events
1,200,000 Events
¼ Events
These channels have not been extensively studied
and are supposed to couple to some missing
baryons.

47. The Machinery Very CPU intense process.
Can impose constraints at the event level between different final states
E.g.  and 0 have to couple to the same states
Get total and differential cross sections as a free by-product. They
are projected out.
Get partial wave intensities and phases as a function of energy,
which can then be fit to a resonance picture.
We have the ability to put some models directly into the fit and
se how well they describe the data.

48. Preliminary Results  p and 0 showing similar coupling to the same
partial wave.
 p

49. Exotic Mesons