1. 1 Excited Baryon Program - in part based on N* Workshop, Nov. 6-7, 2006, JLab - Volker D. Burkert Jefferson Lab Town Meeting on QCD and Hadrons, Rutgers University, January 12-14, 2007

2. 2 Why excited baryons are important  Baryons (nucleons) make up most of the mass of the visible universe.  The 3-quark system are at the foundation of the development of the quark model.  Understanding the existence of the lowest excited Δ ++ baryon required introduction of a new quantum number (later called ‘color’) by O. Greenberg.  Baryon represent the simplest system where the non-abelian character of QCD is manifest.  Study of the excited baryon states is necessary to fully understand the ground state nucleon and to explore quark confinement. Lattice QCD calculation of gluon flux distribution in a system of 3 heavy quarks. gluon self coupling

3. 3 SU(6) SF x O(3) Classification of Baryons Quark orbital angular momentum Harmonic Oscillator-Potential - Principal Energy Levels S 11 (1535) P 33 (1232) P 11 (1440) F 15 (1680) D 13 (1520) Predicted states

4. 4 Why study hadron structure with e.m. probes? resolution of probe low high N π γN may excite states not seen in πN. What are the appropriate degrees-of-freedom describing hadron structure at varying distance? e.m. probe q LQCD - P.O. Bowman DSE - C. Roberts dressed quark (glue, qq) bare quark

5. 5 Electromagnetic Excitation of N*’s The experimental N* Program has two major components: 1) Transition form factors of known states to probe their internal structure and confining mechanism 2) Search for undiscovered states. Both parts of the program are being pursued in various decay channels with CLAS, e.g. Nπ, pη, pπ + π -, KΛ, KΣ, pω, pρ 0 using cross sections and polarization observables. vv N  p   p  e e’ γvγv N N’N’ N*,△ A 3/2, A 1/2, S 1/2 M l+/-, E l+/-, S l+/- 

6. 6 Inclusive Electron Scattering Need to measure exclusive processes in full phase space to separate resonances from each other and from non-resonant contributions. ep → eX CLAS

7. 7 The γ*NΔ(1232) Quadrupole Transition pQCD limit pQCD limit Shape at low Q 2 Non-zero values at higher Q 2 reveal intrinsic quadrupole charge distribution. SU(6): E 1+ =S 1+ =0 ~ -0.03 -0.1

8. γ*NΔ Multipole Ratios R EM, R SM before JLab Sign @ Q 2 > 0 ? Q 2 dependence?

9.  R EM = -2 to -4% at 0 ≤ Q 2 ≤ 6 GeV 2.  R SM < 0, increasing in magnitude.  R EM < 0 favors oblate shape of Δ(1232).  Pion contributions needed to explain shape, magnitude. γ*NΔ Multipole Ratios R EM, R SM with JLab  No trend towards asymptotic behavior R EM → +100%.

10. γ*pΔ + - Magnetic Transition Form Factor G * M Quark core contribution e e e e Pion contribution predicted to drop more rapidly with Q 2 than the quark core. Probe core at sufficiently high Q 2. Pion cloud contribution Large pion contribution needed to explain NΔ transition. Connection with elastic form factors and GPDs => Paul Stoler, Friday session T.-S. H. Lee N. Sato

11. Lattice QCD results for P 11 (1440), S 11 (1535) F. Lee, N*2004 M π 2 (GeV 2 ) Both states are considered as possible nucleon-meson molecular states: P 11 (1440) = |Nσ >, S 11 (1535) = |YK>. Masses of both states are well reproduced in quenched LQCD with valence quarks. For a (Q 3 Q 2 ) system one expects a faster drop of the transition form factors with Q 2.

12. Legendre Moments W(GeV) σ T +εσ L for γ*p → π + n Q 2 =3GeV 2 ~const. ~cosΘ ~ (a + bcos 2 Θ) The Roper P 11, S 11 and D 13 states become dominant contributions at high Q 2 Δ(1232) D 13 (1520) S 11 (1535) D 13 (1520) no Roper with Roper Δ no Roper with Roper CLAS

13. 13 |Q 3 G> nr|Q 3 > |Q 3 G> r|Q 3 > LC  Exclude gluonic excitation Q 3 G.  At short distances consistent with Q 3 - radial excitation.  At large distances meson couplings may be important. preliminary Nature of the Roper N(1440)P 11 ? zero crossing LC Models: S. Capstick & B. Keister; S. Simula; I. Aznauryan C LAS

14. 14 Photocoupling amplitudes N(1535)S 11 N(1535) in the CQM is a L 3Q = 1, P=-1 state. It has also been described as a bound (KΣ) molecule with a large coupling to pη. The slow falloff of the A 1/2 amplitude seen in pη and Nπ suggests a small Q 3 system rather than a large KΣ molecule. NπNπ pηpη C LAS What is the nature of the N(1535) ? preliminary

15. 15 Photocoupling amplitudes N(1520)D 13 A 1/2 is dominant amplitude at high Q 2 as expected from asymptotic helicity conservation. Q 2 (GeV 2 ) C LAS A 1/2 amplitudes P 11, S 11, D 13, (F 15 ) appear to behave similarly at high Q 2. preliminary

16. 16 Test helicity conservation Q 3 A 1/2 Q 5 A 3/2 S 11 P 11 F 15 D 13 No scaling seen for helicity non-conserving amplitude A 3/2 F 15 D 13 Helicity conserving amplitude appears to approach scaling, but needs to be confirmed at higher Q 2. → Expect approach to flat behavior for Q 3 A 1/2, Q 5 A 3/2 at high Q 2 C LAS

17. 17 SU(6)xO(3) Classification of Baryons Predicted states Quark orbital angular momentum

18. Summary of recent N* and Δ* findings  Does not support several N* and  * reported by PDG2006: ***  (1600)P 33, N(1700)D 13, N(1710)P 11,  (1920)P 33 ** N(1900)P 13,  (1900)S 31, N(1990)F 17,  (2000)F 35, N(2080)D 13, N(2200)D 15,  (2300)H 39,  (2750)I 313 *  (1750)P 31,  (1940)D 33, N(2090)S 11, N(2100)P 11,  (2150)S 31,  (2200)G 37,  (2350)D 35,  (2390)F 37 R. Arndt, W. Briscoe, I. Strakovsky, R. Workman Analysis of elastic πN → πN (2006)

19. 19 Discover new baryon states |Q3>|Q3> |Q2Q>|Q2Q>  SU(6) symmetric quark model |Q 3 > predicts many states that have not been seen in elastic πN scattering analysis.  Discovery of new states could have significant impact on our understanding of the relevant degrees of freedom in baryonic matter.  The diquark-quark model |Q 2 Q> has frozen degrees of freedom → fewer states. It accommodates all observed **** states.  Search for new states in different final states, e.g. Nππ, KΛ, KΣ, pω, pη’. Analyses are more complex and channel couplings are likely important.

20. 20 Predicted SU(6) x O(3) States SU(6) x O(3) Partial wave L 2J,2I Mass (MeV) Decays [N 1/2 + ] 4 P 11 1880Δπ, ∑K [N 1/2 + ] 5 P 11 1975Δπ, Nω, Nρ [N 3/2 + ] 2 P 13 1870Nπ, ∑K, Δπ [N 3/2 + ] 3 P 13 1910Δπ, Nω, Nρ [N 1/2 - ] 3 S 11 1945Nρ, Δπ, KΛ * [N 3/2 - ] 3 D 13 1960Δπ, ΛK, Nρ Examples of states predicted in the symmetric quark model with masses near 1900 MeV. ( S. Capstick, W. Roberts )

21. 21 K+K+ New N* states in KY production?

22. 22 PWA of data on  p→ K + , K + , K 0  + K + Σ  K + Λ J. McNabb et al, PRC69 (2004) New N* states in KΛ/KΣ production? Analyses find needs for various new candidate states. Solutions based on unpolarized cross sections alone have ambiguities; demonstrates the need for polarization measurements. A. Sarantsev et al., C. Bennhold, et al.,

23. 23 N* candidate at 1720 MeV in pπ + π - ? electroproduction W(GeV) no 3/2 + (1720) full M. Ripani et al, Phys.Rev.Lett. 91, 2003 photoproduction W(GeV) Background Resonances Interference full calculation no 3/2 + C LAS

24. 24 Search for New Baryon States reactions beam pol.target pol. recoil status _____________________________________________________________ γp → Nπ,pη,pππ,KΛ/Σ - - Λ,Σcomplete γp → p(ρ,φ,ω) linear- -complete --------------------------------------------------------------------------------------------- γp → Nπ, pη, pππ, KΛ lin./circ.long./trans. Λ,Σ2007 γD → KΛ, KΣ circ./lin.unpol. Λ,Σ 2006/2009 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, and nearly complete information on the Nπ final states. C LAS

25. 25 Instrumentation for Excited Baryon Search Frozen Spin Target FROST CLAS Photon Tagger LInearly polarized photon beam P(H ) P(D ) 0 20 40 60 (%) days BNL - Fall’06 Polarized HD - Target Considered to be used at CLAS.

26. 26 γp →K + Λ Projected Accuracy of Data (4 of over 100 bins) →→→

27. 27 γn → K 0 Λ Projected Accuracy of Data (4 of over 100 bins) →→→

28. 28 Need for Theory Support For small resonance cross sections, channel couplings due to unitary constraints can lead to strong distortions of amplitudes. Requires coupled-channel computation that includes all major channels. The Excited Baryon Analysis Center (EBAC) was established in 2006 at JLab to provide theoretical support for the excited baryons experimental program.

29. CLAS12 1m JLab Upgrade to 12 GeV Luminosity > 10 35 cm -2 s -1 General Parton Distributions Transverse parton distributions Longitudinal Spin Structure N* Transition Form Factors Heavy Baryon Spectroscopy Hadron Formation in Nuclei Solenoid, ToF, Central Tracker Forward Tracker, Calorimeter, Particle ID

30. 30 NΔ Transition - Future Program Transition towards asymptotic behavior? +100 ??

31. Projections for A 1/2 @ 12 GeV CLAS12 Full transition to quark core behavior ? CLAS published CLAS preliminary CLAS12 projected

32. DVCS - a new tool in N* physics ep e  N*  t, ξ dependence of N* transition - map out Transition-GPDs GPDs p N*N* e e   hard process Bjorken regime x+  x-   Decouple γ virtuality from momentum transfer to the nucleon  Nucleon dynamics at the parton level  N*’s M n  (GeV) CLAS (preliminary) ep→e  + n

33. 33 Strangeness = -2 Ξ Baryons Advantage - Narrow widths, easier to separate from background. Disadvantage – No s-channel production, low cross sections. Flavor SU(3) predicts same number of Ξ’s as N*’s and Δ*’s. Only 3 Ξ’s have established J P. γp -> K + K + X - Needs higher energy for spectroscopy -> 2007/2008. JLab @ 12 GeV is a good place for cascade spectroscopy. γp -> K + K + Ξ 0 π - Ξ(1320) Ξ(1530)

34. 34 Conclusions Exclusive electroproduction of mesons has become a precise tool to map out the intrinsic structure of established baryon states. With large acceptance detectors in use, and the development of highly polarized electron/photon beams and polarized targets the search for new baryon states has advanced to a much higher level of sensitivity. Planned precision measurements with polarized beams, targets, and recoil polarization measurements with CLAS will provide the basis for unraveling the S=0 baryon spectrum in the critical mass region near 2 GeV. Making full use of the precise data produced by the new equipment requires sound theoretical methods in the search for complex resonance structure, and in understanding the physics at the core of baryons. This effort is underway with the Excited Baryon Analysis Center at JLab and with continuing efforts in Lattice QCD. Jlab @ 12 GeV and CLAS12 allows extension of N* transition form factors to much higher Q 2, and spectroscopy of heavy strange baryons.