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Excited Baryon Program - in part based on N* Workshop, Nov. 6-7, 2006, JLab -

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. Why excited baryons are important. Baryons (nucleons) make up most of the mass of the visible universe.

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Excited Baryon Program - in part based on N* Workshop, Nov. 6-7, 2006, JLab -

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  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. 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++ baryonrequired 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. gluon self coupling Lattice QCD calculation of gluon flux distribution in a system of 3 heavy quarks.

  3. SU(6)SF x O(3) Classification of Baryons F15(1680) Predicted states D13(1520) P11(1440) P33(1232) Quark orbital angular momentum S11(1535) Harmonic Oscillator-Potential - Principal Energy Levels

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

  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, pρ0 using cross sections and polarization observables. e’ p, h, pp,.. γv e lgp=1/2 N*,△ gv N N’ N lgp=3/2 A3/2, A1/2, S1/2 Ml+/-, El+/-, Sl+/-

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

  7. The γ*NΔ(1232) Quadrupole Transition SU(6): E1+=S1+=0 pQCD limit pQCD limit ~ -0.03 -0.1 Shape at low Q2 Non-zero values at higher Q2 reveal intrinsic quadrupole charge distribution.

  8. Sign @ Q2 > 0 ? Q2 dependence? γ*NΔMultipole RatiosREM, RSMbefore JLab

  9. γ*NΔMultipole RatiosREM, RSMwith JLab • REM= -2 to -4% at 0 ≤ Q2 ≤ 6 GeV2. • RSM < 0, increasing in magnitude. • REM < 0 favors oblate shape of Δ(1232). • Pion contributions needed to explain shape, magnitude. • No trend towards asymptotic behavior REM→+100%.

  10. e e e e γ*pΔ+ - Magnetic Transition Form Factor G*M T.-S. H. Lee N. Sato Pion cloud contribution Quark core contribution Large pion contribution needed to explain NΔ transition. Pion contribution predicted to drop more rapidly with Q2 than the quark core. Probe core at sufficiently high Q2. Connection with elastic form factors and GPDs => Paul Stoler, Friday session

  11. Lattice QCD results for P11(1440), S11(1535) F. Lee, N*2004 Both states are considered as possible nucleon-meson molecular states: P11(1440) = |Nσ >, S11(1535) = |YK>. Masses of both states are well reproduced in quenched LQCD with valence quarks. For a (Q3Q2) system one expects a faster drop of the transition form factors with Q2. Mπ2 (GeV2)

  12. with Roper S11(1535) D13(1520) with Roper Δ no Roper Δ(1232) no Roper D13(1520) CLAS Legendre Moments σT +εσL for γ*p→π+n Q2=3GeV2 ~ (a + bcos2Θ) ~cosΘ ~const. W(GeV) W(GeV) W(GeV) The Roper P11, S11 and D13 states become dominant contributions at high Q2

  13. zero crossing CLAS Nature of the Roper N(1440)P11 ? nr |Q3> r |Q3>LC preliminary nr|Q3> |Q3G> r|Q3>LC preliminary |Q3G> LC Models: S. Capstick & B. Keister; S. Simula; I. Aznauryan • Exclude gluonic excitation Q3G. • At short distances consistent with Q3- radial excitation. • At large distances meson couplings may be important.

  14. Photocoupling amplitudes N(1535)S11 CLAS What is the nature of the N(1535) ? preliminary N(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 slow falloff of the A1/2 amplitude seen in pη and Nπ suggests a small Q3 system rather than a large KΣ molecule. Nπ pη

  15. Photocoupling amplitudes N(1520)D13 CLAS preliminary preliminary Q2(GeV2) Q2(GeV2) A1/2 is dominant amplitude at high Q2 as expected from asymptotic helicity conservation. A1/2 amplitudes P11, S11, D13, (F15) appear to behave similarly at high Q2.

  16. Test helicity conservation CLAS → Expect approach to flat behavior for Q3A1/2, Q5A3/2 at high Q2 Q3A1/2 S11 D13 Q5A3/2 P11 F15 F15 D13 Helicity conserving amplitude appears to approach scaling, but needs to be confirmed at higher Q2. No scaling seen for helicity non-conserving amplitude A3/2

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

  18. Summary of recent N* and Δ* findings R. Arndt, W. Briscoe, I. Strakovsky, R. Workman Analysis of elastic πN→πN (2006) · Does not support several N* and D* reported by PDG2006: ***D(1600)P33, N(1700)D13, N(1710)P11, D(1920)P33 ** N(1900)P13, D(1900)S31, N(1990)F17, D(2000)F35, N(2080)D13, N(2200)D15, D(2300)H39, D(2750)I313 * D(1750)P31, D(1940)D33, N(2090)S11, N(2100)P11, D(2150)S31,D(2200)G37, D(2350)D35, D(2390)F37

  19. Discover new baryon states |Q3> |Q2Q> • SU(6) symmetric quark model |Q3> predicts many states that have not been seen in elastic πN scattering analysis. • The diquark-quark model |Q2Q> has frozen degrees of freedom → fewer states. It accommodates all observed **** states. • Discovery of new states could have significant impact on our understanding of the relevant degrees of freedom in baryonic matter. • 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. Predicted SU(6) x O(3) States Examples of states predicted in the symmetric quark model with masses near 1900 MeV. ( S. Capstick, W. Roberts )

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

  22. New N* states inKΛ/KΣproduction? A. Sarantsev et al., C. Bennhold, et al., • PWA of data on gp→ K+L, K+S, K0S+ J. McNabb et al, PRC69 (2004) K+Λ K+Σ0 • Analyses find needs for various new candidate states. • Solutions based on unpolarized cross sections alone have ambiguities; • demonstrates the need for polarization measurements.

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

  24. Search for New Baryon States CLAS reactions beam pol. target pol. recoil status _____________________________________________________________ γp→Nπ,pη,pππ,KΛ/Σ - - Λ,Σ complete γp→p(ρ,φ,ω) linear - - complete --------------------------------------------------------------------------------------------- γp→Nπ, pη, pππ, KΛ lin./circ. long./trans. Λ,Σ 2007 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.

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

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

  27. γn →K0Λ → → → Projected Accuracy of Data (4 of over 100 bins)

  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. 1m CLAS12 JLab Upgrade to 12 GeV • Luminosity > 1035cm-2s-1 • General Parton Distributions • Transverse parton distributions • Longitudinal Spin Structure • N* Transition Form Factors • Heavy Baryon Spectroscopy • Hadron Formation in Nuclei Forward Tracker, Calorimeter, Particle ID Solenoid, ToF, Central Tracker

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

  31. CLAS published CLAS preliminary CLAS12projected CLAS12 Projections for A1/2 @ 12 GeV Full transition to quark core behavior ?

  32. DVCS - a new tool in N* physics ep egN* CLAS (preliminary) e e g* g hard process x+x x-x Bjorken regime ep→egp+n GPDs N* p • t, ξ dependence of N* transition - map out Transition-GPDs N*’s D • Decouple γ virtuality from momentum transfer to the nucleon Mnp+ (GeV) • Nucleon dynamics at the parton level

  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 JP. γp -> K+K+Ξ0π- γp -> K+K+X- Ξ(1320) Ξ(1530) Needs higher energy for spectroscopy -> 2007/2008. JLab @ 12 GeV is a good place for cascade spectroscopy.

  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 Q2, and spectroscopy of heavy strange baryons.

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