Electroexcitation of Nucleon Resonances BARYONS 02

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1 Electroexcitation of Nucleon Resonances Volker D. Burkert Jefferson Lab BARYONS 02 9th International Conference on the Structure of Baryons March 3-8,

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 2

3 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 why the world is the way it is Nucleons represent the simplest system where the non-abelian character of QCD is manifest 3

4 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 why the world is the way it is Nucleons represent the simplest system where the non-abelian character of QCD is manifest Nucleons/baryons are complex enough to reveal physics hidden from us in mesons Gell-Mann & Zweig - Quark Model O. Greenberg - The ++ problem/color 4

5 OUTLINE Why electroproduction? Experimental results Quadrupole deformation in the N- 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 5

6 Why N* Electroproduction? Light quark baryon spectrum for N* Nπ Internal structure of baryons Helicity amplitudes vs Q 2 => Relevant degrees of freedom vs distance scale Meson production mechanism 6

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8 missing states CLAS: ep epx, E=4GeV

9 N- (1232) Quadrupole Transition SU(6): E 1+ =S 1+ =0 9

10 Multipole Ratios R EM, R SM - before

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12 Multipole Analysis for γ*p pπ ο CLAS Q 2 = 0.9 GeV 2 M 1+ 2 Re(E 1+ M 1+ *) M 1+ 2 Re(S 1+ M 1+ *) L.C. Smith 12

13 Multipole Ratios R EM, R SM - before

14 Multipole Ratios R EM, R SM

15 Multipole Ratios R EM (Q 2 ), R SM (Q 2 ) Bonn(2002) Sato Ernst 15

16 Multipole Ratios R EM (Q 2 ), R SM (Q 2 ) Bonn(2002) LQCD

17 Multipole Ratios R EM (Q 2 ), R SM (Q 2 ) Bonn(2002) LQCD 2002? Moore s law) 17

18 Beam spin asymmetry Polarized Beam Observable ep epπ ο σ lt response function / CLAS Mami/A2 Botto Joo Kuhn 18

19 The 2nd Resonance Region The Roper N (1440)P 11 In CQM assigned as a N=2 radial excitation of the nucleon Poor description of properties such as mass, photocouplings, Q 2 evolution Strong gluonic component? Quark core with meson cloud? Nσ molecule? 19

20 The 2nd Resonance Region CLAS ep enπ + UnitaryIsobar fit 20

21 The 2nd Resonance Region The Roper N (1440)P 11 In CQM assigned as a N=2 radial excitation of the nucleon CLAS (preliminary) σ(π ο,π + ), Α e (π ο,π + ), unitary isobar fit Poor description of properties such as mass, photocouplings, Q 2 evolution Strong gluonic component? Quark core with meson cloud? Nσ molecule? H. Egiyan 21

22 The 2nd Resonance Region N * (1535)S 11 CQM assigns state to the [70,1 - ] multiplet Speculation if it is not a q 3 > state but a KΣ> molecule 22

23 The 2nd Resonance Region N * (1535)S 11 CQM assigns state to the [70,1 - ] multiplet Speculation if it is not a q 3 > state but a KΣ> molecule Hard e.m. formfactor LQCD indicates clear q3> behavior 23

24 The 2nd Resonance Region N * (1535)S 11 CQM assigns state to the [70,1 - ] multiplet Speculation if it is not a q 3 > state but a KΣ> molecule Hard e.m. formfactor LQCD indicates clear q3> behavior Strong coupling to pη 24

25 The 2nd Resonance Region N * (1535)S 11 CLAS ep epη CQM assigns state to the [70,1 - ] multiplet Speculation if it is not a q 3 > state but a KΣ> molecule Hard e.m. formfactor LQCD indicates clear q3> behavior Strong coupling to pη 25

26 The 2nd Resonance Region Photocoupling amplitude A 1/2 N * (1535)S 11 Consistent Q 2 evolution from η production H. Denizli 26

27 The 2nd Resonance Region Photocoupling amplitude A 1/2 N * (1535)S 11 Consistent Q 2 evolution from η production Giannini and Santopinto 27

28 The 2nd Resonance Region Photocoupling amplitude A 1/2 N * (1535)S 11 Consistent Q 2 evolution from η production Discrepancy with Νπ analysis 28

29 The 2nd Resonance Region Photocoupling amplitude A 1/2 N * (1535)S 11 Consistent Q 2 evolution from η production Discrepancy with Νπ analysis CLAS pπ ο,nπ + (preliminary) CLAS pη and Nπ data consistent 29

30 Single Quark Transition Model Transition [56,0 + ] -> [70,1 - ] described by 3 amplitudes, e.g. determined from S 11, D 13 30

31 Single Quark Transition Model Transition [56,0 + ] -> [70,1 - ] described by 3 amplitudes, e.g. determined from S 11, D 13 Predicts all other amplitudes in same supermultiplet 31

32 Test of the Single Quark Transition Model Transition [56,0 + ] -> [70,1 - ] described by 3 amplitudes, e.g. determined from S 11, D 13 Predicts all other amplitudes in same supermultiplet Tests model in the large N c limit Good description of Q 2 =0 Insufficient Q 2 = 0 data 32

33 Higher mass and missing states Higher mass states tend to couple strongly to Nππ 33

34 Missing Resonances? Symmetric CQM predicts many more states than observed in elastic πn scattering analysis q 3 > => predicted to couple to Νππ ( π, Nρ), Nω, KY which model is closer to reality? q 2 q> => fewer excitation degrees of freedom fewer states Klempt, Vijande 34

35 Resonances in γ*p pπ + π - CLAS Total cross section Genova-Moscow Isobar model fit Γ Nππ PDG Γ Nγ AO/SQTM missing resonance strength 35

36 Isobar fit to D 13 (1700) and new P 13 CLAS Total cross section Genova-Moscow Isobar model fit Γ Nππ PDG Γ Nγ AO/SQTM P 13 D 13 (1700) W(GeV) 36

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38 Isobar fit - hadronic decays W = 1.74GeV CLAS P 13 M π+p D 13 (1700) Data described best by new P 13 M π+π M = / GeV Γ T = 88 +/- 17 MeV π : / Νρ : / θ π - (deg) 38

39 Isobar fit - A new state? W = 1.74GeV CLAS P 13 M π+p D 13 (1700) Data described best by new P 13 M π+π M = / GeV Γ T = 88 +/- 17 MeV π : / ~ 0 Νρ : / θ π - (deg) consistent with missing P 13 state, but mass low F. Klein known P 13 39

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

41 Resonances in γ*p pω? σ γ ω above resonance region p p CLAS γ p N * p ω in resonance region -1 cosθ pω +1 F. Klein 41

42 Resonances in Virtual Compton Scattering Hall A - E93-50 ep epγ First measurement through entire resonance region advantage over mesons, the lack of final state interaction strong resonance excitations Fonvieille Todor (1232) Ν (1520) N * (1650) 42

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

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

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

46 Outlook Transition amplitudes for several states under study CLAS, Hall A/C, OOPS New instrumentation/facilities - BLAST, MAMI upgrade The (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! 46