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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 1 Transition Form Factors at JLab: The Evolution of Baryonic Degrees of Freedom Ralf W. Gothe University of South Carolina Workshop on Partial Wave Analysis and Dalitz Plot Analysis January 25-26, 2007 Beijing, China Introduction N N Roper, and N N* Transitions 1 and 2 Production

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 2 Physics Goals Understand QCD in the full strong coupling regime transition form factors to nucleon excited states allow us to study relevant degrees-of-freedom wave function and interaction of the constituents ? ! ? ! ? ? ! ! Models Quarks and Gluons as Quasiparticles ChPT Nucleon and Mesons pQCD q, g, qq 1.0 fm <

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 3 Lattice QCD indicates a small oblate deformation of the (1232) and that the pion cloud makes E 1+ /M 1+ more negative at small Q 2. Data at low Q 2 needed to study effects of the pion cloud. Need data at low Q 2 N (1232) Transition Form Factors

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 4 C. Alexandrou et al., PRL, 94, 021601 (2005) R EM (%) R SM (%) Quenched LQCD describes R EM within error bars, but shows discrepancies with R SM at low Q 2. Pion cloud effects? Low-Q 2 Mutipole Ratios for R EM, R SM Need data at low Q 2

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 5 Low-Q 2 Mutipole Ratios for R EM, R SM C. Alexandrou et al., PRL, 94, 021601 (2005) preliminary Quenched LQCD describes R EM within error bars, but shows discrepancies with R SM at low Q 2. Pion cloud effects? Significant discrepancy between CLAS and Bates/MAMI results for R SM. C. Smith

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 6 Data at even lower Q 2 are needed to investigate the pion cloud further. Data at high Q 2 are needed to study the transition to pQCD. Preliminary Multipole Ratios R EM, R SM preliminary Need data at low Q 2

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 7 Constituent Counting Rule A 1/2 1/Q 3 A 3/2 1/Q 5 G M 1/Q 4 * S 11 Q 3 A 1/2 F 15 Q 5 A 3/2 P 11 Q 3 A 1/2 D 13 Q 5 A 3/2 F 15 Q 3 A 1/2 D 13 Q 3 A 1/2 Quark mass extrapolated to the chiral limit, where q is the momentum variable of the tree-level quark propagator using the Asquat action. Bowman et al. (LQCD) (GeV )

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 8 N → Multipole Ratios R EM, R SM New trend towards pQCD behavior does not show up. CLAS12 can measure R EM and R SM up to Q²~12 GeV². R EM +1 M. Ungaro G M 1/Q 4 *

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 9 electro-production (UIM, DR) PDG estimation , 2 combined analysis electro-production (UIM, DR) PDG estimation , 2 combined analysis K. Park (Data) I. Aznauryan (UIM) preliminary Roper Electro-Coupling Amplitudes A 1/2, S 1/2 nr |q 3 |q 3 +g LF |q 3 |q 3 +qq LF |q 3 nr |q 3 |q 3 +qq |q 3 +g LF |q 3 nr |q 3 |q 3 +g LF |q 3 |q 3 +qq nr |q 3 |q 3 +qq |q 3 +g

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 10 preliminary S 11 (1535) Electro-Coupling Amplitudes A 1/2, S 1/2 electro-production (UIM, DR) PDG estimation S 11, D 13 combined analysis (SQTM) K. Park (Data) I. Aznauryan (UIM) production (UIM, DR) nr |q 3 LF |q 3 nr |q 3 LF |q 3 nr |q 3

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 11 Energy-Dependence of + Multipoles for P 11, S 11 imaginary partreal part Q 2 = 0 GeV 2 The study of some baryon resonances becomes easier at higher Q 2. Q 2 = 2.05 GeV 2 preliminary I. Aznauryan (UIM)

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 12 Legendre Moments of Structure Functions Q 2 =2.05GeV 2 M 1- = 0 I. Aznauryan UIM fit The dominating final state multipole amplitude M 1- of the P 11 (1440) resonance is at high Q 2 are much more prominent than at small Q 2. K. Park preliminary CLAS

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 13 invariant mass / MC phase space BES/BEPC, Phys. Rev. Lett. 97 (2006) BESBing-Song Zou and

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 14 Fermion Helicity Conservation Helicity Conservation ´ for q M >> Quark mass extrapolated to the chiral limit, where q is the momentum variable of the tree-level quark propagator using the Asquat action. Bowman et al. (LQCD) (GeV )

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 15 preliminary D 13 (1520) Helicity Asymmetry A hel = A 1/2 2 – A 3/2 2 A 1/2 2 + A 3/2 2 A 1/2 A 3/2

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 16 Nucleon Resonances in 2 Electroproduction p(e,e’)X p(e,e’p) p(e,e’ + )n p(e,e’p + ) 2 channel is sensitive to N*’s heavier than 1.4 GeV Provides complementary information to the 1 channel Many higher lying N*’s decay preferably to N final states Q 2 < 4.0 GeV 2 Trigger W in GeV

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 17 direct 2 production Full calculations p - ++ p+0p+0 pppp p - P ++ 33 (1600) p + F 0 15 (1685) p + D 13 (1520) Combined fit of various single differential cross sections allowed to establish all significant mechanisms Isobar Model JM05 Contributing Mechanisms to p → p + - JM05

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 18 CLAS photo-production W(GeV) Background Resonances Full calculation no 3/2 + N* contributions are much smaller than non-resonant mechanisms electro-production W(GeV) M. Ripani 3/2 + (1720) P ?3 (1720) Full JM05 calculation with and without the 3/2 + (1720) state Resonances in p → p + -

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 19 Resonances CLAS Background Full JM05 no 3/2 + 3/2 + (1720) P ?3 (1720) Resonances and Background in p → p + electro-production photo-production JM05

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 20 Combined 1 -2 Analysis of CLAS Data PDG at Q 2 =0 Previous world data 2 analysis 1 -2 combined at //// Q 2 =0.65 GeV 2 Many more examples: //// P 11 (1440), D 13 (1520), S 31 (1650), //// S 11 (1650), F 15 (1685), D 13 (1700), //// … EBAC at JLab: //// Full coupled channel analysis D33(1700) P13(1720) preliminary JM05

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 21 Combined 1 -2 Analysis of CLAS Data PDG at Q 2 =0 2 analysis 1 -2 combined at Q 2 =0.65 GeV 2 Previous world data

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 22 v p → 0 p 1 Data Description by N * Electro-Couplings of the Combined Analysis W=1.52 GeV Q 2 =0.65 GeV 2 W=1.68 GeV Q 2 =0.65 GeV 2 JM05CLAS

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 23 W=1.54 GeV Q 2 =0.65 GeV 2 W=1.66 GeV Q 2 =0.65 GeV 2 The successful description of all 1 and 2 observables measured with CLAS at Q 2 =0.65 GeV 2 demonstrates the credibility of the N * background separation. 2 Data Description by N * Electro-Couplings of the Combined Analysis JM05CLAS

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 24 preliminary Roper Electro-Coupling Amplitudes A 1/2, S 1/2 PDG at Q 2 =0 1 analysis (UIM) 1 -2 combined at ////Q 2 =0.65 GeV 2 Newest 2 analysis ////at low Q 2 (JM 06) CLAS

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Ralf W. Gothe Nucleon Transition Form Factors Beijing 2007 25 Conclusion: Do Exclusive Electron Scattering Q 2 = 2.05 GeV 2 D 13 (1520)... to Learn QCD!

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