1. N* Production in α-p and p-p Scattering (Study of the Breathing Mode of the Nucleon) Investigation of the Scalar Structure of baryons (related to strong non-valence quark excitations) H.P. Morsch, MENU2004

2. Comparison with operator sum rules: Cross section covers maximum monopole strength Extraction of the baryon compressibility, K B ~1.3 GeV First evidence for the breathing mode of the nucleon from α-p scattering at SATURNE (Phys.Rev.Lett. 69,1336 (1992) Strong L=0 excitation in the region of the P 11 (1440) Projectile Δ excitation

3. Points to be discussed: 1.Theoretical studies of a low lying P 11 resonance 2.Results of α-p experiment 3.What are the properties of the Roper resonance? Roper resonance contains 2 structures: 1. Radial mode 2. Second order Δ excitation 4.New analysis of p-p scattering at beam momenta 5-30 GeV/c 5.What can we learn about the baryon structure from this excitation? Comparison with the longitudinal e-p amplitude S 1/2 6.Summary

4. 1.Theoretical studies of a low lying P 11 resonance Constituent quark model: Gluon exchange Pion exchange Relativistic quark model Bag model Skyrmion model Algebraic models Hybrid structure P 11 generated by strong σ-N interaction Lattice QCD calculations 1s  2s transition mass of P 11 high mass of P 11 lower mass of P 11 right (adjusted) (oscillation of the bag) P 11 is the lowest N* excitation (flat top) J π =1/2 + is lowest state not confirmed by new e-p data P 11 contains valence quark contribution!

5. 2. Saturne experiment α-p scattering Observation of a strong monopole excitation in the P 11 (1440) region Analysis in terms of operator sum rules S1=energy weighted sum S-1= energy inversely weighted sum H.P.Morsch, Z.Phys. A350, 61 (1994) Results of DWBA calculations: P 11 excitation covers the full sum S1 Transition density not compatible with valence quark picture! H.P.Morsch et al., Phys.Rev. C67, 064001 (2003)

6. 3. What are the properties of the Roper resonance? Shape of the resonance in a-p different from pi-N: m o ~1440 MeV, Γ~300-360 MeV in π-N m o ~1390 MeV, Γ~190 MeV in α-p T-matrix description of α-p and π-N scattering H.P.Morsch and P.Zupranski, Phys.Rev.C61,024002 (99) Two resonance picture consistent with γ-p  2π o p (radial mode not excited) new helicity amplitudes from Mainz!

7. 4.New analysis of p-p scattering at beam momenta 5-30 GeV/c Contibuting resonances Δ 33 (1232) D 13 (1520), F 15 (1680), strong res. at 1400 MeV Strongest resonance at 1400 MeV, width 200 MeV No other resonance seen (high selectivity)

8. What are position and width of the strong resonance? Resonance parameters: m o =1400±10 MeV Γ = 200 ± 20 MeV Position and width consistent with Saturne resonance observed in α-p scattering What is the evidence for P 11 ? What are the decay modes? change of m o change of width

9. What is the evidence for P 11 ? Information from the t-dependence of the p-p differential cross section Resonance at 1400 MeV is strongly peaked at small momentum transfer –t Characteristic for L=0 excitation !  P 11 resonance Calculation of the differential cross sections in DWBA using an effective interaction described by multi-gluon exchange (adjusted to fit elastic p-p p-p scattering) Cross section covers the full energy weighted sum rule, consistent with α-p!

10. What are the decay modes of the P 11 resonance at 1400 MeV ? Information from exclusive experiments: 2 prong events: p-p  p N* with N*  p π o and n π + Large yields observed for D 13 (1520) and F 15 (1680) consistent with π-N (elast. width 60% and 70%, resp.) 4 prong events: p-p  p N* with N*  p π + π - Strong peak above 2π threshold Description of the π + π - invariant mass spectrum consistent with the inclusive p-p  p N* spectra: Strong contribution from the P 11 resonance at 1400 MeV Estimated 2π branching B 2π = 75±20%

11. 5. What can we learn about the baryon structure from excitation of the P 11 (1400)? Sensitivity of the calculated differential cross sections to the nucleon transition density Quantitative description of the data requires a surface peaked transition density ρ tr (r) (consistent with the results from α-p) Transition density not consistent with pure valence quark excitation (constituent quark model) How can we understand the surface peaked transition density?

12. How can we understand the observed transition density? 1. Excitation of valence quarks 2. Strong sea quark contribution Sea quark contribution much stronger (~factor 4) than that of the valence quarks! The „breathing“ of the sea quark contribution indicates its existence also in the g.s. density What do we learn from the longitudinal electron scattering amplitude S 1/2 ? (data from JLab)

13. Comparison with the longitudinal e-p amplitude S 1/2 for the Roper resonance excitation C.Smith, NSTAR2004, I.G.Aznauryan, V.D.Burkert, et al., nucl-th/0407021, L.Tiator, Eur.J.Phys.16 (2004) Transition density for the description of S 1/2 requires: 1. valence quark excitation 2. sea quark component Direct relation of the nucleon transition density to the amplitude S 1/2 Difference between (p,p‘) and (e,e‘): (p,p‘) samples the matter densities (e,e‘) samples the charge densities For a better determination of the charge transition density more precise data on S 1/2 (at different q 2 ) needed!

14. Multi-gluon potential and compressibility From operator sum rules  baryon compressibility K B deduced. From the description of the (p,p‘) cross sections of scalar excitations a scalar multi-gluon potential V N (r) is derived! From this the compressibility K N is defined: Obtained compressibility consistent with sum rule estimate!  Scalar modes can be interpreted as vibrations of the multi-gluon field

15. 6. Summary 1.Evidence for the compression mode from α-p (Saturne resonance) 2.Properties of the Roper resonance P 11 (1440) 3.Analysis of p-p scattering at beam momenta 5-30 GeV/c 4.Transition density derived from (p,p‘) indicates strong sea-quark effects 5.From multi-gluon potential  compressibility K N (consistent with that from operater sum rules)  scalar modes interpreted as vibrations of the multi-gluon potential Full energy weighted sum rule observed  Extraction of baryon compressibility K B ~1.3 GeV Two structures: 1. breathing mode 2. second order Δ excitation (consistent with γ-induced reactions) Strongest resonance P 11 at 1400 MeV (mass, width and sum rule strength consistent with α-p) Decay of P 11 (1400) dominant into 2π-N channel similar effect observed in the charge transition density derived from the longitudinal electron scattering amplitude S 1/2