Status of Baryon Spectroscopy

1. Status of Baryon Spectroscopy D. Mark Manley Kent State University Kent, OH 44242 USA GHP2004 First Meeting of the APS Topical Group on Hadronic Physics Fermilab, Batavia, IL October 26, 2004

2. Outline • Introduction: Baryons as 3-Quark States • Experimental Issues • Problems with Quark-Model Predictions • Recent Developments • Summary

3. Introduction: Baryons as 3-Quark States

4. ExperimentalIssues • Most modern experimental efforts focus on photoproduction or electroproduction experiments – needed are high-precision complementary measurements with hadron beams (pions and kaons) • Partial-wave analyses are best way to determine N* properties–Multichannel approaches can help resolve inconsistencies • New measurements with polarized photon beams and polarized targets should help reduce ambiguities in competing PWA solutions

5. Problems with Quark-Model Predictions Center of Gravity of Oscillator Bands: Harmonic-oscillator bands: - N=0 ground state - N=1 first negative-parity excited states - N=2 first positive-parity excited states Pattern of experimental states within a band is produced reasonably, but the “center of gravity” is about 50 MeV too low for the N=1 band and about 40 MeV too high for the N=2 band. Problem is present whether model uses single-gluon-exchange forces between quarks or model is based on exchange of Goldstone bosons.

6. Spin-Orbit Problem: Little experimental evidence for spin-orbit interaction. For example, consider some N* states... N=1 states with L=1 and S=1/2: S11(1535) and D13(1520) N=1 states with L=1 and S=3/2: S11(1650), D13(1700), and D15(1675)

7. Spin-Orbit Problem (Continued): What about some Λ* states? N=1 octet states with L=1 and S=1/2: S01(1670) and D03(1690) But... N=1 singlet states with L=1 and S=1/2: S01(1405) and D03(1520) Problem: Are these states misidentified as members of a spin-orbit doublet? If not, why is the spin-orbit splitting so large?

8. Recent Developments: • E2/M1 Ratio for Δ(1232) • New Data from Accelerator Facilities • Pentaquarks* and Other Exotics • Lattice QCD Calculations • Missing Resonances *See plenary talks by K. Hicks, A. Dzierba, and A. Manohar.

9. E2/M1 Ratio for Δ(1232) • E/M ratios should vanish in simple quark model for baryon “stretched states” • Nonzero ratios tell us about configuration mixing effects (due, for example, to color hyperfine interactions – or to deformation of the baryon) • Best studied case is for Δ(1232): PDG estimate* is E2/M1 = -0.025 ± 0.008. • Model dependence of ratio was investigated by BRAG**. Result was E2/M1 = -0.0238 ± 0.0027. *S. Eidelman et al., Review of Particle Physics, Phys. Lett. B 592, 1 (2004). **R. A. Arndt et al., Proceedings of NSTAR 2001, (World Scientific, 2001) p. 467.

10. Helicity-Amplitude Ratios for Baryon Stretched States Assume simple quark-model prediction that E/M is zero for baryon stretched states. Then we have following predictions for the helicity- Amplitude ratios:

11. Properties of the S11(1535) Resonance • S11(1535) is unique in having large decay branch to ηN. • A1/2=0.060 ± 0.015 GeV-1/2 from γp→N. • A1/2=0.120 ± 0.011 ± 0.015 GeV-1/2 from γp→ηp. • Needs coupled-channel analysis to obtain consistent results. • Dynamically generated or ordinary q3 state? Total cross section for -p→ηn based on η→2γ decay. The dashed line indicates the η production threshold at p=685 MeV/c. A. Starostin et al., Crystal Ball Collaboration, to be submitted to PRC.

12. Exclusive Photoproduction of Cascade Hyperons • Little is known experimentally about the Cascade baryons. • X-(1321) and X-(1530) have been identified with CLAS detector at JLab in the gpK+K+X- reaction. • Potential exists to investigate new Cascade states using photo-production. J. W. Price et al., nucl-ex/0409030

13. Baryon Mass Spectrum from LatticeQCD • Lattice results for baryon ground states reproduce experimental masses to within about 5-10% • Old puzzle: low mass of the Roper resonance, N(1440), can’t be explained by quark potential models • Question: Is Roper resonance a 3-quark state or something more exotic? Needs lattice QCD calculation.

14. Lattice QCD Results for the Roper Resonance • Recent lattice calculations show level switching between the N*(1535) and N’(1440) should happen in lattice simulations with large enough spatial size. • Conclusion: Roper resonance can be described by the simple 3-quark excitation of sizable extent. S. Sasaki, Prog. Theor. Phys. Suppl. 151, 143 (2003).

15. Missing Resonances: Structure in Kaon Photoproduction near 1900 MeV • SAPHIR data for p(γ,K+)Λ show resonance structure near 1900 MeV. • Various proposals suggest state might be a new D13, P13, or D15 state. Theory: T.Mart and C. Bennhold, PRC 61, 012201 (2000). Data: M. Q. Tran et al., SAPHIR Collaboration, PLB 445, 20 (1998).

16. Missing Resonances:A possible new 3/2+ state • Cross section for ep→e’pπ+ π- was measured at JLab for 1.4 < W < 2.1 GeV and 0.5 < Q2 < 1.5 GeV2/c2. • Data show a resonant structure near 1.7 GeV that is not seen in prior experiments. • May indicate new 3/2+ state with narrow width. M. Ripani et al., hep-ex/0304034.

17. (1580) Resonance does not exist! Solid line: with D13(1580) resonance; Dashed line: w/o D13(1580) resonance J. Olmsted et al., PLB 588, 29 (2004).

18. Summary • Many new data are becoming available from JLab, Mainz, Bonn, Graal, BES, BNL, etc. • Spin observables will help constrain PWAs. • High-precision hadronic data are needed to help interpret the data from electromagnetic facilities. • Multichannel PWAs are needed to obtain consistent results. • Understanding baryon structure is still the basic goal. • Search continues for missing resonances, hybrids, pentaquarks, and other exotics.

19. Acknowledgments • This work was supported in part by DOE Grant No. DE-FG02-01ER41194. • The assistance of John Tulpan and Hongyu Zhang is greatly appreciated.