Qiang Zhao Theory Division Institute of High Energy Physics, CAS Email: zhaoq@ihep.ac

1. Oct. 06, 2008 Institute of High Energy Physics, CAS Update of quark model calculations for vector meson photoproduction Qiang Zhao Theory Division Institute of High Energy Physics, CAS Email: zhaoq@ihep.ac.cn

2. Outline • Effective Lagrangian approach for meson photoproduction • Updated results for , , and K* production • Summary

3. 1. Baryon spectroscopy and status of S11(1535) • The non-relativistic constituent quark model (NRCQM) makes great success in the description of hadron spectroscopy: meson (qq), baryon (qqq). • However, it also predicted a much richer baryon spectrum, where some of those have not been seen in N scatterings. • – “Missing Resonances”. , 0 P33(1232)  P11(1440) S11(1535) D13(1520) … N*, L2I,2J N, ½+

4. Dilemma: • The NRCQM is WRONG: quark-diquark configuration? … • The NRCQM is CORRECT, but those missing states have only weak couplings to N, i.e. small gN*N. (Isgur, 1980) • Looking for “missing resonances” in N* N, K, K, N, N, N, N … • (Exotics …) To pin down the underlying effective degrees of freedom and understand the property of QCD at low-energy limit u  d u N* u d d n u d

5. Photo- & electroproduction • Difficulties • Perturbative QCD cannot be applied • A lot of resonances could be present in a relatively narrow energy region • Non-resonance background is almost equally complicated M (,, , , , K …)  N*,* Strong EM N N (N, ,  …) • Experiment • Jefferson Lab (USA) • MAMI (Germany) • ELSA (Germany) • ESRF (France) • SPring-8 (Japan) • BES (China)¶   + p D13 F15 ¶ A unique way of studying baryon spectrum is via BES: J/ N N*,…

6. PDG2004: 22 nucleon resonances (uud, udd) (**) not well-established

7. 18 Lambda resonances (uds)

8. Kinematics for vector meson photoproduction x Invariant amplitudes: z V, q,V y x , k, cm z y N, Pi , 1 N, Pf , 2 • Spin observables: • Unpolarized cross sections • Polarized asymmetries See e.g. Pichowsky, Savkli, and Tabakin, PRC53, 593(1996)

9. Helicity amp. for an intermediate resonance excitation can be expressed as i.e. where the parity conservation gives

10. Spin observables in terms of density matrix elements Vector meson decay distribution:  z   y  x z V, q,V cm , k, N, Pi , 1 N, Pf , 2

11. Unpolarized decay distribution: Linearly-polarized decay distribution: e.g. The polarized beam asymmetry: Zhao, Al-Khalili & Cole, PRC71, 054004 (2005); Pichowsky, Savkli & Tabakin, PRC53, 593 (1996)

12. Two additional differential pol. beam asymmetries: Zhao, PRC63, 025203 (2001)

13. Kinematic features of the production mechanism • Forward angle peaking is predominant due to the diffractive or t-channel light meson exchanges. • S-channel resonance excitations contribute to the cross sections at middle and backward angles. • Quark-hadron duality argument makes the s- and t-channel transitions obscure at some kinematics. d/d t-channel s-channel u-channel Interferences from different transition mechanisms 0 90 180 Scattering angle

14. Three ingredients in our quark model approach: • s- and u-channel resonance excitations • Vector meson production via an effective Lagrangian for quark-vector-meson interactions in the s- and u-channel; • 2. t-channel natural parity exchange • Pomeron exchange for neutral vector meson (, 0, ) production in the t-channel, and t-channel scalar meson exchange; • 3. t-channel unnatural parity exchange • Light meson exchanges in the t-channel, e.g. 0 exchange for  production. Refs. Z., Li, & Bennhold, PLB436, 42(1998); PRC58, 2393(1998); Z., Didelez, Guidal, & Saghai, NPA660, 323(1999); Z., PRC63, 025203(2001); Z., Saghai, Al-Khalili, PLB509, 231(2001); Z., Al-Khalili, & Bennhold, PRC64, 052201(R)(2001); PRC65, 032201(R) (2002); Z., Al-Khalili, & Cole, PRC71, 054004(2005); Z. and Close, PRD74, 094014(2006)

15. 1.Effective Lagrangian for quark-vector-meson interactions: Vector meson fields in the SU(3)-flavour symmetry: s-channel , k , k V, q V, q + N, Pf N,Pi N, Pf N, Pi N() () N() ()

16. u-channel seagull , k , k V, q , k V, q V, q + N, Pf N, Pi N, Pf N, Pi N, Pf N, Pi N() () N() () Transition amplitude: M=M(s) + M(u) + M(seagull) + M(t) where with

17. S-channel quark correlation operators

18. 2.Pomeron exchange: Donnachi & Landshoff’s Pomeron: Pomeron mediates the long range interactions between two confined quarks and behaves rather like a C=+1 isoscalar photon. Vertices: Transition amplitude: Pomeron trajectory: Thus, Loop tensor: Lee and Pichowsky

19. 3.Pseudoscalar meson exchange (): t-channel Vertices: , k V 0,, q 0, N, Pf N, Pi Form factor: • Advantage: • A complete set of NRCQM resonances is included with very few parameters as leading contributions. • The same parameters for the production of SU(3) multiplets. • Disadvantage: Neither covariant nor unitary.

20. I. Theoretical results for  production -- Data from SAPHIR Single polaration asymmetries

21. Theoretical results for  production -- data from GRAAL Collaboration + … • Total cross sections   p    p N  2 Born terms + N > 2 degenerate in N a = 3.67, b = 3.85 GRAAL Collaboration, PRL96, 132003(2006)

22. Differential cross section   p    p Full dot: GRAAL Empty circle: SAPHIR

23. Polarized beam asymmetry • Without s- and u-channel, the asymmetry should be zero due to helicity conservation. • Effects without contributions from: • D13(1520) [dashed]; • P13(1720) [dotted]; • F15(1680) [dot-dashed]

24. New data from CBELSA, 0807.0594[nucl-ex] Consistent with the GRAAL results.

25. Density matrix elements above resonance region E = 2.8 GeV 4.7 GeV 9.3 GeV E = 2.8 GeV 4.7 GeV 9.3 GeV Data from ABBHHM collaboration

26. -- Data from Hall C (JLab) Diff. X-section at small Q2 Backward enhancement: Evidence for s-channel resonance excitations via   N  N*    N e  Q2, * e W N N a = 3.67, b = 3.85 Same as extracted from GRAAL data Ambrozewicz et al., [JLab E91-016 Colla.], PRC70, 035203 (2004)

27. II. Theoretical results for  production • Diffractive pomeron exchange dominant. • t-channel unnatural  exchange included. • s- and u-channel account for large angle cross sections. Nucleon Born term is important. Parameter “a” can be related to the NN coupling and compared with the NN coupling extracted from  production.

28. Predictions for the  production on the neutron The cross section difference at large angles is mainly due to the difference between p and n.

29. Prediction of A

30. III. Theoretical results versus data from: K* production on the proton Zhao, Al-Khalili and Bennhold, PRC64, 052201(R)(2001)

31. Experimental data from JLab CLAS, PRC75, 042201(R)(2007), & erratum

32. Experimental data from CBELSA Black dot: CBELSA Red empty square: CLAS Solid: a = 2.7 b = 1.7 Dash-dotted: a =  2.2 b = 0.8 CBELSA/TAPS, Euro. Phys. J. A35, 333 (2008)

33. Total cross sections CBELSA/TAPS, Euro. Phys. J. A35, 333 (2008)

34. Summary - I • The forward peaking is dominated by the t-channel natural or unnatural exchanges. The measurement of the parity asymmetry is sensitive to their interferences at forward angles. • Resonance signals appear in vector meson productions. Polarized beam asymmetry at middle and large angles are essential for determining the underlying mechanisms. • Further coherent studies of all vector meson production channels as well as pseudoscalar meson productions are needed and they are in progress. • e.g. photoproduction of , , ; and N  N; Kp  • Q. Z., PRC 63, 035205 (2001) ; • Q. Z., J.S. Al-Khalili, Z.P. Li, and R.L. Workman, PRC 65, 065204 (2002); • Q. Z., B. Saghai and Z.P. Li, JPG 28, 1293 (2002); • X.H. Zhong, Q. Zhao, J. He, and B. Saghai, PRC 76, 065205 (2007)

35. Summary - II 4. For the purpose of searching for individual resonance excitations, it is essential to have a quark model guidance for both known and “missing” states. And then allow the data to tell: i) which state is favored; ii) whether a state beyond quark model is needed; iii) how quark model prescriptions for N*NM form factors complement with isobaric models; 5. Further questions and comments: i) be aware of the collective effects arising from several closing states; ii) how to recognize the mixture of t-channel and s-channel transitions due to the quark-hadron duality argument; iii) how to introduce the relativistic effects; iv) and more ……

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