Coherent  -meson Photo-production from Deuterons Near Threshold

1. Coherent -meson Photo-production from Deuterons Near Threshold Wen-Chen Chang for LEPS collaboration Institute of Physics, Academia Sinica, Taiwan

2. Outline • Features of photo-production of -mesons from protons. • What we learned from the results of photo-production of -mesons from protons. • Features of coherent  production from deuterons. • Measurement of differential cross section and decay asymmetry of DD in the forward direction near threshold. • Summary

3. Pomeron exchanges0.08 g r, w,f g r, w Pomeron Meson exchanges-0.45 p,h,s・・・ p uud p Vector Meson Photoproduction from Protons

4. This fact makes the -photoproduction a unique process to determine Pomeron contribution and possible new processes near threshold!  Diffractive Photoproduction of  (ss) Mesons • Pseudo-scalar particle: • Negative power-law scaling of s. • Showing up at small energy. • Un-natural parity (= –1). • Exchange particles like ,, quark exchange. • OZI suppressed • Pomeron: • Positive power-law scaling of s. • Dominating at large energy. • Natural parity (=+1). • Exchange particles unknown; likely to be gluonic degree of freedom.

5. Glueball Hunt in  PhotoproductionT. Nakano and H. Toki (1997) P1: Pomerons0.08 ; P2: daughter Pomeron s-1.73 , inspired by (0+ glueball, M23 GeV2) P1+P2 P1

6. f →K+K- K+ eg Decay Plane // g natural parity exchange (-1)J (Pomeron, 0+ glueball, scalar mesons) K- Photon Polarization K+ eg Decay Planeg unnatural parity exchange -(-1)J (Pseudoscalar mesons p,h) K- Polarization Observables with Linearly Polarized Photon

7. Decay Asymmetry  11-1 - Im 21-1 • Azimuthal angle distribution of : • For spin-conserving processes: • Pomeron exchange: 11-1= –Im 21-1 = +0.5,  11-1-Im 21-1 = +1 • , exchanges: 11-1= –Im 21-1 = – 0.5,  11-1-Im 21-1 = 1

8. d/dt at t=tmin of pp Peak Off Peak LEPS(2005)

9. Decay Angular Distributions of pp Forward angles; -0.2 < t+|t|min <0. GeV2 Curves: fit to the data. Peak 11-1=0.197 ±0.030 Off Peak 11-1=0.189 ±0.024 • W ∝ sin2qhelicity-conserving processesare dominating. • 11-10.2N/(N+UN) ~70%

10. Natural-parity exchange Coherent  Photo-production from Deuteron DD • Beside the smallness of coupling constants g andgNN, can we further suppress the unnatural-parity exchange processes?  Study coherent production from deuterons, iso-scalar target, where iso-vector -exchange is forbidden. Unnatural-parity exchange We expect a strong dominance of natural-parity exchange processes in diffractive DD.

11. LEPS Run Summary Experimental detail is referred to the talk by M. Sumihama in this session.

12. Measurement of  Photo-production from Deuterons via K+K Decay Mode  DX

13. LD2 LH2 Disentangle Coherent and Incoherent Interactions in Missing Mass Spectra MMd(,KK) DX MX

14. Differential Cross Section of Coherent Interactions • The fitted b becomes smaller as we move to the large |t-tmind| region. • At |t-tmind|<0.1 GeV2, • b=20.52.1 GeV-2 Consistent with: b=b(F) + b(gp→ fp) =18.6+3.4=22 GeV-2

15. Intercept: d/dt at t=tmind Consistency between SLH2 and LLH2. Preliminary

16. Stronger asymmetry Decay Angular Distributions at |t-tmind|<0.1 GeV2 with MMd Cuts

17. Pure natural-parity exchange Preliminary Decay Asymmetry of Coherent Interactions Coherent interaction DD is mostly contributed by natural-parity exchanges.

18. Energy Dependence of Optical Points of DD P1: Pomerons0.08 ; P2: daughter Pomeron s-1.73 , inspired by (0+, M23 GeV2) Preliminary

19. Summary • The first measurement differential cross section and decay asymmetry of DD in the forward direction near threshold. Disentanglement of coherent and incoherent events is done by the fit in the missing mass spectra. • Large exponential slope about 20. • The intercept of d/dt at t=tmind increases with beam energy. • Close-to-one decay asymmetry for the coherent interaction with LD2 target: significant dominance of natural-parity exchange processes. Consistent with theoretical prediction based on the elimination of unnatural-parity -exchange. • Combining with the measurements at higher energies, the optical points of DD near threshold is consistent with the standard Pomeron exchange near the threshold assuming the same energy dependence.

20. Backup Slides

21. Consistent with the scenario: • not due to unnatural-parity processes ONLY. • possible presence of additional natural parity exchange • signature of 0+ glueball trajectory?? Peak and Off Peak

22. Coherent  Photoproduction from Deuteron Titov et al., PRC 66, 022202 (2002) • Deuteron form factor leads to a steeper exponential slope in t distribution. • In scattering amplitude, the unnatural-parity iso-vector  exchange is completely eliminated due to Tn= Tp. Decay asymmetrygets closer to +1.

23. Isospin Effect of Quasi-free  Photoproduction from Nucleons Due to isospin factor 3: • gpp and gpp are of the same sign: constructive interference between -exchange and -exchange. • gnn (= gpp )and gnn (=gpp ) are of opposite sign: destructiveinterference between -exchange and -exchange. • Value of decay symmetry gets closer to +1 in nn, compared with pp. Titov et al., PRC 59, R2993 (1999)

25. Measurements by CLAS (T. Mibe’s talk in APS meeting, 2006)

26. Stronger asymmetry LD2 |I0U|2=0 LH2 Preliminary Missing Mass Spectra MMd(,KK) and Decay Asymmetry DX MX