Polarization observables in the photoproduction of Φ mesons with linearly polarized photons

1. NSTAR 2011 Newport News, VA May 19, 2011 Polarization observables in the photoproduction of Φ mesons with linearly polarized photons Julian Salamanca* Philip Cole + The CLAS Collaboration **PhD Dec. 2009 (CLAS Analysis Note)

2. OZI evading/respecting process • Okubo Zweig Iizuka rule:In the strong interaction, processes with final state particles which can only be reachedthrough quark-antiquark annihilationwill be suppressed. 49.2 ± 0.6 % 34 ± 0.5 % Experimentally this decay mode is (15.3 ± 0.4) % ~84% of phi-meson decay is OZI respecting

3. Processes • Yougseok Oh and H. C. Bhang, Phys. Rev. C 64 055207(2001)

4. Previous Measurements J. Ballam, G.B. Chadwick et al., Phys. Rev. D 7 3150(1972).

5. Previous Measurements Spring-8 used a beam of linearly polarized photons (|t|<0.4 GeV2) • T. Mibe, “Measurement of  meson photoproduction near production threshold with linearly polarized photons,” PhD Thesis, Osaka University, Japan (2004), unpublished. • T. Mibe et al., Phys. Rev. Lett. 95, 182001 (2005).

6. Experiments SLAC: ~53 Events ------------------------------------------ Forward direction. No central region (Low “t”) -0.28 < t < -0.12 GeV2 Spring-8: ~5000 Events ------------------------------------------- Central region ( mid- to higher “t” ) -1.2 < t < -0.24 GeV2 g8b: ~8200 Events

7. CEBAF(Continuous Electron Beam Accelerator Facility) Hall B Hall C Hall A

8. CLAS

9. Coherent Bremsstrahlung Facility

10. PHOTON TAGGER • There are 384 energy counter and 61 timing counters. • The energy counters are 1/3 overlapping. • By conservation of energy: Eγ=Ee-Ee’ • This means we have effectivity doubled the number of channels to ~ 768 energy bins Because of hodoscope spans the energy region 0.20 ≤ Ee’ ≤ 0.95Ee implies Photon energy resolution is 0.1% of the incident electron energy, Ee !

11. Φ-meson Photoproduction:g8b experiment Linearly Polarized Photon Beam p(’,ϕ), K Target: Hydrogen

12. Event Selection Energy cut: 1 .68 to 1 .92 GeV 1 .88 to 2 .102 GeV 1.9 GeV CohEdge 2.1 GeV CohEdge

13. Φ-meson reconstruction was fitted by a Breit-Wigner convoluted with a Gaussian + 2nd order polynomial (We fix ГΦ = 4.26 MeV from PDG) Event Selection PARA PERP 1.9 GeV Coh Edge 2.1 GeV Coh Edge

16. HELICITY FRAME Results @ 2.1 GeV Coherent Edge Results @ 1.9 GeV Coherent Edge Parallel Perpendicular Parallel Perpendicular

17. GOTTFRIED-JACKSON FRAME Results @ 2.1 GeV Coherent Edge Results @ 1.9 GeV Coherent Edge Parallel Perpendicular Parallel Perpendicular

18. The Decay Angular Distribution • Those are calculated in ϕ-meson rest frame (e.g. Helicity Frame) Linearly polarization gives access to six more density matrix elements

19. Density Matrix Elements IF VDM: All density matrix elements should be equal to ZERO except for two ( ρ11-1and Im{ρ21-1} ) • ρ11-1, Im{ρ21-1} = (1/2, -1/2 : Pomeron) • ρ11-1, Im{ρ21-1} = (-1/2, 1/2 : Meson) If any of the other SDMEs are nonzero: • Knockout processes taking place? • Interesting physics beyond VDM

21. SDMEs parametrization Acceptance Acceptance effects cancels out

22. From the Angular Distributions in the rest frame of the phi-meson we can extract:SPIN DENSITY MATRIX ELEMENTS

23. SPIN DENSITY MATRIX ELEMENTS

24. Helicity Frame* tests for s-channel helicity conservation • ρ000is sensitive to single helicity-flip transitions, and we find that this SDME is small for both • 1.7 < Eg < 1.9 GeV • 1.9 < Eg < 2.1 GeV (this means the probability of finding the phi-meson with longitudinal polarization photoproduced by a transversely polarized photon in the schannel is small). *Quantization axis is opposite the recoil proton in the cms of the proton and phi meson. We then boost along this quantization axis to the rest frame of the phi meson. The Kaon angular distributions are measured in this frame.

25. Gottfried-Jackson Frame* tests for t-channel helicity conservation • ρ000has the same value – within error bars – for both • 1.7 < Eg < 1.9 GeV and • 1.9 < Eg < 2.1 GeV • <ρ000> = 0.3875 ± 0.061 averaged over the the datasets (this means the probability of finding the phi-meson with longitudinal polarization in the tchannel is high). *Quantization axis is in the direction of the photon’s travel. We then boost along this quantization axis to the rest frame of the phi meson. The Kaon angular distributions are measured in this frame.

26. Gottfried-Jackson Frame • ρ01-1 < 0.1 (but nonzero), which suggests a small contribution from non-helicity conserving processes. Perhaps from a possible f2’ exchange as pointed out by A.I. Titov and T.-S.H. Lee, PRC 67, 065205 (2003). • For 1.7 < Eg< 1.9 GeV, ρ3 = 0.100 ± 0.057 averaged over the parallel and perpendicular datasets • For 1.9 < Eg< 2.1 GeV, ρ3 = 0.134 ± 0.053 averaged over the parallel and perpendicular datasets – this SDME agrees with the LEPS experiment* where ρ3 = 0.19 ± 0.03 at low t for 1.97 < Eg< 2.17 GeV *Mibe et al., PRL 95, 182001 (2005)

27. Gottfried-Jackson Frame • ρ11-1 = 0.206 ± 0.039 (1.9 < Eg< 2.1 GeV) • ρ11-1 = 0.102 ± 0.036 (1.7 < Eg< 1.9 GeV) • Im{ρ21-1} = 0.064 ± 0.039 (1.9 < Eg< 2.1 GeV) • Im{ρ21-1} ≈ −ρ11-1(1.7 < Eg< 1.9 GeV) • Nota Bene: Vector Meson Dominance says • −Im{ρ21-1} = ρ11-1 = +0.5 for natural-parity exchange • −Im{ρ21-1} = ρ11-1 = −0.5 for unnatural-parity exchange

28. Remarks • Natural-parity exchange mechanisms are larger than those for unnatural-parity exchange in both the s- and t-channels. • The relative contributions for natural- and unnatural-exchange processes are energy dependent in the t-channel and not in the s-channel • A small oscillation in W(ϕ) suggests a small pseudoscalar-meson-exchange component and that no pomeron exchange is present in the t channel.

29. More Remarks…. • Polarization ~70% • Over 8200 ϕ-meson events were analyzed. Highest ever at threshold with linearly polarized photons • Spin Density Matrix Elements (SDMEs) were extracted and are found to have non-VDM values. • Φ-meson azimuthal distribution has a considerable amplitude which suggests that one or more additional mechanisms beyond VDM are involved. **This work was made possible through a grant from the National Science Foundation, NSF-0555497

30. Thanks

31. Event Selection → γ p → pϕ →pK+K- Mode: pK+(X) → pK+(K-) 1.9GeV CohEdge 2.1GeV CohEdge

32. Event Selection π + background subtraction i.e. PERP 1.9 GeV CohEdge i.e.PARA 2.1 GeV CohEdge