1. Measurement of Differential Cross Section of φ-meson Photoproduction From Deuterons near Threshold at SPring-8/LEPS
Advisors: Rurng-Sheng Guo
Wen-Chen Chang
Graduate: Po-Ju, Lin
2006/05/29, NKNU

2. Outline Introduction Experimental Apparatus Data Analysis
Results and Discussions
Summary

3. Introduction

4. The flow chart Physics Results Physics motivation Physics events
Detectors
Raw data
DAQ
Electronic devices
Data analysis
Physics Results

5. Cross section The cross section σ is defined as：
Density of final states, phase space
Transition matrix element, dynamical information contained.
And can be understand as：
Number of reactions per unit time
Beam particles per unit time × scattering centers per unit area

6. Cross section
The differential cross section measured in the experiment is referred to：
Differential cross section for a point-like scattering case.
Form factor
t = four-momentum transfer squared
q = three-momentum transfer
Form factor, which is the Fourier transform of the charge distribution, determines how the scattering rate is reduced from its value for a point-like scatting.
We measure the cross section experimentally and confront the result with the theoretical presications.

7. The Standard Model
The Standard Model is the most widely accepted approach describing the fundamental particles and the interaction between them. It consists of two major parts: the spin ½ fermions, and the integer-spin bosons.
Particles participating strong interaction
Force carriers
Constituents of Matter.

8. The hadrons
Under the framework of Quantum Chromo-dynamics, quarks interact with each other by exchanging gluons and are bonded to form hadrons.
(強子)
Consist of three quarks
(重子)
Consist of quark, anti-quark pair
(介子)

9. Intermediate-energy nuclear physics
The perturbative QCD (pQCD) approach which works well in the extremely high energy regime is not applicable in the relatively low energy region.
Modifications and Model construction emphasizing the most aspects of QCD need to be made and tested with experimental data.
GeV-photons are chosen to be our probe to investigate the property of strong interaction in intermediate-energy resion.

10. Vector meson photoproduction and vector meson dominance model
Vector-meson photoproduction with small four-momentum transfer squared has originally been described with vector meson dominance (VMD) model, which assumes that photons interact with hadrons by first changing into neutral vector mesons having the same quantum number with photon, such as ρ,ω, and φ.

11. φ-meson photoproduction
Why is φ- meson photoproduction of special interest?

12. Vector Meson Photoproduction
Pomeron exchange g p
r, w, φ
uud
Pomeron
Meson exchange (OZI suppressed for φ) g p
p,h,s・・・
r, w
Speculate new Pomeron trajectory
by 0+ glueball exchange ??
r, w, φ g p
uud
???

13. What’s the Pomeron
In the pre-QCD approach, Regge theory, particles exchanged are summed coherently to give the exchange of so-called Regge trajectories.
Small-angle scattering is characterized by the exchange of the Pomeron trajectory which dominates in the high total energy region

14. Differential Cross section of  p  φ p
A.I.Titov et. al.
PRC 59, R2993 (1999)
(t=0)
M.A. Pichowsky and T.-S. H. Lee
PRD 56, 1644 (1997)
gp fp
gp rp
Pomeron
Meson

15. OZI rule
Observed by Okubo, Zweig, and Iizuka：If the Feynman diagram expressing a specific reaction can be cut in two by slicing only gluon lines, the process is suppressed.
Suppressed
Allowed

16. Photoproduction of φ-mesons at forward region
Pomeron:
Positive power-law scaling of s.
Dominating at large energy.
Natural parity (=+1).
Exchange particles unknown; likely to be glueball : P1(J=2+), P2 (J=0+, negative power-law scaling of s, Ref: T. Nakano and H.Toki, 1998)
Pseudo-scalar particle:
Negative power-law scaling of s.
Showing up at small energy.
Un-natural parity (= –1).
Exchange particles like ,.
OZI suppressed
This fact makes the -photoproduction an unique process to determine
Pomeron contribution and possible new trajectories near threshold!

17. Φ-photoproduction using LD2 target.γ φ d p n γ φ d
Incoherent Interaction：
Deuteron breaks up to be a proton and a neutron in the final state.
Coherent Interaction：
Deuterons remains intact.
 + d  φ + pn
 + d  φ + d

18. Isospin
Isospin is a symmetry of the strong interaction introduced by Heisenberg as it applies to the interactions of the proton and neutron.
this symmetry was in certain respects similar to the mathematical formulation of spin, the proton and the neutron are therefore assign to a doublet:
p：isospin ½ with I3=1/2
n：isospin ½ with I3=-1/2
In the framework of the Standard Model, the isospin symmetry of the proton and neutron are reinterpreted as the isospin symmetry of the up and down quarks.

19. Coherent production Conjecture： Pseudo-scalar Exchange
Primary components of meson-exchange channel：π, η
Pseudo-scalar Exchange
 : isoscalar
D : isoscalar
η: isoscalar
Allowed
π: isovector
Forbidden
Conjecture：
By isospin conservation we should be able to extract isovector part (π) from the whole exchanging channels.

20. Isotopic effect in incoherent interaction
LH2
The destructive inteference would occur between π and η exchange in the reaction  n  φ n
Isotopic effect can be investigated by comparing with results from proton target experiments.

21. Previous experiment New production mechanism near threshold?
Solid curve :
Pomeron + Pseudo scalar exchange by A. Titov, et al.
(PRC 67 (2003), )
New production mechanism
near threshold?

22. Previous experiment
There was only one measurement of deuteron φ photoproduction at = GeV and no separation of in coherent and incoherent contribution was made.

23. Experimental Apparatus

24. Super Photon Ring at 8 Gev (SPring - 8)

25. SPring-8 beamline map

26. Backward-Compton scattering
By shooting a few eV photons to 8 GeV electrons, photons with maximum energy 2.4 GeV are produced.

27. LEPS beamline

28. The laser system Ar ion laser (MLUV, 5W) Polarization rotator
Focusing lens

29. Eγ collision in storage ring
Tagging
counter
Bending magnet
Straight section g e’
Laser
e- (8GeV)

30. The charged particle spectrometer
Dipole Magnet (0.7 T)
TOF wall
Start counter
Target 160mm-long g
Aerogel
Cerenkov
(n=1.03)
MWDC 3
Silicon Vertex
Detector
MWDC 1
MWDC 2 1m

31. Particle identification
β = particle velocity
p = particle momentum
Momentum
Path of flight
Time of Flight
Length
Time
Velocity
Mass

32. Data Analysis

33. Data samples target runs r23690 (2002.05.23) – r24058 (2002.07.09)

34. Invariant mass and missing mass
This study focus on the reaction,  d  φ X, followed by the φ K+K-
Invariant mass：reconstruct the mass of the mother particle by the detectable decay produced daughters.
Missing mass：identify the undetected particle by all the other particles relating to the reaction

35. Disentanglement by MMd
Coherent interaction
Clear peak around GeV/c2 is shown in LD2 events, but absent in LH2 one.
Coherent events and incoherent events can be disentangled by fitting the MMd distribution with Monte-Carlo-simulated events

36. Results and Discussions

37. Disentanglement of Coherent and Incoherent events
Fitting of MMd of LD2 real data using Mote-Carlo generated incoherent, coherent events.
coherent
incoherent
sum

38. The Acceptance LD2 coherent LD2 incoherent LH2
Acceptance obtained by Monte Carlo simulation：
Implemented t-distribution：

39. Fitting of t distribution
LD2 coherent
LD2 incoherent
LH2
Fitting Formula：
intercept
slope

40. The slope parameter
The trend of steady slope distribution is observed in three different reaction machenisms.
The averaged slope parameters：
Data set
b (GeV-2)
χ2/ndf
LD2 coherent
± 0.830
2.2215
LD2 incoherent
4.136 ± 0.136
1.8940
LH2
3.619 ± 0.192
1.6520
LD2 coherent

41. The slope parameter
LD2 incoherent
LH2

42. Differential cross section
N0 need to be background-subtracted to obtain
The differential cross section can than be deduced:
LD2 coherent

43. Differential cross section
LD2 incoherent
LH2

44. Discussions No strong energy dependence is shown.
Slope of LD2 coherent events is significantly higher than the LD2 incoherent and LH2 events. This can be understood by the form factor of deuteron.
Coherent
Incoherent
LH2

45. Discussions
Constantly increase with photon energy in coherent interaction
Clear isotopic effect is seen in the LD2 incoherent events.
Peak structure is seen in LD2 incoherent and LH2 events.
Coherent
Incoherent
LH2
Previous SLH2 exp.

46. Discussions
Differential cross section of LD2 coherent events corrected with form factor comparing with the results with LH2 events.
Preliminary
No dropping with decreasing photon energy is seen.
：LH2
：Corrected coherent
：Previous SLH2 exp.

47. Summary

48. Summary LD2 coherent events
Differential cross section of the photoproduction of φmeson from deuterium target has been studied and compared with the results from hydrogen target in the energy range from the production threshold to Eγ=2.4 GeV.
By fitting the MMd spectra with those of Monte-Carlo-simulated coherent and incoherent events, disentanglement was achieved.
No strong energy dependence was observed in three different reaction mechanisms.
LD2 coherent events
The LD2 coherent differential cross section was observed to have a large t-slope. And the differential cross section at t=-|t|min shows a constantly increase with photon energy.

49. Summary LD2 incoherent and LH2 events
Consistent exponential slope were obtained for the differential cross section of LD2 and LH2 events.
Strong Isotopic effect of amplitude interference is shown.
Peaking structure around Eγ=2.2 GeV observed in the previous LEPS analysis is seen.
Together with the study on decay symmetry, the presence of the peaking structure may be interpreted as the manifestation of additional natural-parity exchange process, such as the daughter Pomeron trajectory.

50. Summary Thank you for your paying attention Future works
Comparison of the predicted differential cross section of  p  φ p from Pomeron exchange and that of the LD2 coherent interaction can be proceeded to discriminate the possible natural-parity contribution.
By using polarized target, more informative observables can be obtained.
New measurement at higher energies is crucial to establish the non-monotonical-increase structure of the cross section near threshold.
Thank you for your paying attention

51. Backup slide