Transcript $\phi$-meson Photoproduction By Using a Beam of Linearly

**PhD Dec. 2009 (CLAS Analysis Note)
OZI evading/respecting process

Okubo Zweig Iizuka rule:
interaction,
with
which can
be
In the strong
49.2 ± 0.6 %
34 ± 0.5 %
Experimentally this decay
mode is (15.3 ± 0.4) %
~84% of phi-meson decay is
OZI respecting
Processes
Yougseok Oh and H. C. Bhang, Phys. Rev. C 64 055207(2001)
Previous Measurements
J. Ballam, G.B. Chadwick et al., Phys. Rev. D 7 3150(1972).
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unpublished.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).
Experiments
-----------------------------------------Forward direction.
No central region (Low “t ”)
-0.28 < t < -0.12 GeV2
------------------------------------------Central region ( mid- to higher “t ” )
-1.2 < t < -0.24 GeV2
(Continuous Electron Beam Accelerator Facility)
Hall B
Hall C
Hall A
CLAS
PHOTON TAGGER
•
By conservation of energy:
Eγ=Ee-Ee’


There are 384 energy counter
and 61 timing counters.
The energy counters are 1/3
overlapping.
• 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 !
Φ-meson Photoproduction:
g8b experiment
Linearly Polarized Photon Beam
p(’,ϕ), K
Target: Hydrogen
Event Selection
Energy cut:
1 .68 to 1 .92 GeV
1 .88 to 2 .102 GeV
1.9 GeV CohEdge
2.1 GeV CohEdge
Φ-meson reconstruction was fitted by a Breit-Wigner convoluted with
a Gaussian + 2nd order polynomial (We fix ГΦ = 4.26 MeV from PDG)
PARA
Event Selection
PERP
1.9 GeV
Coh Edge
2.1 GeV
Coh Edge
Results @ 2.1 GeV
Coherent Edge
Parallel
Perpendicular
Results @ 1.9 GeV
Coherent Edge
Parallel
Perpendicular
Results @ 2.1 GeV
Coherent Edge
Parallel
Perpendicular
Results @ 1.9 GeV
Coherent Edge
Parallel
Perpendicular
The Decay Angular Distribution
Linearly polarization gives access to six more density matrix elements

Those are calculated in ϕ-meson rest frame (e.g. Helicity Frame)
Density Matrix Elements
IF VDM:
All density matrix elements should be equal to
except for two ρ11-1 and 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
• Knockout processes taking place?
• Interesting physics beyond VDM
:
Acceptance
SDMEs parametrization
Acceptance effects cancels out
tests for s-channel helicity conservation
is 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 s channel 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.
tests for t-channel helicity conservation
has the same value – within error bars – for both
• 1.7 < Eg < 1.9 GeV
and
• 1.9 < Eg < 2.1 GeV
averaged over the the datasets
(this means the probability of finding the phi-meson
with longitudinal polarization in the t channel 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.
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,
the parallel and perpendicular datasets
averaged over
• For 1.9 < Eg < 2.1 GeV,
averaged over
the parallel and perpendicular datasets – this SDME agrees
with the LEPS experiment* where
at low t
for 1.97 < Eg < 2.17 GeV
*Mibe et al., PRL 95, 182001 (2005)
(1.9 < Eg < 2.1 GeV)
(1.7 < Eg < 1.9 GeV)
(1.9 < Eg < 2.1 GeV)
(1.7 < Eg < 1.9 GeV)
• Nota Bene:
says
for natural-parity exchange
for unnatural-parity exchange
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.
 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
Event Selection
→
γ p → pϕ →pK+KMode: pK+(X) → pK+(K-)
1.9GeV CohEdge
2.1GeV CohEdge
Event Selection
i.e. PERP
1.9 GeV CohEdge
i.e.PARA
2.1 GeV CohEdge
π + background subtraction