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Measurement of polarization observables in
photoproduction with linearly polarized photons at
BL33LEP/SPring-8Spring-8 LEPS seminar
5th February, 2003
Tsutomu Mibe†‡
for the LEPS collaboration
† Research Center for Nuclear Physics‡ Advanced Science Research Center, JAERI
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Outline
Physics probed by photoproduction with linearly polarized photonsExperiment at LEPSCurrent status of data analysisFuture planSummary
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Regge theory• T. Regge Nuovo Cimento 14, 951 (1950)
– Soft processes in two-body elastic/quesi-elastic scattering
Exchange of a family of particles on Regge-trajectories
1 2
3 4
Total cross section 1)0(
0
))0(Im(1
s
stA
stotal
(t)=(0)+’(0)t
A simple power of s
J(=
(t))
1
2
3
M2(=-t)(GeV2)
Reggeon
1 3
Scattering amplitude
)(),( tstsA
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Total cross sections
• Donachie and Landshoff Phys.Lett. B296(1992)227
YsXstotal
Flavor blind
Additive-quark rule
trajectory =a(0)-1 = -0.4525Pomeron trajectory =a(0)-1 = 0.0808
)p()Kp(),pp()pp(
32)pp()p(
Pomeron Glueball ?s
Tot
al c
ross
sec
tion
(mb)
pp
pp
-p
+p
K-p
K+p
p
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Tensor glueball (Jpc=2++) candidate f2(1950) pp→pp
WA91, PLB 324 (1994)509
(2230) J/ radiative decayMark-III(SLAC),PRL56(1986)107
BES, PRL76(1996)3502
Lattice QCD: M(2++) = 2400 ±25±120 MeV (SDQCD) *
Pomeron trajectoryJ(
=
(t))
1
2
3
M2(=-t)(GeV2)1 3
(t)=1.08 + 0.25tScalar glueball (Jpc=0++) candidate f0(1500), f0(1710) Lattice QCD: M(0++)=
1730 ±50±80 MeV (SDQCD) *1622 ± 29 MeV (IBMQCD) **
Daughter trajectory for J=0 glueball ?
(t)=-0.75 + 0.25t
Lying on the Pomeron trajectory !
* Morningstar, PRD60(1999)034509** Weingarten, PLD60(1999)014015
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Vector Meson Photoproduction
• Vector Meson Dominance
• Meson Exchange
• Pomeron Exchange
N
N
(~ss)
q
_q
_qq =
Dominant at low energies
Slowly increasing with energyAlmost constant around threshold uud
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p p
p p
Vector Meson Photoproduction
M.A. Pichowsky and T.-S. H. LeePRD 56, 1644 (1997)
Prediction from Pomeron exchangePrediction from meson exchange
Data from: LAMP2('83), DESY('76), SLAC('73), CERN('82),FNAL('79,'82), ZEUS('95,'96)
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photoproduction near production threshold
P2: 2ndpomeron ~ 0+glueball
(Nakano, Toki (1998))
Titov, Lee, Toki Phys.Rev C59(1999) 2993
Data from: SLAC('73), Bonn(’74),DESY(’78)
Natural parity exchange
Unnatural parity exchange
1. Important to detinguish natural parity exchanges from unnatural ones2. Contribution of P2 depends on threshold behavior of P1
0
0
0
1P sas
ss
)s(T ???
P2: 2ndpomeron ~ 0+glueball
(Nakano, Toki (1998))
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Polarization observables with linearly polarized photon
Decay Plane // natural parity exchange (-1)J (Pomeron, Scalar mesons)
Polarizationvector of
K+
K+
K-
In meson rest frame
Decay Plane unnatural parity exchange -(-1)J
(Pseudoscalar mesons )
Relative contributions from natural, unnatural parity exchanges
Decay angular distribution of meson
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Decay angular distribution of meson
K+
K+
K-
p’
meson rest frame (Gottfried-Jackson(GJ) frame)
K+
K+
K-
polProduction
planez
Decayplane
z-axis
K+-pol
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Decay angular distribution
• W0,W1,W2 are parameterized by the 9 spin density matrix elements.
Re(
)
Im() andIm(
)
),(),,( 0 WW
),()2sin(),()2cos( 21 WPWP Unpolarized part
Polarized part
K.Schilling et al. Nucl. Phys. B15(1970) 408
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Helicity conserving amplitudes
Prediction (D+ps+N,N*) E = 2.2 GeV by A. Titov
|t|=
K+-pol K+-pol
Yie
ld(A
rbita
ry U
nit
)
Pomeron, 0+glueball, scalar meson (natural parity)pseudoscalar meson exchange (un-natural parity)
Pure naturalparity exchange
Pure unnaturalparity exchange
0 0
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Helicity flip amplitudes• Diffractive ‘soft’ Pomeron exchange
– Helicity is conserved
• Non-perturbative 2-gluon exchange– Different from ‘soft’ Pomeron exchange at larger angles– Helicity flip mechanism due to spin-orbital interaction (A. Titov)
P
p
p
+
W(cos) ≈ sin2
W(cos) ≈ 1 + b cos2
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Helicity flip amplitudes
Single spin-flip,000 ( →
|t|=
Double spin-flip,01-1 ( →
|t|=
0
0
0
1P sas
ss
)s(T
No helicity flip mechanism for the 0++ glueball and scalar m
eson trajectories
Helicity flip amplitudes may give an information on the threshold behavi
or of the Pomeron
Prediction (D+ps+N,N*) with TPomeron~(s/s0) E = 2.2 GeV (A. Titov)
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Published data in 1972
J. Ballam et al. PLD 7 (1972)3150
K+
- pol
(degree)
cos(K+)
53 events in E=2.8,4.8 GeV
“Natural-parity exchange in the t channel seems to be the major process.”
Precise measurementsnear threshold at
LEPS @Spring-8 (pol) CLAS @J-lab (unpol,pol)
SAPHIR@Bonn (unpol)
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The LEPS beamline
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Linearly polarized Photon
• Backward Compton scattering by using UV laser light• Intensity (typ.) : 2.5 * 106 cps• Tagging Region : 1.5 GeV< E < 2.4 GeV• Linear Polarization : 95 % at 2.4 GeV
E (Tagger) (GeV) E (GeV)
Co
unt
s
Lin
ear
pol
ariz
atio
n
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Charged particle spectrometer
1m
TOF wall
MWDC 2
MWDC 3
MWDC 1
Dipole Magnet (0.7 T)
Liquid Hydrogen Target50mm-long (2000 Dec.-2001June)150mm-long (2002May-July)
Start counter
Silicon VertexDetector
AerogelCerenkov(n=1.03)
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Summary of data taking
• Trigger condition : TAG*STA*AC*TOF• Run period
I (50mm-long LH2) 2000,Dec. – 2001, June
II(150mm-long LH2) 2002,May - 2002.July
• Total number of trigger 1.83*108 trigger (~50% Horizontal, ~50% Vertical
pol.)
• Number of events with charged tracks 4.37*107 events
Present analysis
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Event selections
• PID• Decay-in-flight cut• Vertex position cut• Invariant mass cut• Missing mass cut
Decay angular distribution of mesonin meson rest frame
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Charged particle identification
Mass(GeV)
Mo
men
tum
(G
eV
)
K/ separation (positive charge)
K++
Mass/Charge (GeV)
Eve
nts
Reconstructed mass
d
p
K+
K-
+-
(mass) = 30 MeV(typ.) for 1 GeV/c Kaon4 cut for K+/K-/proton PID
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Vertex distribution
z vertex (mm)
Eve
nts ToF start counter
LH2 target(50mm) Vacuum
Window
Vertex distribution (KK,Kp tracks)
z vertex (mm)
x ve
rtex
(m
m)
LH2 target : -1100 < z < -910 mmBG from target cell : ! (z<-960mm, x<-15 mm)
BG from target cell
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Reconstructed events (K+K- event)
Missing mass (,K+K-)X (GeV)
eve
nts
/2
.5M
eV
Proton(938)
=10 MeV
Invariant mass (K+K-) (GeV)
even
ts /2
.5M
eV
Inva
rian
t mas
s sq
uar
e (
K+K
- ) (G
eV2)
Invariant mass square (K-p) (GeV2)
Selections for event (KK mode) |M(KK)-M |< 10 MeV |MM((,K+K-)X)-Mproton|< 30 MeV
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Reconstructed events (K-p event)
Missing mass (,K-p)X (GeV)
eve
nts
/2
.5M
eV
K+(494)
Invariant mass (K+K-) (GeV)
even
ts /2
.5M
eV
Inva
rian
t mas
s sq
uar
e (
K+K
- ) (G
eV2)
Invariant mass square (K-p) (GeV2)
Backgroundfrom (1520)
Background study is underway.
Selections for event (Kp mode) |M(KK)-M |< 20 MeV |MM((,K-p)X)-MK|< 60 MeV
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Kinematical coverage for events
High acceptance at forward angles
~5000 ’s (2000,Dec-2001,June)
Golden region (High polarization, acceptance ~ flat)
2.2 < E < 2.4 GeV (P ~0.95)
-0.2 < t < |t|min
E (GeV)
t (G
eV
2)
t (G
eV
2)
E (GeV)
KK event Kp event
E (GeV)
t (G
eV2 )
Real data
Phase space(Monte Carlo)
Present analysis
High acceptance at forward angles ~5000 ’s (2000,Dec-2001,June) Present analysis
2.2 < E < 2.4 GeV (P ~0.95) -0.2 < t < -|t|min
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Geometrical acceptance
K+
(lab) (degree)
t (G
eV2 )
Present analysis
Monte Carlo
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cosK+ distribution in GJ frame
-0.2< t < -|t|min GeV2 , 2.2 < E < 2.4 GeV
w/o Acceptance CorrectionRaw data
cosK+
Num
ber
of
even
t
Dominance of spin conserving amplitudes
2sin)(cosW
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K+-pol distribution in GJ frame
-0.2< t < -|t|min GeV2 , 2.2 < E < 2.4 GeV
w/o Acceptance CorrectionRaw data
Num
ber
of
even
t/3
0 d
eg
. Horizontally polarized beam Vertically polarized beam
K+
- pol
(degree)
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Status at most forward angles(2.2<E<2.4,-0.2<t<|t|min)
• Major controbution from natural parity exchange– Contradiction with the model
which predicts large amount of meson exchange at W ≈ 2.3 GeV.
– Compensation by natural parity exchange processes (Pomeron, glueball, scalar mesons).
p p
p p
M.A. Pichowsky and T.-S. H. LeePRD 56, 1644 (1997)
Titov, Lee, Toki Phys.Rev C59(1999) 2993
W=2.3 GeV
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On-going analysis
• Acceptance studies• Decay angular distributions at larger |t|• Extraction of full spin density matrix
elements (Maximum likelihood fit)• Differential cross section• Analysis of other data set
– Long LH2 target run
– Nuclear target (Li, C, Al, Cu) run
– LD2 target run (Data taking underway)
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Statistical significance (short LH2 run)
• Estimate of statistical errors on spin density matrix elements
|t| (GeV2) |t| (GeV2)
E=2.3 GeV
w/ linearly polarized beam
Diff.+PS+N,N*(A. Titov)
SPring-8 (2.2<E<2.4 GeV)
JLAB
JLAB
w/ pol. or unpol. beam
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Statistical significance (short LH2 run)
• Estimate of statistical errors on differential cross section
d/d
t (b
/GeV
2 )
(degree)
E=2.0 GeV E=1.7 GeV
Bonn(1974) (E=2.0GeV)
SPring-8 (1.9<E<2.1 GeV)
SPring-8 (1.6<E<1.7 GeV)
JLAB (?)JLAB
Diff.+PS+N,N*(A. Titov)
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Summary
photoproduction at low energies provides unique information about Pomeron and exotic components (glueball or/and scalar meson trajectories).
~5000photoproduction events have been identified with linearly polarized photon beam from E= 1.6GeV(threshold) to 2.4GeV at LEPS/Spring-8.
An angular distribution of decay was studied at forward angles (-0.2 <t<-|t|min) .
The major contribution from spin conserving amplitudes, a larger fraction of natural-parity exchange were observed.