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

Outline

Physics probed by photoproduction with linearly polarized photonsExperiment at LEPSCurrent status of data analysisFuture planSummary

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

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

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

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

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)

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))

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

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

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

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

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

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)

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)

The LEPS beamline

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

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)

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

Event selections

• PID• Decay-in-flight cut• Vertex position cut• Invariant mass cut• Missing mass cut

Decay angular distribution of mesonin meson rest frame

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

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

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

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

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

Geometrical acceptance

K+

(lab) (degree)

t (G

eV2 )

Present analysis

Monte Carlo

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

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)

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

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)

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

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)

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.