Production of Mesons on Protons near Threshold by Linearly Polarized Photon

at SPring-8/LEPS

Wen-Chen Chang for LEPS collaboration

Institute of Physics, Academia Sinica Taipei 11529, Taiwan

XVI Particles and Nuclei International ConferenceSeptember 30 - October 4, 2002, Osaka, JapanPaNic02

Outline

• Photo-production of mesons at high energy and near production threshold.– Pomeron exchange.– Pseduo-scaler particle exchange.

• How linearly polarized photon beam can help us in distinguishing different sources of contribution?– Spin density matrix element.– Decay asymmetry in Helicity Frame.

• Measurement by LEPS experiment at SPring-8.– Reconstruction of mesons in KK modes.– Angular distribution of K+ in Helicity Frame at forward direction.

• Summary.

Total Cross Section of Hadronic Interaction

XsYsT

Universal Scaling : = 0.5, = 0.08

A. Donnachie and P.V. Landshoff

Photo-Production of Mesons

at Forward Region (small |t|)

Pomeron Exchange Pseudo-scaler Exchange

(OZI Suppressed!)

Exotic knock-out process

Photo-Production of Mesons

at Backward region (large |t|)

Emission from Nucleon Exotic shake-off process

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)

– Successful phenomenology for diffractive process.

• Pseudo-scaler particle: – Negative power-law scaling of

s.

– Showing up at small energy.

– Un-natural parity (= –1).

– Exchange particles like ,.

– OZI suppressed.

Differential Cross section of p p

with un-polarized photon beams

A.I.Titov et. al., PRC 59, R2993

(t=0

)

With Linearly-Polarized Photon Beam

• Experimentally we measure W’s functions and density matrix

could be determined by spherical harmonics decomposition.

• The contributions of natural and un-natural parity exchange in the t-channel are represented by the matrix element of Re(0), Re(1).

),(cos2sin),(cos2cos

),(cos),,(cos21

0

HHHH

HHHHL

WPWP

WW

H: polar angle of K+ in Helicity frame H : azimuthal angle of K+ in Helicity frame: angle between the photon polarization vector and CMS production plane.

Spin Density Matrix Elements

011

000

2211

210

2111

110

2100

2111

2011

010

2000

000

21

)]2sinsinImsin2sinIm2(

2sin

)2cossincos2sin2cossin(

2cos

2cossincos2sinRe2

cos)13(2

1)1(

2

1[

4

3

),,(cos

HHHH

HHHHHH

HHHH

H

HHL

P

P

W

K.Schilling et. al., Nucl. Phys. B15 (1970) 397

The Spin Density Matrix Elements at small |t| regions

•Two important spin-conserving processes:•Pomeron exchange: 1

1-1= –Im 21-1 = +0.5, and all diagona

l ’s = 0 at =0., exchange: 1

1-1= –Im 21-1 = – 0.5, and all diagonal ’

s = 0 at =0.

A.I.Titov et. al., PRC60, 035205

.1when ,5.0 ;0when ,5.0

2

21

2

||||

1

111

111

2221

11

2

UN

UN

TT

TTT

Angular Distribution of K+ in Helicity Frame

=0, 11-1=+0.5 =1, 1

1-1= – 0.5

)))(2cos(21(4

3),( 1

11

HH PW

SLAC MeasurementJ.Ballam, et. al, PRD 7(1973), 3150

LEPS CollaborationRCNP, Osaka University, JapanH. Fujimura, M. Fujiwara, T. Hotta, H. Kohri, T. Matsumura, N. Matsuoka,T. Mibe, M. Morita, T.Nakano, T. YoritaOsaka University, JapanN. Nomachi, A. Sakaguchi, Y. Sugaya, M. SumihamaAcademia Sinica, TaiwanW.C. Chang, D.S. Oshuev, C.W. Wang, S.C. WangChiba University, JapanH. Kawai, T. Ooba, Y. ShiinoIHEP, RussiaP. ShaginJAERI, JapanY. Asano, N. Muramatsu, R.G.T. ZegersJASRI, JapanS. Date, N. Kumagai, Y. Ohashi,H. Ookuma

Konan University, JapanH. AkimuneKyoto University, JapanK. Imai, T. Ishikawa, M.Miyabe, M.Niiyamma ,M. YosoiNagoya University, JapanS. Fukui, T. Iwata, Y. Miyachi, A. WakaiOhio University, U.S.K. HicksPusan National University, S. KoreaJ.K. AhnSaskatchewan University, CanadaC. RangacharyuluTohoku University, JapanH. ShimizuWakayam Med. University, JapanS. Makino

LEPS detectors

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)

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-

+-

Signal of K+K

Reconstructed events

Large acceptance at forward angles ~5000 ’s (Dec,2000 - June, 2001) Runs with 150mm-long LH2 target (May-July, 2002 ).

KK event Kp event

Experimental Acceptance at -0.2< t < |t|min GeV2

2.2 < E < 2.4 GeV

Simulation at -0.2< t < |t|min GeV2 2.2 < E < 2.4 GeV

=0 =0.5 =1

2222 :)1(|:||| UNN TT

Decay angular distribution of K+ in rest frame (Helicity frame)

|M(KK)-M |< 10 MeV , |M((KK)X)-Mproton|< 30 MeV

-0.2< t < |t|min GeV2 , 2.2 < E < 2.4 GeV

w/o Acceptance Correction

Raw Data

Vertically polarized beamHorizontally polarized beam

Summary

• About 5000 events of photo-production of mesons by linearly polarized photons from E= 1.6GeV(threshold) to 2.4GeV are measured by LEPS experiment at SPring-8/Japan.

• In comparison with measurement at high energy, our preliminary results show that the contribution of natural-parity exchange still composes a larger fraction in the total scattering amplitude while that of un-natural parity component becomes more significant at E=2.3 GeV and t>-0.2 region.

• With the ongoing studies of the acceptance correction and systematic errors, the cross section, differential cross section and spin density matrix elements at the forward regions will be reported in the near future.

A presentation of detailed results will be shown in the poster session:“Polarization observables in the \phi(1020) meson photoproduction withlinearly polarized photons” by T. Mibe (RCNP, Osaka Univ.).

Decay Angular Distribution in p p

Example

Super Photon Ring 8 GeV (SPring-8)