Coherent Coherent -meson Photo-production from -meson Photo-production from Deuterons Near ThresholdDeuterons Near Threshold

Wen-Chen ChangWen-Chen Chang for LEPS collaboration

Institute of Physics, Academia Sinica,Institute of Physics, Academia Sinica, Taiwan Taiwan

Outline

Features of photo-production of -mesons from protons.What we learned from the results of photo-production of -mesons from protons.Features of coherent production from deuterons.Measurement of differential cross section and decay asymmetry of DD in the forward direction near threshold.Summary

Vector Meson Photoproduction from Protons

p

uudPomeron

Pomeron exchange s0.08

p ・・・

Meson exchange s -0.45

Diffractive Photoproduction of (ss) Mesons

Pomeron: – Positive power-law scaling of s.

– Dominating at large energy.

– Natural parity (=+1).

– Exchange particles unknown; likely to be gluonic degree of freedom.

Pseudo-scalar particle: – Negative power-law scaling of s.

– Showing up at small energy.

– Un-natural parity (= –1).

– Exchange particles like ,, quark exchange.

– OZI suppressed

This fact makes the -photoproduction a unique process to determinePomeron contribution and possible new processes near threshold!

Glueball Hunt in PhotoproductionT. Nakano and H. Toki (1997)P1: Pomeron s0.08 ;P2: daughter Pomeron s-1.73 , inspired by (0+ glueball, M23 GeV2)

P1

P1+P2

Polarization Observables with Linearly Polarized Photon

Decay Plane // natural parity exchange (-1)J (Pomeron, 0+ glueball, scalar mesons)Photon Polarization

K+

K+

K-

Decay Plane unnatural parity exchange -(-1)J

(Pseudoscalar mesons )

→K+K-

K-

Decay Asymmetry 11-1 - Im 2

1-1

•Azimuthal angle distribution of :

•For spin-conserving processes:

•Pomeron exchange: 11-1= –Im 2

1-1 = +0.5, 1

1-1-Im 21-1 = +1

, exchanges: 11-1= –Im 2

1-1 = – 0.5, 1

1-1-Im 21-1 = 1

))](2cos(*)Im(*1[),( 211

111 HH PW

20

20

20

201

11211

111 ||||

||||

2

1 ;Im

UN

UN

II

II

H

d/dt at t=tmin of pp Peak Off Peak

LEPS(2005)

Decay Angular Distributions of pp Curves: fit to the data.

•W sin∝ 2 helicity-conserving processes are dominating.1

1-10.2N/(N+UN) ~70%

Forward angles; -0.2 < t+|t|min <0. GeV2

11-1=0.197 ±0.030

11-1=0.189 ±0.024

Peak

Off Peak

Coherent Photo-production from Deuteron DD

Beside the smallness of coupling constants g and gNN, can we further suppress the unnatural-parity exchange processes?

Study coherent production from deuterons, iso-scalar target, where iso-vector -exchange is forbidden.

Unnatural-parity exchange Natural-parity exchange

We expect a strong dominance of natural-parity exchange processes in diffractive DD.

LEPS Run SummaryPeriod Target Detectors Integrated Flux

Dec. 2000-Jun. 2001 LH2 50mm Spectrometer 5x1012

2001-2002 Nuclei Spectrometer +

Gamma detector

May 2002-Apr. 2003 LH2 150mm Spectrometer 1.4x1012

Oct. 2002-Jun. 2003 LD2 150mm Spectrometer 2x1012

2003 Nuclei Spectrometer +

Gas Cherenkov

2004 C/CH2/Cu TPC

2005 C/CH2/Cu TPC +

Spectrometer

Experimental detail is referred to the talk by M. Sumihama in this session.

Measurement of Photo-production from Deuterons via K+K Decay Mode

DX

Disentangle Coherent and Incoherent Interactions in Missing Mass Spectra MMd(,KK)

LD2

LH2

DX

MX

Differential Cross Section of Coherent Interactions

min||exp||min

ttbttdt

d

dt

d

•The fitted b becomes smaller as we move to the large |t-tmind| region.•At |t-tmind|<0.1 GeV2, b=20.52.1 GeV-2

Consistent with:b=b(F) + b(p→ p) =18.6+3.4=22 GeV-2

Intercept: d/dt at t=tmind

Preliminary

Consistency between SLH2 and LLH2.

Decay Angular Distributions at |t-tmind|<0.1 GeV2 with MMd Cuts

Stronger asymmetry

Decay Asymmetry of Coherent Interactions

Preliminary

Pure natural-parityexchange

Coherent interaction DD is mostly contributed by natural-parity exchanges.

Energy Dependence of Optical Points of DD

Preliminary

P1: Pomeron s0.08 ;P2: daughter Pomeron s-1.73 , inspired by (0+, M23 GeV2)

SummaryThe first measurement differential cross section and decay asymmetry of DD in the forward direction near threshold. Disentanglement of coherent and incoherent events is done by the fit in the missing mass spectra.– Large exponential slope about 20.– The intercept of d/dt at t=tmind increases with beam energy.– Close-to-one decay asymmetry for the coherent interaction with L

D2 target: significant dominance of natural-parity exchange processes. Consistent with theoretical prediction based on the elimination of unnatural-parity -exchange.

Combining with the measurements at higher energies, the optical points of DD near threshold is consistent with the standard Pomeron exchange near the threshold assuming the same energy dependence.

Backup Slides

Peak and Off Peak

Consistent with the scenario:• not due to unnatural-parity processes ONLY.• possible presence of additional natural parity exchange signature of 0+ glueball trajectory??

Coherent Photoproduction from Deuteron

Deuteron form factor leads to a steeper exponential slope in t distribution.

In scattering amplitude, the unnatural-parity iso-vector exchange is completely eliminated due to Tn

= ・Tp

. Decay asymmetry gets closer to +1.

Titov et al., PRC 66, 022202 (2002)

Isospin Effect of Quasi-free Photoproduction from NucleonsDue to isospin factor 3:

gpp and gpp are of the same sign: constructive interference between -exchange and -exchange.gnn (= gpp )and gnn (=・gpp ) are of opposite sign: destructive interference between -exchange and -exchange.Value of decay symmetry gets closer to +1 in nn, compared with pp. Titov et al., PRC 59, R2993 (1999)

Measurements by CLAS (T. Mibe’s talk in APS meeting, 2006)

Missing Mass Spectra MMd(,KK)

and Decay Asymmetry

LD2

LH2

DX

MX

Stronger asymmetry

20

20

20

202

111

11

211

111

||||

||||Im

))](2cos(*)Im(*1[

UN

UN

H

II

II

PW

Preliminary

|I|I00UU||22=0=0