Coherent  -meson Photo-production from Deuterons Near Threshold Wen-Chen Chang Wen-Chen Chang for LEPS collaboration Institute of Physics, Academia Sinica, 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  D  D in the forward direction near threshold. Summary

Vector Meson Photoproduction from Protons  p  uud Pomeron Pomeron exchange  s 0.08  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 determine Pomeron contribution and possible new processes near threshold! 

Glueball Hunt in  Photoproduction T. Nakano and H. Toki (1997) P1: Pomeron  s 0.08 ; P2: daughter Pomeron  s -1.73, inspired by (0+ glueball, M 2  3 GeV 2 ) 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+K+ K-K- Decay Plane   unnatural parity exchange -(- 1) J (Pseudoscalar mesons  )    →K + K - K-K-

Decay Asymmetry  Im  Azimuthal angle distribution of : For spin-conserving processes: Pomeron exchange:  = –Im  = +0.5,   Im  = +1 ,  exchanges:  = –Im  = – 0.5,   Im  =  1

d  /dt at t=t min of  p  p PeakOff Peak LEPS(2005)

Decay Angular Distributions of  p  p Curves: fit to the data. W ∝ sin 2   helicity-conserving processes are dominating.   0.2  N/(N+UN) ~70% Forward angles; -0.2 < t+|t| min <0. GeV 2  =0.197 ±0.030  =0.189 ±0.024 Peak Off Peak

Coherent  Photo-production from Deuteron  D  D Beside the smallness of coupling constants g  and g  NN, 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 exchangeNatural-parity exchange We expect a strong dominance of natural-parity exchange processes in diffractive  D  D.

LEPS Run Summary PeriodTargetDetectorsIntegrated Flux Dec Jun. 2001LH2 50mmSpectrometer5x NucleiSpectrometer + Gamma detector May 2002-Apr. 2003LH2 150mmSpectrometer1.4x10 12 Oct Jun. 2003LD2 150mmSpectrometer2x NucleiSpectrometer + Gas Cherenkov 2004C/CH2/CuTPC 2005C/CH2/CuTPC + 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   D  X

Disentangle Coherent and Incoherent Interactions in Missing Mass Spectra MM d ( ,KK) LD2 LH2  D  X MXMX

Differential Cross Section of Coherent Interactions The fitted b becomes smaller as we move to the large |t-t min d | region. At |t-t min d |<0.1 GeV 2, b=20.5  2.1 GeV -2 Consistent with: b=b(F) + b(  p→  p) = =22 GeV -2

Intercept: d  /dt at t=t min d Preliminary Consistency between SLH2 and LLH2.

Decay Angular Distributions at |t-t min d |<0.1 GeV 2 with MMd Cuts Stronger asymmetry

Decay Asymmetry of Coherent Interactions Preliminary Pure natural-parity exchange Coherent interaction  D  D is mostly contributed by natural-parity exchanges.

Energy Dependence of Optical Points of  D  D Preliminary P1: Pomeron  s 0.08 ; P2: daughter Pomeron  s -1.73, inspired by (0+, M 2  3 GeV 2 )

Summary The first measurement differential cross section and decay asymmetry of  D  D 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=t min d increases with beam energy. –Close-to-one decay asymmetry for the coherent interaction with LD2 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  D  D 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 T n  =  T p . Decay asymmetry gets closer to +1. Titov et al., PRC 66, (2002)

Isospin Effect of Quasi-free  Photoproduction from Nucleons Due to isospin factor  3 : g  pp and g  pp are of the same sign: constructive interference between  - exchange and  -exchange. g  nn (= g  pp )and g  nn (=  g  pp ) are of opposite sign: destructive interference between  -exchange and  - exchange. Value of decay symmetry gets closer to +1 in  n  n, compared with  p  p. Titov et al., PRC 59, R2993 (1999)

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

Missing Mass Spectra MM d ( ,KK) and Decay Asymmetry LD2 LH2  D  X MXMX Stronger asymmetry Preliminary |I 0 U | 2 =0