Coherent Coherent -meson Photo-production from -meson Photo-production from Deuterons Near Threshold Deuterons Near Threshold Wen-Chen Chang Wen-Chen Chang for LEPS collaborat ion Institute of Physics, Academ Institute of Physics, Academ ia Sinica, Taiwan ia Sinica, Taiwan
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Coherent -meson Photo-production from Deuterons Near Threshold Wen-Chen Chang Wen-Chen Chang for LEPS collaboration Institute of Physics, Academia Sinica,
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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
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)