Coherent -meson Photo-production from Deuterons Near Threshold
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Transcript of Coherent -meson Photo-production from Deuterons Near Threshold
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