Exotic state searches at SPring-8:
Evidence for Narrow S=+1 Baryon Resonance
Yuji Ohashi (SPring-8)
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Introduction
Experiment
Evidence for Q+
Results from other labs
Other exotics searches
Conclusion & Outlook
Xth Workshop on High Energy Spin Physics
SPIN03
September 17, 2003 @ JINR
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The LEPS collaboration
Research Center for Nuclear Physics, Osaka University
T. Nakano, D.S. Ahn, M. Fujiwara, T. Hotta, K. Kino,
H. Kohri, T. Matsumura, T. Mibe, A. Shimizu, M. Sumihama
Pusan National University
J.K. Ahn
Konan University
H. Akimune
Japan Atomic Energy Research Institute / SPring-8
Y. Asano, N. Muramatsu
Institute of Physics, Academia Sinica, Taiwan
W.C. Chang, T.H. Chang, D.S. Oshuev, C.W. Wang, S.C. Wang
Japan Synchrotron Radiation Research Institute (JASRI)
/ SPring-8
S. Date, H. Ejiri, N. Kumagai, Y. Ohashi, H. Ookuma,
H.Toyokawa, T. Yorita
Ohio University
K. Hicks
Kyoto University
K. Imai, M. Miyabe, M. Niiyama, T. Sasaki, M. Yosoi
Chiba University
H. Kawai, T. Ooba, Y. Shiino
Yamagata University
T. Iwata
Wakayama Medical University
S. Makino
Nagoya University
T. Fukui
Osaka University
H. Nakamura, M. Nomachi, A. Sakaguchi, Y. Sugaya,
University of Saskatchewan
C. Rangacharyulu
Institute for High Energy Physics (IHEP), Moscow
P. Shagin
Laboratory of Nuclear Science, Tohoku University
H. Shimizu, T. Ishikawa
University of Michigan
K. Yonehara
Michigan State University
R.G.T. Zegers
Seoul National University
H. Fujimura
Miyazaki University
T. Matsuda, Y. Toi
51 collaborators / 20 institutions
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Q+(Z+) Baryon
Q+(1530)
D. Diakonov, V. Petrov, and M. Polyakov,
Z. Phys. A 359 (1997) 305.
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Exotic: S=+1
Low mass: 1530 MeV
Narrow width: < 15 MeV
Jp=1/2+
M = [1890-180*Y] MeV
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Exotic S=+1 Baryon
NOTE ON THE S = + 1 BARYON SYSTEM
(PDG 1986; Phys. Lett. B170, 289)
The evidence for strangeness +1 baryon resonances was reviewed
in our 1976 edition,1 and more recently by Kelly2 and by Oades.3 Two
new partial-wave anaIyses4 have appeared since our 1984 edition.
Both claim that the P13 and perhaps other waves resonate.
However, the results permit no definite conclusion- the same story
heard for 15 years. The standards of proof must simply be much
more severe here than in a channel in which many resonances are
already known to exist. The general prejudice against baryons not
made of three quarks and the lack of any experimental activity in
this area make it likely that it will be another 15 years before the
issue is decided.
References
•1. Particle Data Group, Rev. Mod. Phys. 48, SI88 ( 1976).
•2. R.L. Kelly, in Proceedings of the Meeting on Exotic Resonances (Hiroshima, 1978), ed. I. Endo et al.
•3. G.C. Oades, in Low and Intermediate Energy Kaon-Nucleon Physics (1981), ed. E. Ferrari and G. Violini.
•4. K. Hashimoto, Phys. Rev. C29, 1377 (1984); and R.A. Arndt and L.D. Roper, Phys. Rev. D31, 2230 (1985).
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Possible Q+ Production Reactions
LEPS/SPring-8
CLAS/JLAB
Q+
Q+
Q+
Q+
DIANA/ITEP
KEK-PS/E522
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Laser Electron Photon facility at SPring-8
in operation since 2000
g
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SPring
Energy Spectrum
0.7
0.6
0.5
0.4
0.3
2.4 GeV
8 GeV
0.2
0.1
0
00
.5
11
.522
.5
3
Eg (GeV)
Intensity (Typ.) : ~3 * 106 photons/sec
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5
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LEPS detector
Aerogel
Cerenkov
(n=1.03)
TOF
wall
Dipole Magnet
(0.7 T)
Start counter
Liquid Hydrogen
Target (50mm thick)
g
Silicon Vertex
Detector
MWDC 1
MWDC 3
MWDC 2
1m
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LH2 Target
Start Counter
g
SSD
Drift
Chamber
Cerenkov
Detector
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Charged particle identification
K/p separation (positive charge)
Events
p- p+
p
K+
d
K-
Mass/Charge (GeV)
Momentum (GeV)
Reconstructed mass
p+
K+
Mass(GeV)
s(mass) = 30 MeV(typ.) for 1 GeV/c Kaon
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Summary of data taking
30 Hz for 800 kHz@tagger
•Total number of trigger
1.83*108 trigger
Charge veto
Target
(LH2)
Dec, 2000 to Jun, 2001
AC(n=1.03)
Start counter (SC)
Optimized for g p  f p  K+ K- p
Small distance between LH2 and SC
High index of AC
•Number of events with
reconstructed charged tracks
4.37*107 events
•About a half of events were
produced in SC
g p  K0 Q+  p+ p- K+ n
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Identification of Q+
g n → K- Q+ → K- K+ n
• K- missing mass gives Q+ mass
• K+K- missing mass gives n
Problems:
• no neutron target
•CH start counter
(n is part of C!)
• “background”
 fK+K(produced from n & p)
Non-resonant KK
L(1520)
Q +?
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Fermi motion correction
K+p- missing mass
L
K+ missing mass
S-
K+ missing mass (corrected)
L
K+ missing mass
LH2 target
Correction works
better when
longitudinal
Fermi momentum
is small.
Start counter (CH)
Test-case:
g n → K+ S-→K+ p- n
g p → K+ L→K+ p- p
K+ missing mass (corrected)
Correction: MMgK+ (corrected) = MMgK+- MMgK+p-+ Mn
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Proton-recoil cut
K+
tof
dc
dc
ssd
dc
 gnK+K-n no recoil proton (proton is a spectator)
 gpK+K-p slow recoil proton is present
 proton is too slow to be seen in full detector,
but might be seen in SSD vertex detector.
g
target
(SC)
Kp
dipole
Remove all events for which proton is detected in
SSD, or for which predicted nucleon track does not
hit SSD.
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Effectiveness of proton recoil cut
g p(n)K+K-p(n)
• select KK events from
start counter
• construct K+ missing mass
(corrected) plot
• apply p-recoil cut
MMcgK+ = MMgK+- MMgK+K-+ Mn
gp L(1520)K+
K-p
• reverse p-recoil cut
（proton is present)
No L(1520) peak in events with
a spectator proton.
The cut enhances g nK+K-n
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n(g,K-) missing mass
g n → K- Q+ → K- K+ n
• select KK events from SC
• apply KK invariant mass cut (f)
• apply KK missing mass cut
(0.9<MMkk<0.98)
• apply proton recoil cut
• construct K- missing mass plot
Background shape can be determined
from events from LH2 target using the
same cuts except for:
• Proton-recoil cut is removed
• L(1520) events are removed
(only from p)
Q+
Background?
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Q+ identification
Background level is estimated by a fit
in a mass region above 1.59 GeV.
M = 1.540.01 MeV
G < 25 MeV
Gaussian significance 4.6s
Assumption:
• Background is from non-resonant
K+K- production off the
neutron/nucleus
• … is nearly identical to non-resonant
K+K- production off the proton
background
Phys.Rev.Lett. 91 (2003) 012002
hep-ex/0301020
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Confirmation from other labs
DIANA/ITEP
K+ Xe  K0 p X
(K+ n K0 p)
M = 15392 MeV
G < 9 MeV
hep-ex/0304040
CLAS/JLAB
g d  p K+ K- n
M = 15425 MeV
G < 21 MeV
hep-ex/0307018
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Mysteries
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Why is it so light?
Why is the width so narrow?
Pentaquark or KN molecule?
Excited S=+1 states?
Is this really an I=0, Jp=1/2+ state?
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Theoretical activities
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Exotic baryon states in topological soliton models
Walliser, H ; Kopeliovich, V B, hep-ph/0304058
Interpretation of the Theta+ as an isotensor resonance with weakly decaying
partners
Capstick, Page, Roberts, hep-ph/0307019
Stable $uudd\bar s$ pentaquarks in the constituent quark model
Stancu, Fl ; Riska, D O, hep-ph/0307010
The Constituent Quark Model Revisited - Quark Masses, New Predictions for
Hadron Masses and KN Pentaquark
Karliner, Marek; Lipkin, Harry J, hep-ph/0307243
Pentaquark states in a chiral potential
Hosaka, Atsushi hep-ph/0307232
Group theory and the Pentaquark
Wybourne, B G, hep-ph/0307170
Diquarks and Exotic Spectroscopy
Jaffe, R L ; Wilczek, F, hep-ph/0307341
Understanding Pentaquark States in QCD
Zhu, Shi-Lin, hep-ph/0307345
The anticharmed exotic baryon Theta_c and its relatives
Karliner, Marek; Lipkin, Harry J hep-ph/0307343
Determining the $\Theta^+$ quantum numbers through the $K^+p\to
\pi^+K^+n$ reaction
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Hyodo, T ; Hosaka, A ; Oset, E nucl-th/0307105
Very recent results with proton target
M = 154042 MeV
G < 25 MeV
M = 153710 MeV
G < 32 MeV
CLAS/Jlab
hep-ex/0307088
SAPHIR/ELSA
hep-ex/0307083
Require cos qK* > 0.5
g
K*
K*(1430) and/or K0
p
Q+
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Unofficial results
g
K*
K*(1430) and/or K0
p
Q+
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Photoproducion by linearly polarized
photon
Helicity frame
p+
p+
Polarization
vector of g
Decay Plane // g
if natural parity
exchange (-1)J
KK-
Decay Plane
g
if unnatural parity exchange
-(-1)J (Pseudoscaler mesons)
gp  K*Q+
Eth = 2.65 GeV
4p detector
TPC
Decay Angular distribution
of K*
Decomposition of
• natural parity exchange
• unnatural parity exchange
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To determine Spin and Parity
• Polarize Q+ and measure the K+ direction
and the neutron spin.
• Double or triple polarization experiment?
Polarized target
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Other Exotics Searches
• f meson photoproduction w/ linearly
polarized g
2nd Pomeron / Glueball exchange?
• L(1405) production w/ linearly polarized g
hybrid?
• gA
p0p0A , s meson search
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(q=0 degree)
f photoproduction near threshold
Titov, Lee, Toki Phys.Rev C59(1999) 2993
Natural parity
exchange
Unnatural parity
exchange
Data from: SLAC('73),
Bonn(’74),DESY(’78)
P2: 2ndpomeron ~ 0+glueball
(Nakano, Toki (1998))
Important to distinguish
natural parity exchanges from unnatural ones
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f photoproduction with linearly
polarized photon
Number of event
-0.2< t < |t|min GeV2 , 2.2 < Eg < 2.4 GeV
w/o Acceptance Correction
W (q ) =
cos(qK+)
Helicity conserving amplitudes are dominant.
Vertically polarized beam
Number of event
Horizontally polarized beam
3
0
(sin 2 q +  00
(3 cos 2 q - 1))
8p
W (f , F) 
3
(1 + 2 Pg 11-1 cos( 2(f - F pol )))
4p
Major contribution from
natural-parity exchange
fK+ - Fpol (degree)
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Invariant mass of pS
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Conclusion & Outlook
• Observation of Narrow Peak in Missing
Mass of g n → K- X.
– Evidence for Narrow S=+1 baryon at LEPS at 1.54
GeV with a narrow width.
– Confirmation from other facilities (CLAS(d)/Jlab,
DIANA/ITEP, CLAS(p)/Jlab, SAPHIR/ELSA).
– Narrow S=+1 baryon state at 1.54 GeV is well
established.
– Further data taking with LD2 target finished at
LEPS and scheduled at CLAS.
• Next things to do.
– Determination of Spin and Parity. Is this really Q+?
– Other pentaquark resonances ? (S=+1 or not)
– More theoretical works including lattice are needed. 33
Conclusion & Outlook (cont.)
• f data favors soft Pomeron contribution
• For further experimental study
– p0p0 run will start tomorrow
– 4p Coverage .  A new TPC (Readout system
will be ready in a few month.)
– Photon energy upgrade (Max. 3 GeV) to study Q
(and L(1405)) in K*(892) photo-production. (Use
linearly polarized photons as a parity filter.)
– Measurement of a recoiled nucleon polarization
OR a Polarized target. (Technically the latter is
easier.)
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Experimental Setup
with TPC
Dipole Spectrometer
Solenoid
Time Projection
g
Chamber
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