EXOTIC STATE SEARCHES AT THE SPRING-8:
OBSERVATION OF A PENTAQUARK STATE Θ+ BARYON†
Yuji Ohashi on behalf of the LEPS collaboration
SPring-8
Japan Synchrotron Radiation Research InstituteMikazuki, Sayo, Hyogo, 679-5198, Japan
E-mail: ohashi@spring8.or.jp

Abstract
A narrow resonance peak was observed at 1.54±0.01 GeV/c2 as an exotic baryon
with strangeness S = +1 in the γ + n → K + + K − + n reaction on 12 C. The newstate have a width smaller than 0.025 GeV/c2 and a Gaussian significance of 4.6σ.
It can not be interpreted as three quark state and is consistent with the lightest
member of an antidecuplet of baryons predicted by the chiral soliton model.

INTRODUCTION
The constituent quark model has been remarkably successful to describe most of theexperimentally observed mesons and baryons. Beyond conventional hadrons, exotic statessuch as glueballs, hybrid mesons, and other multi-quark states are not prohibited in QCD.
There is a different approach by the Skyrme model [1] that is originated in the same QCD.
In this direction, following the pioneering works [2, 3], Diakonov, Petrov and Polyakov(DPP) proposed a possible existence of the S = +1 J P = 1/2+ resonance at 1.53 GeV/c2
with a width less than 0.015 GeV/c2 using the chiral soliton model [4]. The Θ+ (originallydenoted the Z + ) is the lightest member of the anti-decuplet which appears as the third
rotational excitation states. Their model simply assumes the relatively well establishednucleon resonance N (1710, 1/2+ ) as a nucleon-like member of the anti-decuplet. The mass
formula for the members of the antidecuplet is proposed as
M = [1.89 − Y × 0.18 ] GeV /c2(1)

where Y is a hypercharge.
Motivated by the resonance-like structure appeared around K + momentum of 1 GeV/cin both K + p and K + n total cross-sections, experimental studies on the K + N reactions
started from 1960’s. It is to be noted that most of the searches were performed in themass range between 1.74 and 2.16 GeV/c2 . Although the possible Z ∗ baryon resonanceswere reviewed in the 1986 baryon listings by the PDG [5], evidence of their existence wasconcluded as poor.
†

Invited talk at the Xth Advanced Research Workshop on High Energy Spin Physics (SPIN-03),
Dubna, 16-20 September, 2003

1

One of our experimental goal is to search various kinds of exotic states through photonuclear reactions. The present experiment was motivated in part by the DPP prediction
mentioned above, and the experimental result was published in the article [6]. We describe
this paper including some issues that were not mentioned at the workshop or appeared
afterward.

EXPERIMENTAL METHOD

dσ/dEγ (b/GeV)

The experiment was carried out at theLaser-Electron Photon facility at the SPring0.45
8 (LEPS) [7]. Photon beam is produced byElectron energy Ee = 8GeV
Compton scattering of laser photons on the
0.4
275 nmcirculating 8 GeV electrons and beam energy
351 nm0.35
488 nmis tagged by analyzing scattered electron mo0.3
mentum. The maximum photon energy is de0.25
termined by the wave length of the laser, and
0.2
it is 2.4 GeV when a 351 nm Ar laser is used.
0.15
Energy spectrum of a photon beam is rel0.1
atively flat as shown in Fig. 1 and photons
are highly polarized when laser photons are
0.05
polarized. Un-collimated beam size in stan0
0
0.5
1
1.5
2
2.5
3
3.5
Eγ (GeV)dard deviation(σ) is 3 mm vertcally and 8 mmhorizontally at the target position about 70 Figure 1. Energy spectrum of a Laserm downstream of the Compton scattering re- Electron Photon beam.
gion. Typical tagged photon flux above 1.5
GeV was ∼ 1 × 106 per second.
Experimental setup was designed for the study of φ meson
production from protons. Photons hit a liquid hydrogen target (LH2 ) of 5 cm thick after
passing through a charged particle veto counter (VC). The LEPS
detector consisted of a start trigger counter (SC), a silica aerogelČerenkov counter (AC), a siliconstrip vertex detector (SSD), an
upstream drift chamber, a 0.7T
dipole magnet, two sets of downstream drift chambers, and a timeof-flight scintillator array(TOF) as
shown in Fig. 2. The detector acceptance covered forward angle of Figure 2. The LEPS detector setup.
±0.4 and ±0.2 rad in the horizontal and vertical directions, respectively. Typical detector resolutions in σ for tagged Eγ , momentum, time-of-flight, and2

Figure 3: The Mass distribution.
mass (momentum dependent) were 0.015 GeV/c2 , 0.006 GeV/c at 1 GeV/c, 150 psec
for a flight length of ∼ 4 m from the target to the TOF, and 0.01 GeV/c2 for 1 GeV/ckaon, respectively. Particle identification was applied within 3 σ in this analysis, and its
effectiveness can be viewed in the mass spectrum in Fig. 3.
e+ e− pairs produced in the very forward angles were blocked by horizontal lead bars
which were set in the median plane inside the magnet gap. The trigger simply required
a hadron event by applying a tagging counter hit, no charged particle before the target,
charged particles after the target, no signal in the AC, and at least one hit on the TOF
wall. AC fires for electrons and positrons, and for pions with momentum higher than 0.6
GeV/c by the choice of a refractive index of 1.03. A typical trigger rate was about 20
events per second. The data taking was carried out during December, 2000 to June, 2001.

DATA ANALYSIS
From the total amount of 4.3 × 107 triggers,
8.0 × 103 events with a K + K − pair were reconstructed. Data reduction was carried out in thefollowing scheme.

3

SC

1000

Events/2.0 mm(1) Vertex position cut
The SC, which was used as a neutron target
for the search of Θ+ , was a 0.5 cm thick plastic
scintillator (polystyrene) and was placed at 9.5 cm
downstream of LH2 . K + K − pair events from the
SC were identified by selecting vertex position along
beam direction. Events that originate from a reaction in the SC were clearly seen in Fig. 4. Eventsfrom maximum photon energy region above 2.35
GeV were omitted in consideration of increasing
background contribution to the signal with increasing energy. These cuts reduced the data sample to
3.2 × 103 K + K − events.
(2) Missing mass cut

1200

800

600

400

LH2
200

0
-1050-1025-1000 -975 -950 -925 -900 -875 -850 -825 -800

K+K- vtz position (mm)Figure 4. Vertex position for K + K −events along the photon beam direction. Arrows indicate the cut position
for SC or LH2 events. SC events wereused for exotic state search.

+MM(γ,K ) (GeV)

Events/0.005 GeV

In the reaction γN → K + K − X, MX was
700
required to be nucleon and the rest of car600
bon nucleus be spectator. The missing mass
M MγK + K − in the above reaction was calculated
500
by assuming a qusi-free γN scattering. A to400
tal of 1.8 × 103 events survived after applying
300
0.90 < M MγK + K − < 0.98 GeV/c2 .
200(3) φ meson cut+ −Most of the remaining K K events ( 85%),100
which were from φ meson photoproduction and
00.95
11.05 1.1 1.15 1.2 1.25 1.3
were uninteresting events in this report, were cut+ K K Invariant Mass (GeV)
by applying 1.00 < M (K + K − ) < 1.04 GeV/c2
as in Fig. 5.
Figure 5. Invariant K + K − mass distribu(4) Proton cut
tions for the SC events. φ meson contriK + K − events, of which reactions were orig- butions were cut for exotic state search as
inated from protons, were eliminated by requir- indicated by arrows.
ing SSD hit. The direction and momentum of
the nucleon in the final state was calculated from the K + and K − momenta. Events were
rejected if the recoiled nucleon was out of the SSD acceptance or was the momentum
smaller than 0.35 GeV/c since an uncertainty in the calculation becomes large for low
momentum case. 108 events were eliminated by requiring that the hit position in the SSD
agreed with the expected hit position within 45 mm. This distance was chosen to take theFermi motion in 12 C into account. A total of 109 events satisfied all the selection criteria(”signal sample”).
A raw missing mass calculation done above
1.3
was obtained without considering the Fermi motion in 12 C. To evaluate this effect, well known
1.25
process γn → K + Σ− → K + π − n was studied.
Clear correlations are seen in the missing mass
1.2
plots Fig. 6, where M MγK + and M MγK + π−
1.15
were obtained for the γN → K + X and γN →+ −K π N channels by assuming the nucleons at
1.1
rest. This is understood by the fact that thenucleons in the two channels are identical. Con1.05
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98
sidering the above missing mass correlation and+ MM(γ,K K ) (GeV)
setting the nucleon mass to MN , the Fermi moction corrected missing mass M MγK+ is obtained
Figure 6. The scatter plot of M MγK + vs.
as+ −M MγK + π− for K π(2) in the SC.

c
M MγK+ = M MγK + − M MγK + π − + MN .

photoproductions

Eq. (2) compensates not only the Fermi motion but also the experimental resolutions andthe binding energy of the nucleon in 12 C. The Fermi motion effect is the major contribution
in the analysis. Although there is only a smeared distribution in the raw M MγK + (dashedline in Fig.7), the Λ and the Σ− peaks are separated after the correction (solid line in
Fig.7). Note that this correction is not a good approximation for events with M MγK + π−
far from the nucleon rest mass, and is good in case of a decay with a small Q value. The
4

45
40

Σ−

Λ

Events/(0.01 GeV/c2)

35
30
25
20
15
10
5
0
1.051.075 1.1 1.1251.151.175 1.2 1.2251.251.275 1.3c +MMγK(GeV/c2)c+ −Figure 7: The corrected missing mass distribution M MγKevents+ [Eq.(2)] for the K π+ −from the SC (solid line) and for Monte Carlo events for the γn → K Σ channel (dottedcurve). The dashed line shows the missing mass distribution M MγK + which was obtained
from the projection of the events in Fig. 6 on to the vertical axis without the Fermi-motion
correction.

latter effect is seen in the small width for the Λ and the large width for the Σ− becauseof the imperfection of the correction due to a large Q value for the Σ− .
c
+ −The corrected missing mass M MγKevents is given as± for K Kc
M MγK
± = M MγK ± − M MγK + K − + MN .

(3)

Events/0.02 GeV

cFig. 8 shows no obvious peaks in a M MγK+ distribution of the signal sample (109
events) that satisfied all the selection criteria (dotted line), and shows a clear peak due tothe γp → K + Λ(1520) → K + K − p process in the 108 events that agreed the proton hit in
the SSD (solid line). Effectiveness of a proton cut is verified that the Λ(1520) peak does
not show up in the dotted line plot.

22.5
20
17.5
15
12.5
10
7.5
5
2.5
0

1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8+MM(γ,K ) (GeV)c+ −Figure 8: The corrected missing mass distribution M MγKpro+ [Eq.(3)] for the K K
ductions for the signal sample (dotted line) and for events from the SC with a proton hit
in the SSD (solid line).

5cFig. 9 shows the M MγK− distribution of the signal sample. A prominent peak at 1.54
2
GeV/c is observed with 36 events in the peak region between 1.51 and 1.57 GeV/c2 . To
estimate the background due to the non-resonant K + K − production in the peak region
defined above, the missing mass distribution of the signal sample in the region above 1.59
GeV/c2 was fitted by a distribution of events from the LH2 . LH2 sample required thesame selection criteria except for the shifted vtz window and a proton hit in the SSD.
Since the Λ(1520) contribution was removed from the signal sample, it was removed fromc2
LH2 sample for 1.51 ≤ M MγK
The best fit with a χ2 of 7.2 for 8+ < 1.53 GeV/c .
degrees of freedom is obtained with a scale factor of 0.2 as shown with a dotted line in
Fig. 9. Resulting background level in the peak region is estimated to be 17.0 ± 2.2 ± 1.8,
where the first uncertainty is the error in the fitting in the region above 1.59 GeV/c2 andthe second is a statistical uncertainty in the peak region. The combined uncertainty ofthe background level is ±2.8. The estimated number of the signal events after subtracting backgroundis 19.0 ± 2.8, which corresponds to a Gaussian significance of 4.6+1.2−1.0 σ√2(19.0/ 17.0). The signal region between 1.47 and 1.61 GeV/c after background subtraction was compared with Monte Carlo simulations assuming a Breit-Wigner function
for a resonance shape. The best fit was 1.54 ± 0.01 (statistical) GeV/c2 . The systematic
error was speculated to 0.005 GeV/c2 by referring the known particle Σ− , of which peak
position was 0.005 GeV/c2 smaller than the PDG value of 1.197 GeV/c2 . The width Γ ofthe resonance cannot be determined by the fitting since the zero width gives the minimum
χ2 of 1.6 for 4 degrees of freedom. The upper limit for the width was determined to be
0.025 GeV/c2 with a confidence level of 90%.

Events/(0.02 GeV/c2)

15

10

5

0

1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8
MMcγK− (GeV/c2)cFigure 9: The corrected missing mass distribution M MγK− [Eq.(3)] for the signal sample(solid line) and for events from the LH2 (dotted line) normalized by a hit in the region
above 1.59 GeV /c2 .

6

To make sure that the observed peak is not due to a fake one, the mass cut on oneof the kaons was tightened from 3 σ to 2 σ, and also the kaon mass was intentionallyassigned for the pions in the missing mass calculations. The peak was not affected inboth cases, and it was also confirmed that the peak is not due to the effect of the tails of
the φ mesons.

DISCUSSION
After we announced the evidence of Θ+ , significant deal of activities have been reported in both experimental and theoretical fields. Because of the peculiar nature andrelatively small statistical significance of the new state, experimental confirmation wasseriously awaited. Several experimental results appeared in succession from the DIANA
collaboration at the ITEP [8], CLAS collaboration at the JLab [9], SAPHIR collaboration at the ELSA [10], and a neutrino experiment [11]. In all cases the mass is near
1.54 GeV/c2 and the width is smaller than 0.025 GeV/c2 . There are informal announcesof again consistent observations of Θ+ from HERMES and ZEUS. It is instructive thatthe DIANA collaboration reanalyzed almost 15 years old data which was obtained in the
direct s-channel K + N measurements with Xe bubble chamber.
The most surprising point was the narrowness of the Θ+ width which is claimed to besmaller than 0.009 GeV/c2 [8]. Recent partial wave analysis on K + N elastic scattering
by Arndt, Strakovsky and Workman indicated that the width of Θ+ is likely smaller than
0.001 GeV/c2 [12]. One of the speculation required Θ+ to be isotensor, and the isospin
violating decay causes the narrow width [13]. This explanation seems unlike if isospin of
Θ+ is confirmed to be zero [10, 14].
In fact we only know the Θ+ mass but not other quantum numbers. Spin and parityof Θ+ can be determined in principle by measuring the decay angular distribution andnucleon polarization as in the textbooks. This is extremely difficult to do experimentallyin straightforward way. To determine quantum numbers is our crucial task now on [15,16, 17].
It is natural to ask whether Θ+ is a resonance or a baryon-meson molecular like state.
It seems unusual to assume Θ+ to be a baryon-meson molecular state because of its narrow
width according to Jaffe and Wilczek (JW) [18].
A discovery of the Θ+ was certainly guided by the prediction based on the chiralsoliton model by DPP. On the other hand quark model approaches have been reported
recently with highly correlated diquak-diquark [18] and diquark-triquark [19] schemes. If
Θ+ mass is set to experimentally measured value of 1.54 GeV/c2 in each model, expected
exotic Ξ mass is quite different between correlated quark model and chiral soliton model
predictions. Exotic Ξ mass is at 1.75 GeV/c2 by a quark model of JW, although it is at
2.07 GeV/c2 from eq.(1) with S = -2 by a chiral soliton model by DDP. DDP predicts thewidth to be greater than 0.14 GeV/c2 in addition. Recently reported observation of exotic
Ξ by NA49 collaboration indicates that the mass is at 1.862 GeV/c2 with a width narrower
than 0.018 GeV/c2 [20]. The measured mass does not fit in any predictions, and Diakonov
and Petrov proposes a revised model [21]. There have been significant discoveries recently
in meson spectroscopy field from the B-collider experiments. Unusually narrow mesonstates were observed in the BABAR, BELLE and other detectors [22]. These states may
7

be attributed to exotic tetraquark states. Further researches on the Θ+ and exotic Ξbaryons together with the exotic mesons will boost in understanding the nature of theexotic states.

CONCLUSION
The LEPS measurement provides the first evidence for the existence of a S = +1
narrow baryon resonance at 1.54 ± 0.01 GeV/c2 in the K − missing mass spectrum of the
γn → K + K − n reaction on 12 C. The Gaussian significance is estimated to be 4.6 σ andthe width is smaller than 0.025 GeV/c2 . The result is surprisingly coincided with the
DPP’s prediction. This state is certainly an exotic baryon with a quark configuration of
uudds̄.

ACKNOWLEDGMENTS
I am grateful to the organizers of Spin2003 for their invitation. I would like to thankDr. Atsushi Hosaka for stimulating discussions on exotic states. This research was supported in part by the Ministry of Education, Science, Sports and Culture of Japan, by
the National Science Council of Republic of China (Taiwan), and by KOSEF of Republicof Korea.

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[11] A. E. Asratyan, A. G. Dolgolenko, and M. A. Kubantsev, hep-ex/0309042 (submitted
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