arXiv:hep-ex/0301020 v2 8 Jul 2003
Evidence for Narrow S=+1 Baryon Resonance in
Photo-production from Neutron
T. Nakano,
1
D.S. Ahn,
2
J.K. Ahn,
2
H. Akimune,
3
Y. Asano,
4, 5
W.C. Chang,
6
S. Dat´e,
7
H. Ejiri,
7, 1
H. Fujimura,
8
M. Fujiwara,
1, 5
K. Hicks,
9
T. Hotta,
1
K. Imai,
10
T. Ishikawa,
11
T. Iwata,
12
H. Kawai,
13
Z.Y. Kim,
8
K. Kino,
1
H. Kohri,
1
N. Kumagai,
7
S. Makino,
14
T. Matsumura,
1, 5
N. Matsuoka,
1
T. Mibe,
1, 5
K. Miwa,
10
M. Miyabe,
10
Y. Miyachi,
15, ∗
M. Morita,
1
N. Muramatsu,
5
M. Niiyama,
10
M. Nomachi,
16
Y. Ohashi,
7
T. Ooba,
13
H. Ohkuma,
7
D.S. Oshuev,
6
C. Rangacharyulu,
17
A. Sakaguchi,
16
T. Sasaki,
10
P.M. Shagin,
1, †
Y. Shiino,
13
H. Shimizu,
11
Y. Sugaya,
16
M. Sumihama,
16, 5
H. Toyokawa ,
7
A. Wakai,
18, ‡
C.W. Wang,
6
S.C. Wang,
6, §
K. Yonehara,
3, ¶
T. Yorita,
7
M. Yoshimura,
19
M. Yosoi,
10
and R.G.T. Zegers
1
1
Research Center for Nuclear Physics,
Osaka University, Ibaraki, Osaka 567-0047, Japan
2
Department of Physics, Pusan National University, Busan 609-735, Korea
3
Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
4
Synchrotron Radiation Research Center,
Japan Atomic Energy Research Institute, Mikazuki, Hyogo 679-5198, Japan
5
Advanced Science Research Center,
Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
6
Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
7
Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japan
8
School of Physics, Seoul National University, Seoul, 151-747, Korea
9
Department of Physics and Astronomy,
Ohio University, Athens, Ohio 45701
10
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
11
Laboratory of Nuclear Science, Tohoku University, Sendai, Miyagi 982-0826, Japan
12
Department of Physics, Yamagata University, Yamagata 990-8560, Japan
13
Department of Physics, Chiba University, Chiba 263-8522, Japan
14
Wakayama Medical University, Wakayama, Wakayama 641-8509, Japan
15
Department of Physics and Astrophysics,
1
Nagoya University, Nagoya, Aichi 464-8602, Japan
16
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
17
Department of Physics and Engineering Physics,
University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 5E2
18
Center for Integrated Research in Science and Engineering,
Nagoya University, Nagoya, Aichi 464-8603, Japan
19
Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan
(Dated: Ju ly 8, 2003)
Abstract
The γn → K
+
K
−
n reaction on
12
C has been studied by measur ing both K
+
and K
−
at forward
angles. A sharp baryon resonance peak was observed at 1.54 ± 0.01 GeV/c
2
with a width smaller
than 25 MeV/c
2
and a Gaussian significance of 4.6 σ. The strangeness quantum number (S)
of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon reson an ce
or alternatively as an exotic 5-quark state (uudd¯s) that decays into a K
+
and a neutron. The
resonance is consistent with the lowest member of an anti-decuplet of baryons predicted by the
chiral soliton mo del.
PACS numbers: 13.60.Le, 13.60.Rj, 14.20.Jn
2
The search for baryon resonances with the strangeness quantum number S=+1, that
cannot be formed by three quarks, has a long and interesting history. In f act , the summary
of the S=+1 baryon resonance searches has been dropped from the Particle Data Group
(PDG) listings although the possible exotic resonances were noted in the 1986 bar yon listings
[1]. Most of the previous searches were made using the partial wave a na lyses of kaon-nucleon
(KN) scatter ings [2]. These searches result ed in two possibilities, the isoscalar Z
0
(1780) and
Z
0
(1865), for which the evidence of the existence was reviewed to be poor by PDG.
The present wor k was motivated in part by the recent work by Diakonov, Petrov a nd
Polyakov [3] who studied anti-decuplet baryons using the chiral soliton model. The mass
splittings of the established octet and decuplet were reproduced within accuracy of 1 % in
this model, and those of the new anti-decuplet were also estimated using the nucleon sigma
term [4] and the current quark-mass ratios. The anti-decuplet was anchored to the mass of
the P
11
(1710) nucleon r esonance, giving the Z
+
(spin 1/2, isospin 0 and S=+1) a mass of
∼ 1530 MeV/c
2
and a total width of less than 15 MeV/c
2
. The S=+1 baryon resonances in
this mass region have not been searched for in the KN scattering experiments in the past
because momenta of kaons were too high as pointed out in Refs. [3, 5].
The concept of a molecular meson-baryon bound state has been proposed by Refs. [6 , 7, 8]
in conjunction with the well-known Λ(1405) particle. The mass spectrum of the Λ(1405)
can be dynamically generated [6, 7] suggesting that this “particle” can be described as
a molecular meson-baryon bound state with a quark configuration uuds¯u. However, the
validity of this assumption is not solid and should be checked by measuring the Λ(1405)
decay [6, 7], o f which experimental data are scarce. Moreover, the same quantum numbers
for the Λ(1405) can be achieved with a quark configuration uds. This ambiguity is not
present in the case of the proposed Z
+
resonance with a quark configuration uu dd¯s. In this
letter we report the experimental evidence for a nar row resonance with S = +1 which can
be interpreted a s the predicted exotic Z
+
state.
The experiment was carried o ut at the Laser-Electron Photon facility at SPring-8
(LEPS) [9, 10, 11]. Photons were produced by Compton back-scattering of laser photons
from 8 GeV electrons in the SPring-8 storage ring. Using a 351-nm Ar laser, photons with a
maximum energy of 2.4 GeV were produced. Electrons that were participants in the back-
scattering process were momentum-analyzed by a bending magnet of the SPring-8 storag e
ring, and detected by a tagging counter inside the ring to get the photon energy with a
3
resolution (σ) of 15 MeV. Only photons with energies above 1.5 GeV were tagged. The
typical photon flux was ∼ 10
6
/s.
A 0.5-cm thick plastic scintillator (SC) which was composed of hydrogen and carbon
nuclei (C:H ≈ 1:1) was used as a targ et in the present experiment (see Ref. [9]). The SC
was located 9.5 cm downstream from the 5 -cm thick liquid-hydrogen (LH
2
) target used
for studying the photo- production of φ-mesons. In fact, the two experiments were carried
out simultaneously. Since this paper concentr ates on the study of events generated from
neutrons in carbon nuclei at the SC, a comparison between events from the LH
2
and the
SC selected under the same conditions, with only a change in the software condition on t he
reconstructed vertex position (vtz) along the beam axis, provides a good tool to distinguish
contributions from protons and neutrons.
A silicon-strip ver tex detector (SSD ) and 3 drift chambers were used to t r ack charged
particles through a dipole magnet with a field strength of 0.7 Tesla. The SSD consists of
single-sided silicon-strip detectors (vertical and horizontal planes) with the strip pitch of 12 0
µm. The first drift chamber located before the magnet consists of 6 wire planes (3 vertical
planes, 2 planes at +45
◦
, and 1 plane at −45
◦
), and the other two drift chambers af t er
the magnet consist of 5 planes (2 ver t ical planes, 2 planes at +30
◦
, and 1 plane at −30
◦
).
A time-of-flight (TOF) scintillator array was positioned 3 m behind t he dip ole magnet.
Electron-posit ron pairs produced at very forward angles were blocked by lead bars which
were set horizontally along the median plane inside t he magnet gap. Electrons and positrons
that escaped from t he blocker, and pions with a momentum higher than ∼ 0.6 GeV/c were
vetoed on-line by an aerogel
ˇ
Cerenkov counter located downstr eam of the SC.
The angular coverage of the spect r ometer was about ±0.4 rad and ±0.2 rad in the hor-
izontal and vertical directions, respectively. The momentum resolution (σ) for 1-GeV/c
particles was 6 MeV/c. The timing resolution (σ) of the TOF was 15 0 psec for a typical
flight length of 4 m from the target to the TOF. Particle identification was made within 3σ
of the momentum-dependent mass resolution, which was about 30 MeV/c
2
for a 1-GeV/c
kaon.
The design of the LEPS detector is optimized f or measuring φ-mesons produced near
the threshold energy at f orward angles by detecting the K
+
K
−
pair from the φ decay.
These measurements will be reported in a separate a r t icle. Here we discuss the detection of
K
+
K
−
pairs generated at the SC. From the total set of 4.3 × 10
7
events measured in the
4
LEPS detector, 8.0 × 10
3
events with a K
+
K
−
pair were selected. As shown in Fig . 1(a) ,
a cut on the vtz cleanly selected events that originate f r om a reaction at the SC, which
accounted for about half of the K
+
K
−
-pair events.
To reduce contributions from non-resonant K
+
K
−
productions for which the phase space
increases quadratically with the photon energy from the production threshold, events with
the photon energy above 2.35 GeV were rejected. About 3.2 × 10
3
events remained after
this cut. The missing mass MM
γK
+
K
−
of the N(γ, K
+
K
−
)X reaction was calculated by
assuming that the target nucleon (pro t on or neutron) has the mean nucleo n mass of 0.9389
GeV/c
2
(M
N
) and zero momentum. Subsequently, events with 0.90 < MM
γK
+
K
−
< 0.98
GeV/c
2
were selected. A total of 1.8 × 10
3
events survived aft er this cut. Most of the
remaining events (∼ 85 %) were due to the photo-production of the φ meson. They were
eliminated by removing the events wit h the invariant K
+
K
−
mass from 1.00 GeV/c
2
to 1 .04
GeV/c
2
for the φ (Fig. 1(b)).
In order to eliminate photo-nuclear reactions of γp → K
+
K
−
p on protons in
12
C and
1
H
at the SC, the recoiled protons were detected by the SSD. The direction and momentum of
the nucleon in the final state was calculated from the K
+
and K
−
momenta, and such events
in which the recoiled nucleon wa s out of the SSD acceptance were rejected. Events were
rejected if the momentum of t he nucleon was smaller tha n 0.35 GeV/c since the calculated
direction had a large uncertainty in this case. Finally, we rejected 108 events for which the
hit position in the SSD agreed with the expected hit position within 45 mm in the vertical
or hor izontal direction. The cut points correspond to about ± 2σ resolution for events that
are affected by the Fermi motion. A total of 109 events satisfied all the selection criteria.
We call this set of events the “signal sample”.
In case of reactions on nucleo ns in nuclei, the Fermi motion has to be taken into ac-
count to obtain a ppropriat e missing-mass spectra. To evaluate this effect, we studied the
γn → K
+
Σ
−
→ K
+
π
−
n sequential process as an example, where the K
+
and π
−
were
detected. The missing masses, MM
γK
+
and MM
γK
+
π
−
, were obtained for the N(γ, K
+
)X
and N(γ, K
+
π
−
)N channels by assuming that the nucleon in
12
C is at rest with the mass
equal to M
N
. Both the missing masses a r e smeared out due to the Fermi motion of nucleons
in
12
C. However, since the nucleons in the two channels are identical, the two missing masses
have a strong correlation as shown in Fig. 2(a). Accordingly, t he missing mass corrected for
5