Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 41 (2008)

1(LNS Experiment : #2552, #2565)

Study of Neutral Kaon Photo-Production at LNS-Tohoku
P. Bydžovský1 , N. Chiga 2 , Y. Fujii 2 , K. Futatsukawa2 , Y.C. Han 6 ,O. Hashimoto 2 , K. Hirose 4 , K. Hosomi 2 , A. Iguchi 2 , T. Ishikawa 4 ,H. Kanda 2 , M. Kaneta 2 , M. Kawai 2 , T. Kawasaki 2 , D. Kawama 2 ,S. Kiyokawa 2 , T. Koike 2 , O. Konno 5 , Y. Ma 2 , K. Maeda 2 , N. Maruyama 2 ,A. Matsumura 2 , M. Mimori 2 , K. Miwa 2 , Y. Miyagi 2 , S.N. Nakamura 2 ,T. Ohtani 2 , Y. Okayasu 2 , A. Sasaki 3 , H. Shimizu 4 , K. Shirotori 2 ,M. Sotona 1 , K. Suzuki 4 , T. Tamae 4 , H. Tamura 2 , N. Terada 2 , K. Tsukada 2 ,T.S. Wang 6 , T. Yamamoto 2 , H. Yamazaki 4 , and K. Yokota 2123

Department of Physics, Tohoku University, Sendai 980-8578

Department of Electrical and Electronic Engineering, Akita University, Akita 010-8502
45Nuclear Physics Institute, 25068, Řež, Czech Republic

Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826

Department of Electrical Engineering, Ichinoseki National College of Technology, Ichinoseki021-8511
6

School of Nuclear Science and Technology, Lanzhou University,
Tianshuinan Road 222, Lanzhou 730000, China§ 1. Introduction
The investigation of the kaon production on a nucleon by the electromagnetic interaction provides
invaluable information on the strangeness production mechanism, strength of meson-baryon coupling
constants and structure of hadrons, being labeled by the strangeness degree of freedom. Such studies using beams of real photons and electrons have been conducted experimentally and theoretically since the1950’s, taking advantage of the electromagnetic interaction that is understood better than the hadronicinteraction. Until now, the experimental studies have been carried out in p(γ, K + )Λ , p(γ, K + )Σ0 andp(γ, K 0 )Σ+ reactions among six isospin channels ( [1], [2]). However, there are no data for the otherthree channels on a neutron. Theoretically, phenomenological models have been constructed based on
measured channels so far. The isobar models, Kaon-MAID [3] and SLA [4], were adopted in the presentanalysis. The predictions of the photon energy dependence and the kaon angular distribution of theother three channels on a neutron using these models are quite different.
The lack of the key data for strangeness photoproduction of the three channels on a neutron is dueto the experimental difﬁculties to measure neutral kaons and to prepare a neutron target. The measurement of these three strangeness production channels provides much information on the strangenessphotoproduction mechanism. In particular, the n(γ, K 0 )Λ reaction has following features. (1) Since nocharge is involved, the t-channel Born term does not contribute. (2) It is a mirror reaction to p(γ, K + )Λ. For the hyperon resonance exchange terms, a coupling constant, gKΣN , changes its sign from the2isospin symmetry, gK 0 Σ0 n = −gK + Σ0 p , resulting the different interference effect. Furthermore, the number of resonances to be considered is small in the threshold region. Therefore, the n(γ, K 0 )Λ reaction isexpected to play an essential role to investigate the strangeness photoproduction mechanism.
We have already taken exploratory data quite successfully with use of Neutral Kaon Spectrometer(NKS) at LNS-Tohoku in 2003 and 2004. We intend to extend the previous experiment by considerably upgrading the original neutral kaon spectrometer to a completely new neutral kaon spectrometer(NKS2), fully replacing the spectrometer magnet, tracking detectors and all the trigger counters. The
new spectrometer NKS2 has signiﬁcantly larger acceptance for neutral kaons compared with NKS, particularly covering forward angles and much better invariant mass resolution. The estimated acceptanceof NKS2 is about 3 to 4 times (depend on momentum and model) larger for KS0 than that of NKS. Additionally, it is about 8 to 10 times larger for Lambda. With this advantage, we expect simultaneous
measurements of KS0 and Λ. Additionally, we plan to measure other strangeness production channelsand also Λ hyperon polarization in γ + n and γ + p reactions.
In this report, we present a status of NKS2 experiment. The NKS results are found in elsewhere [5,6].

§ 2. The NKS2 Experiment
The NKS2 spectrometer is located the BM4 beam line of the second experimental hall of Laboratoryof Nuclear Science (LNS), Tohoku University (see Fig. 1). The incident beam from LINAC has 0.2 GeVof the beam energy and is accelerated up to 1.2 GeV in Stretcher-Booster (STB) Ring. The photon beam
is created as bremsstrahlung of electron by a carbon wire at STB Tagger system of Bending Magnet 4(BM4). There is a dipole magnet which is called the sweep magnet for e+ e− from photon conversion.
The sweep magnet is the same one which is used in the previous experiment NKS.
The spectrometer is placed following the sweep magnet. The main magnet is a dipole which is renovated from a cyclotron magnet of Cyclotron RI center of Tohoku University (it is so called 680 magnet).
Detectors of NKS2 are: Inner Hodoscope (IH), Straw Drift Chamber (SDC), Cylindrical Drift Chamber(CDC), Outer Hodoscope (OH), and Electron Veto counter (EV). Figure 2 shows detector position and 3D
views are shown in Figs. 3 and 4. The detail description of the beam line and the spectrometer will beshown in the following sections.

§ 3. Results of the NKS2 experiment
The data taking for commissioning runs was carried out in Jan. Mar, Jun., and Sep. 2006 using thecarbon target. During those runs, we had studied detector performance, data acquisition system, andtrigger rate. The data taking using NKS2 was done in Nov. and Dec. 2006, and Jan. and Jun. 2007.
A preliminary invariant mass distribution of π + π − pairs in 0.8 < Eγ < 1.1 GeV is shown in Fig. 5.
The number of tagged photons for the distribution was 3.2 × 1011 from 2006’s runs.
The events that have two tracks and more was selected. The particle identiﬁcation was done on 2
dimensional cut on TOF and momentum. After positive and negative pions was identiﬁed in the same

3Fig.1.

The outline of the second experimental hall of LNS (see text in detail).

OHV

OHH

CDC

SDC
IH
20 cm

Targetg Beam1mg Beam

Fig.2.

A schematic view of NKS2. The photon beam direction is bottom to top in the ﬁgure. The
target holder is at center of magnet. The detectors are (the order is center to outer): Inner
Hodoscope (IH), Straw Drift Chamber (SDC), Cylindrical Drift Chamber (CDC), Outer
Hodoscope (OH, Vertical (OHV) and Horizontal (OHH) ), and Electron Veto counter (EV).
Note that EV is placed at downstream of OHV but not shown the ﬁgure.

4Fig.3.

A 3D view of the spectrometer viewed from upstream of beam line. We can see a part of
OHH on the magnet yoke and OHV around magnet coil.

Fig.4.

A 3D view of the spectrometer viewed from downstream of beam line. There are OHVaround the magnet coil and OHH on the magnet yoke. The EV counters are placed following OHV. The OHV and EV counters are supported by aluminum chassis.

Number of counts

5Parnirelimy2Invariant mass [ GeV / c ]Fig.5.

The invariant mass distribution of π + π − pairs in 0.8 < Eγ < 1.1 GeV.

event, the decay vertex was reconstructed from the two tracks. We required −0.9 < cos θOA < 0.8(where θOA is an opening angle of two tracks) to select KS0 candidate. We required an opening angle cutof two tracks to remove e+ e− background. A decay volume cut was necessary to remove a background ofresonances (e.g., ρ, ω). Since photoproduced vector mesons and nucleon resonances decay in the target
region and create π + π − background, we did not employ events which have their decay vertex positionin the target cell. We employ events which have the decay vertex position at outside of the target cell toremove large contribution of hadronic background.
We have an upgrade plane of the spectrometer, that we will extend the acceptance by changing SDCwith a 3D tracking chamber. With this advantage, we expect simultaneous measurements of KS0 and Λ.
Additionally, we plan to measure other strangeness production channels and also Λ hyperon polarizationin γ + n and γ + p reactions.

Acknowledgement
This work is supported by Grant-In-Aid for Scientiﬁc Research from The Ministry of Education ofJapan, Nos. 09304028, 12002001, 14740150 and 16GS0201.

References[1]

D.S. Caran et al.: Phys. Rev. Lett. 90 (2003) 131804.

[2]

R.G.T. Zegers et al.: Phys. Rev. Lett. 91 (2003) 092001.

[3]

F.X. Lee, T. Mart, C. Bennhold, and L.E. Wright: Nucl. Phys. A695 (2001) 237.

[4]

T. Mizutani, C. Fayard, G.H. Lamot, and B. Saghai: Phys. Rev. C58 (1998) 75.

[5]T. Watanabe et al.: Phys. Lett. B651 (2007) 269.

[6]

K. Tsuakda et al.: arXiv:0712.0657(nucl-ex).