Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 38 (2005)

1(LNS Experiment : #2446, #2506)

Photoproduction of Neutral Kaons on Liquid Deuterium Target
K. Tsukada1, T. Takahashi1＊ , T. Watanabe1†, P. Bydžovský2, M. Ejima1, Y. Fujii1,
K. Futatsukawa1, O. Hashimoto1, K. Hirose1, T. Ishikawa3, S. Kameoka1‡, H. Kanda1,F. Kato1, S. Kinoshita1, T. Kinoshita3, T. Kon1, O. Konno5, K. Maeda1,A. Matsumura1, Y. Miura1, H. Miyahara3, H. Miyase1, T. Nakabayashi3, K. Nonaka1,A. Ohtani1, Y. Okayasu1, M. Oyamada1, A. Sasaki4, H. Shimizu3, T. Tamae3,H. Tamura1, T. Terasawa3, H. Tsubota1, S. N. Nakamura1, H. Nomura1, M Ukai1§,
D. Uchida1, H. Yamauchi1, H. Yamazaki3, K. Yawata1, and M. Wakamatsu1

34

Department of Physics, Tohoku University, Sendai, 980-8578
2
Nuclear Physics Institute, 25068, R ež, Czech Republic
ˆ1

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

Department of Electrical and Electronic Engineering, Akita University, Akita, 010-8502

We have performed the experiment of K0 photoproduction on the liquid deuterium target with
Neutral Kaon Spectrometer (NKS) at LNS-Tohoku. The K0s were measured via K0S → π＋π－ decay channel
in the threshold region. The momentum spectra of K0 photoproduction cross sections were derived andcompared with the calculations using representative models.

§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 both experimentally and theoreticallysince the 1950' s, taking advantage of the electromagnetic interaction that is understood better than thehadronic interaction. Until now, the experimental studies have been carried out in p(γ, K ＋)Λ, p(γ, K ＋)Σ0
and p(γ, K0)Σ ＋ reactions among six isospin channels [1-5]. However, no data have been measured forthe other three channels on a neutron. Theoretically, phenomenological models have been constructedbased on measured channels so far. The isobar models, Kaon-MAID [6] and SLA [7], were adopted in
the present analysis. The predictions of the photon energy dependence and the kaon angular
distribution of the other three channels on a neutron using these models are quite different.
Present address : Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator＊Research Organization (KEK)†

Present address : Department of Physics, Gifu University, Gfu 501-1193‡

Present address : Computing Research Center, High Enegy Accelerator Research Organization (KEK)§

Present address : Cyclotron and Radioisotope Center (CYRIC), Tohoku University, Sendai, 980-8578

2The 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 thestrangeness photoproduction mechanism. In particular, the n(γ, K0)Λ reaction has following features.
(1) Since no charge is involved, the t-channel Born term does not contribute. (2) It is a mirror reaction top(γ, K ＋)Λ. For the hyperon resonance exchange terms, a coupling constant, gKΣN, changes its sign fromthe isospin symmetry, gK Σ = －gK+Σ , resulting the different interference effect. Furthermore, the number00n0

pof resonances to be considered is small in the threshold region. Therefore, the n(γ, K0)Λ reaction isexpected to play an essential role to investigate the strangeness photoproduction mechanism.
In this paper, we present the results of photoproduction of neutral kaons on deuterons near thethreshold.

§2. Experiment
We carried out the experiment of the d(γ, K0) reaction in the threshold region, Eγ = 0.8 ～ 1.1 GeV.
The K0s were measured in π＋π－ decay mode with Neutral Kaon Spectrometer (NKS) which we installedin LNS-Tohoku. Figure 1 shows a schematic view of NKS which consists of a dipole magnet of 107 cm
diameter and 60 cm gap with 0.5 T, straw drift chambers (SDC), cylindrical drift chambers (CDC), an
inner hodoscope (IH), an outer hodoscope (OH) and electron veto counters (EV). The detector
conﬁguration was symmetric against the beam line. The solid angle of NKS was about π sr. The CDC
and SDC were used to measure particle momenta in the horizontal plane by trajectory reconstruction.
The magnetic field map was calculated by TOSCA. IH and OH were used for the time-of-flightmeasurement and event triggers. The momenta and time-of-flights were used for the particle
identiﬁcation. As a trigger condition, we required more than two charged particles; at least one event
both in the left and right arms in coincidence with a tagger signal. EVs were employed to reduce theserious background triggered by γ → e＋e－ conversion. They covered vertically ± 2.5 cm in the horizontal
plane at OH's positions. The geometrical acceptance was reduced by 8 %.
The photon beam was generated from the 1.2 GeV electron beam via bremsstrahlung and its energy
was tagged by the STB-tagger system [8]. Typical tagged photon intensity was 2 × 106/sec and tagging
efﬁciency was 79 ± 1 %. Total experimental period was about 1000 hours.
In order to investigate the K0 elementary production process on a neutron, the liquid deuterium
target with a thickness of 3.2 cm was used. The target system was developed for this experiment andprovided the stable liquid-state deuterium during the experimental period [9]. The density of the target
was estimated to be 0.17 g/cm3 from the temperature of the target and the pressure.

3Fig.1. Schematic view of the Neutral Kaon Spectrometer (NKS).

§3. Analysis
The momenta of particles were derived from the curvature of trajectories. Besides, the velocities ofparticles were calculated from the time of ﬂight and the ﬂight length. The e＋e－ events were removed byrejecting the events of which the vertex position was upstream the target. Figure 2(a) shows the vertex
point distribution of the π＋π－ events. An opening angle (η) cut of － 0.9 < cosη < 0.8 was applied in orderto remove the e＋e－ or vertex mis-reconstruction events. The vertex resolution was estimated to be 1.3
mm in RMS in the beam direction. Most of events were originated in the target. Backgrounds throughsuch as a multi pion productions, N＊ and ρ, whose production cross sections are much larger than thatof kaon production. Hence, no peak is observed in the invariant mass spectrum as shown in Fig. 2(b) forevents in the target region. K0 events are enhanced by selecting vertex points in the decay volume,
because K0S has a relatively long life time of cτ=2.68 cm. Figure 2(c) shows the invariant mass spectrum
when events in the decay volume are chosen. The peak of K0S is clearly seen as shown in the ﬁgure. The
number of K0 events is about 900 for the present data. In order to obtain the cross section, we deﬁned_ M(π＋ π－ ) < 0.54 GeV/c2. Momentum distributions of K0
the gate in the invariant mass spectrum, 0.46 <were obtained after subtracting background contributions. The origin of the backgrounds wereconsidered as (1) leakage from the target region due to the finite resolution of the vertex point
reconstruction, and (2) wrong combination betweenπ＋ from K0 andπ－ fromΛ. The ﬁrst one was estimatedfrom the events originated in the target region, and the second one was estimated by Geant4 simulation.
These estimations gave the shape of the backgrounds, and the magnitude of the backgrounds wereadjusted by ﬁtting the invariant mass spectrum. Figure 3 shows the ﬁtting result assuming a gaussian
for K0 and the shapes of backgrounds as estimated in the photon energy from 0.9 to 1.0 GeV (ELow) and4Fig.2. (a) Vertex distribution for π＋π－ events. The beam comes from left to right. Events come mainly
from the target region. (b), (c) An invariant mass spectrum of π＋π－ events with the gate thatthe vertex is in the target for (b) and outside the target denoted by decay volume for (c).

form 1.0 to 1.1 GeV (EHigh).
The acceptance of NKS was calculated by a Geant4 simulation. The geometry of NKS wasconsidered realistically, and the position and time resolution of the detectors were taken into account.
Various analysis efﬁciencies were also estimated in the acceptance. Figure 4 shows the contour plot of
the acceptance map. We used the data with momentum (P) and angle (cosθ) of K0 in the effective regions
shown in the ﬁgure because the acceptance is very low out of these regions. In this paper, the results inRegion 1 are presented.

§4. Results and Discussion
The obtained momentum spectra of K0s are shown in Fig.5. The backgrounds contributions arealready subtracted. The integrated momentum range is limited as mentioned in the previous section.
The obtained spectra are compared with the calculations using the representative two models, KaonMAID [6] and Saclay-Lyon A (SLA) [7]. The lines represent calculations in the plane wave impulse
approximation framework using Bonn OBEPQ (One-Boson-Exchange-Potential in Q-space) deuteronwave function [10] by Bydžovský et al. [11]. The Kaon-MAID model is the only model to predict theelementary cross section of K0 photoproduction on a neutron assuming SU(3) symmetry. On the otherhand, SLA has an adjustable parameter for K0Λ photoproduction, since the ratio of the decay width
between the charged and neutral K1 resonance (RK K ) is unknown. In Kaon-MAID, this ratio was0
11γ

obtained from the γ + p → K + Σ process. Therefore, the elementary cross sections of SLA are calculated
0＋assuming various rK K . Kinematical regions are also selected for the calculations similarly to the1 γ

experimental data.
Since we measured only K0, the Σ production process may contribute in addition to K0Λ production.
From the estimation using Kaon-MAID, the contributions of Σ production are negligibly small in the5Fig.3. Fitting results of invariant mass spectra in photon regions from 0.9 to 1.0 GeV (ELow, left)
and from 1.0 to 1.1 GeV (EHigh, right). The contribution around 0.4 GeV/c2 comes from thewrong combination background and that around 0.6 GeV/c2 comes from the leakage of thetarget events. The peak near 0.3 GeV/c2 comes from the e＋e－ events.

Fig.4.

The contour plot of the acceptance map of NKS. The effective regions, Region 1, Region 2and Region 3, are overdrawn by solid lines.

ELow region and sizable large at lower momentum in the EHigh region. The prediction based on theKaon-MAID model is consistent with our data in the ELow region but too large in the EHigh region
even if the Σ contribution is involved. The spectra calculated by SLA with rK K = － 1.9 well account for1 γ

the present results at both energy regions.
In the ELow region, the shape of the momentum spectrum in the laboratory system mainly depends
on the angular distribution in the center of mass system. From the momentum distributions using SLA
with various rK K values, it is favored that the K0 photoproduction on a neutron has a backward peak in1 γ

the center of mass system.

6Fig.5. Momentum dependences after the correction for efﬁciencies and subtraction of backgrounds.
The lines represent the calculations using Kaon-MAID (solid), SLA(rK K = － 1.6, dotted),SLA(rK K =－1.9, solid), SLA(rK Kγ=－2.0, dashed), SLA(rK K =－2.5, solid) and SLA(rK K =－3.0,
dash-dotted). The photon energy ranges are from 0.9 to 1.0 GeV(ELow, left) and from 1.0 to
1.1 GeV(EHigh, right). Error bars are overdrawn for the (statistic) and (statistic ＋systematic) errors.
1 γ1 γ11 γ1 γ§5. Summary
We have successfully measured neutral kaons, bombarding a liquid deuterium target with a photon
beam in the threshold region from 0.8 to 1.1 GeV. It was the ﬁrst data for K0 photoproduction on thedeuteron in this energy region. The momentum spectra of K0 production were compared with theoretical
spectra calculated assuming isobar models for the elementary process and a realistic deuteron wave
function for the target nucleus. The present experiment has demonstrated the usefulness of the neutralkaon measurement for the investigation of photo strangeness production reactions.

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

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