- Email: [email protected]
K0 photoproduction on 12C in the threshold region
Nuclear Physics A 754 (2005) 327c–331c
K0 photoproduction on 12C in the threshold region T. Watanabe a,∗ , S. Endo d , Y. Fujii a , O. Hashimoto a , K. Hirose a , T. Ishikawa b , K. Ito a , H. Kanda a , M. Katoh a , T. Kinoshita b , O. Konno c , K. Maeda a , A. Matsumura a , Y. Miura a , F. Miyahara b , H. Miyase a , K. Mizunuma a , T. Nakabayashi b , S.N. Nakamura a , H. Nomura a , Y. Okayasu a , T. Osaka a , A. Otani a , M. Oyamada a , A. Sasaki d , T. Sato d , H. Shimizu b , T. Takahashi b , T. Tamae b , H. Tamura a , T. Terasawa b , H. Tsubota a , K. Tsukada a , M. Ukai a , M. Utoyama a , M. Wakamatsu a , H. Yamauchi a , Y. Yamaguchi a , Y. Yamamoto a , H. Yamazaki b a Department of Physics, Tohoku University, Sendai 980-8578, Japan b Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan c Department of Electrical Engineering, Ichinoseki National College of Technology, Ichinoseki, Japan d Department of Electrical and Electronic Engineering, Akita University, Akita 010-8502, Japan
Received 10 February 2005; accepted 22 February 2005 Available online 24 March 2005
Abstract The γ n → K 0 Λ process plays an important role in the study of strangeness production by the electromagnetic interaction. We have investigated the quasi-free production reaction on a 12 C target in the threshold region (Eγ = 0.8–1.1 GeV) at the Laboratory of Nuclear Science, Tohoku University. Preliminary results are presented. 2005 Published by Elsevier B.V.
* Corresponding author.
E-mail address: [email protected] (T. Watanabe). 0375-9474/$ – see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2005.02.157
328c
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
Fig. 1. Neutral kaon spectrometer (NKS).
Study of strangeness production by the electromagnetic interaction is important for hadron physics, such as a search for missing resonances, investigation of hadron structure, etc. However, the information has been obtained through limited experimental data in K + production channels. The γ n → K 0 Λ reaction in the threshold region is expected to give keys for further investigating electromagnetic strangeness production although no experimental data are available so far. The reasons are: (1) since no charge is involved, the t-channel Born term does not contribute; (2) the coupling constant, gKΣ 0 N , in the Σ 0 -exchange diagram (u-channel Born term) changes its sign from SU(3) symmetry, so an interference effect differs from γp → K + Λ channel; and (3) number of resonances to be considered is small in the threshold region. Thus, this reaction provides information on this strangeness production process. We have constructed a neutral kaon spectrometer (NKS) at the Laboratory of Nuclear Science (LNS), Tohoku University and have measured photoproduction of K 0 ’s on a 12 C target in the threshold region as a first stage of a series of experiments. With the 12 C target, we intend to investigate the γ n → K 0 Λ reaction through quasi-free production. Photons were generated and energy tagged by the STB-tagger at LNS [1]. The carbon target was located at the center of NKS (Fig. 1). Produced K 0 ’s were detected in the Ks0 → π + π − channel with the NKS by reconstructing the invariant mass of the measured π + π − . The NKS consists of a dipole magnet of 1.1 m diameter with 0.5 T, straw drift chambers (SDC), cylindrical drift chambers (CDC), inner hodoscopes (IH), outer hodoscopes (OH) and electron veto counters (EV). The dipole magnet, CDC and SDC were used to measure particle momentum in the horizontal plane by trajectory reconstruction. IH and OH were used for time-of-flight measurement and making event triggers. These momenta and timeof-flight were used for particle identification (PID). The EV reduced background events by γ → e+ e− conversion in the trigger level. Invariant mass spectra for π + π − events (Mπ + π − ) are shown in Fig. 2. K 0 events are hidden under the large background from N ∗ , ρ and so on (Fig. 2(b)). Vertices of these background events exist in the target region because original particles have a short life
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
329c
Fig. 2. Invariant mass spectra for π + π − events. The upper panel shows decay volume events, and lower one shows target region events.
time. However, the K 0 has a life time, cτ = 2.6 cm, which is long enough to decay outside the target. A K 0 peak is clearly shown by selecting the decay vertex point outside the target (Fig. 2(a)). K 0 events were selected by gating 0.445–0.545 GeV/c2 in Mπ + π − (Fig. 2(a)). The background contribution was estimated taking the background shapes from the data and normalizing it by fitting to the invariant mass spectrum, together with a Gaussian peak for K 0 . The quasi-free production cross section for γ 12 C → KY 11 C process was calculated by 2 dσ Neff 1 |F (s, t, m ˜ N )|2 pK = ρ( p ) d p N N dΩK dpK N (4π)2 4pγ · pN EK EY ˜ × δ(Eγ + EN − EK − EY ), (1) where E˜ N and m ˜ N are the nucleon energy and mass in a nucleus which conserve energy ˜ N ), ρ(pN ) and momentum in γ N → KY and γ A X → KY A−1 X , respectively. F (s, t, m and Neff are the elementary amplitude, momentum distribution for a bound nucleon, and the effective nucleon number, respectively. Kaon-MAID for the elementally amplitude [3], and a Fermi gas model of kF0 = 0.22 GeV/c for the momentum distribution were assumed. Neff was assumed to be 4.2 from Zeff for 12 C(γ , K + ) [2]. Fig. 3 shows the integrated cross section with the photon energy compared with 12 C(γ , K 0 )X (our data) and 12 C(γ , K + )X [2]. Integrated regions about the kaon production angle were almost the same in the laboratory system, (cos θ of 0.8–1.0 (for K 0 ) and 0.766–0.985 (for K + )). The solid line in Fig. 3 shows the quasi-free production cross section for γ 12 C → K 0 Λ11 C calculated by Eq. (1). The integrated region is the same for the K 0 experimental data. The magnitude of the cross section is almost the same, suggesting that the elementary cross section for K 0 production is of the same order as K + production.
330c
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
Fig. 3. Energy dependence of the cross section for γ C → K 0 X (full circle, our data), and γ C → K + X (open triangle, [2]). Solid line shows calculation for γ C → K 0 X.
Fig. 4. Momentum distribution of cross section. Solid line is calculation for 12 C(γ , K 0 )Λ11 C, and dashed line is that for 12 C(γ , K 0 )Σ 11 X data.
Fig. 4 shows momentum spectra of the outgoing kaons. The points show experimental data for C(γ , K 0 ). Solid lines are the calculated cross section of the γ n → K 0 Λ process by Eq. (1). Dashed lines include γp → K 0 Σ + and γ n → K 0 Σ 0 processes. As seen in
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
331c
the figure, the contribution of Σ production processes are small compared with Λ production as expected in the threshold region. Kaon-MAID predictions well represent present the experimental data. In summary, we measured K 0 production cross section with photon energy and K 0 momentum dependence for the first time. The experimental data are compared with elementally cross section of Kaon-MAID using a quasi-free cross section calculation (1).
Acknowledgements This work is supported by Grant-In-Aid for Scientific Research from The Ministry of Education of Japan, Nos. 09304028, 12002001, and by the 21COE program of MEXT Japan.
References [1] H. Yamazaki, et al., Research Report of Laboratory of Nuclear Science, vol. 34, 2001, p. 25. [2] H. Yamazaki, Phys. Rev. C 52 (1995) 1157–1160. [3] C. Bennhold, H. Haberzettl, T. Mart, nucl-th/9909022, web address is http://www.kph.uni-mainz.de/MAID/ kaon/kaonmaid.html.
K0 photoproduction on 12C in the threshold region T. Watanabe a,∗ , S. Endo d , Y. Fujii a , O. Hashimoto a , K. Hirose a , T. Ishikawa b , K. Ito a , H. Kanda a , M. Katoh a , T. Kinoshita b , O. Konno c , K. Maeda a , A. Matsumura a , Y. Miura a , F. Miyahara b , H. Miyase a , K. Mizunuma a , T. Nakabayashi b , S.N. Nakamura a , H. Nomura a , Y. Okayasu a , T. Osaka a , A. Otani a , M. Oyamada a , A. Sasaki d , T. Sato d , H. Shimizu b , T. Takahashi b , T. Tamae b , H. Tamura a , T. Terasawa b , H. Tsubota a , K. Tsukada a , M. Ukai a , M. Utoyama a , M. Wakamatsu a , H. Yamauchi a , Y. Yamaguchi a , Y. Yamamoto a , H. Yamazaki b a Department of Physics, Tohoku University, Sendai 980-8578, Japan b Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan c Department of Electrical Engineering, Ichinoseki National College of Technology, Ichinoseki, Japan d Department of Electrical and Electronic Engineering, Akita University, Akita 010-8502, Japan
Received 10 February 2005; accepted 22 February 2005 Available online 24 March 2005
Abstract The γ n → K 0 Λ process plays an important role in the study of strangeness production by the electromagnetic interaction. We have investigated the quasi-free production reaction on a 12 C target in the threshold region (Eγ = 0.8–1.1 GeV) at the Laboratory of Nuclear Science, Tohoku University. Preliminary results are presented. 2005 Published by Elsevier B.V.
* Corresponding author.
E-mail address: [email protected] (T. Watanabe). 0375-9474/$ – see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2005.02.157
328c
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
Fig. 1. Neutral kaon spectrometer (NKS).
Study of strangeness production by the electromagnetic interaction is important for hadron physics, such as a search for missing resonances, investigation of hadron structure, etc. However, the information has been obtained through limited experimental data in K + production channels. The γ n → K 0 Λ reaction in the threshold region is expected to give keys for further investigating electromagnetic strangeness production although no experimental data are available so far. The reasons are: (1) since no charge is involved, the t-channel Born term does not contribute; (2) the coupling constant, gKΣ 0 N , in the Σ 0 -exchange diagram (u-channel Born term) changes its sign from SU(3) symmetry, so an interference effect differs from γp → K + Λ channel; and (3) number of resonances to be considered is small in the threshold region. Thus, this reaction provides information on this strangeness production process. We have constructed a neutral kaon spectrometer (NKS) at the Laboratory of Nuclear Science (LNS), Tohoku University and have measured photoproduction of K 0 ’s on a 12 C target in the threshold region as a first stage of a series of experiments. With the 12 C target, we intend to investigate the γ n → K 0 Λ reaction through quasi-free production. Photons were generated and energy tagged by the STB-tagger at LNS [1]. The carbon target was located at the center of NKS (Fig. 1). Produced K 0 ’s were detected in the Ks0 → π + π − channel with the NKS by reconstructing the invariant mass of the measured π + π − . The NKS consists of a dipole magnet of 1.1 m diameter with 0.5 T, straw drift chambers (SDC), cylindrical drift chambers (CDC), inner hodoscopes (IH), outer hodoscopes (OH) and electron veto counters (EV). The dipole magnet, CDC and SDC were used to measure particle momentum in the horizontal plane by trajectory reconstruction. IH and OH were used for time-of-flight measurement and making event triggers. These momenta and timeof-flight were used for particle identification (PID). The EV reduced background events by γ → e+ e− conversion in the trigger level. Invariant mass spectra for π + π − events (Mπ + π − ) are shown in Fig. 2. K 0 events are hidden under the large background from N ∗ , ρ and so on (Fig. 2(b)). Vertices of these background events exist in the target region because original particles have a short life
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
329c
Fig. 2. Invariant mass spectra for π + π − events. The upper panel shows decay volume events, and lower one shows target region events.
time. However, the K 0 has a life time, cτ = 2.6 cm, which is long enough to decay outside the target. A K 0 peak is clearly shown by selecting the decay vertex point outside the target (Fig. 2(a)). K 0 events were selected by gating 0.445–0.545 GeV/c2 in Mπ + π − (Fig. 2(a)). The background contribution was estimated taking the background shapes from the data and normalizing it by fitting to the invariant mass spectrum, together with a Gaussian peak for K 0 . The quasi-free production cross section for γ 12 C → KY 11 C process was calculated by 2 dσ Neff 1 |F (s, t, m ˜ N )|2 pK = ρ( p ) d p N N dΩK dpK N (4π)2 4pγ · pN EK EY ˜ × δ(Eγ + EN − EK − EY ), (1) where E˜ N and m ˜ N are the nucleon energy and mass in a nucleus which conserve energy ˜ N ), ρ(pN ) and momentum in γ N → KY and γ A X → KY A−1 X , respectively. F (s, t, m and Neff are the elementary amplitude, momentum distribution for a bound nucleon, and the effective nucleon number, respectively. Kaon-MAID for the elementally amplitude [3], and a Fermi gas model of kF0 = 0.22 GeV/c for the momentum distribution were assumed. Neff was assumed to be 4.2 from Zeff for 12 C(γ , K + ) [2]. Fig. 3 shows the integrated cross section with the photon energy compared with 12 C(γ , K 0 )X (our data) and 12 C(γ , K + )X [2]. Integrated regions about the kaon production angle were almost the same in the laboratory system, (cos θ of 0.8–1.0 (for K 0 ) and 0.766–0.985 (for K + )). The solid line in Fig. 3 shows the quasi-free production cross section for γ 12 C → K 0 Λ11 C calculated by Eq. (1). The integrated region is the same for the K 0 experimental data. The magnitude of the cross section is almost the same, suggesting that the elementary cross section for K 0 production is of the same order as K + production.
330c
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
Fig. 3. Energy dependence of the cross section for γ C → K 0 X (full circle, our data), and γ C → K + X (open triangle, [2]). Solid line shows calculation for γ C → K 0 X.
Fig. 4. Momentum distribution of cross section. Solid line is calculation for 12 C(γ , K 0 )Λ11 C, and dashed line is that for 12 C(γ , K 0 )Σ 11 X data.
Fig. 4 shows momentum spectra of the outgoing kaons. The points show experimental data for C(γ , K 0 ). Solid lines are the calculated cross section of the γ n → K 0 Λ process by Eq. (1). Dashed lines include γp → K 0 Σ + and γ n → K 0 Σ 0 processes. As seen in
T. Watanabe et al. / Nuclear Physics A 754 (2005) 327c–331c
331c
the figure, the contribution of Σ production processes are small compared with Λ production as expected in the threshold region. Kaon-MAID predictions well represent present the experimental data. In summary, we measured K 0 production cross section with photon energy and K 0 momentum dependence for the first time. The experimental data are compared with elementally cross section of Kaon-MAID using a quasi-free cross section calculation (1).
Acknowledgements This work is supported by Grant-In-Aid for Scientific Research from The Ministry of Education of Japan, Nos. 09304028, 12002001, and by the 21COE program of MEXT Japan.
References [1] H. Yamazaki, et al., Research Report of Laboratory of Nuclear Science, vol. 34, 2001, p. 25. [2] H. Yamazaki, Phys. Rev. C 52 (1995) 1157–1160. [3] C. Bennhold, H. Haberzettl, T. Mart, nucl-th/9909022, web address is http://www.kph.uni-mainz.de/MAID/ kaon/kaonmaid.html.