Study of K^+ meson production with electromagnetic probes

用K^+光在原子核上产生的实验观测来研究超核光谱和核奇异度被认为是与强子探针的研究相补充的。在研究核奇异性方面，（γ,K^+）反应与其他可能的反应相比有一个优点，那就是在核物质中具有较长的γ和K^+的平均自由程。这一特性使我们能够以较小的变形研究核子和超子在核内部的行为。（pi^+,K^+）反应与（γ,K^+）反应具有相同的运动条件，建立了λ超核的壳层结构。强子探测器取得的这些成果必须与（gamma,K^+）和（e,e'K^+）反应的电磁过程结合起来，才能统一认识超核和核奇异度的产生。由于缺乏合适的设备，在原子核上产生介子光还没有进行过。CEBAF项目将在未来几年内对核奇异性进行更详细的探索。利用东京大学核研究所1.3 GeV电子同步加速器上的标记光子装置，测量了^<12>C（γ,K^+）反应。从同步加速器中提取的1.2 GeV电子束以10%的占空因数引入光子标记系统。用一组能量分辨率为10 MeV的32ch标记镜对0.7 ~ 1.1 GeV的光子束流进行了标记。利用TAGX系统的磁谱仪测量了带电粒子的事件。安装了快速触发计数器和TOF壁来识别先导颗粒。采用两组空间分辨率为lmm的硅条探测器（SSD）来提高跟踪分辨率。通过质谱选择K^+事件。估计介子和质子事件的污染小于5%。得到^<12>C（γ,K^+）截面（初步）。与p（γ,K^+）截面进行比较。由于K^+产率的E_< γ >依赖性与基本过程相似，因此认为准自由过程占主导地位。推导出了有效质子数N_<eff>=3.5，即参与^<12>C（γ,K^+）反应的质子数。与^<12>C（pi,K^+）反应得到的N_<eff>=1.7相比，光- kaon过程可以被认为是研究原子核内部的良好探针。少

Experimental observations of K^+ photoproduction on nuclei to study hypernuclear spectroscopy and the nuclear strangeness have been considered complementary to that of hadronic probes. An advantage of the (gamma,K^+) reaction compared with other possible reactions in studying the nuclear strangeness is on relatively long mean-free path of gamma and K^+ in the nuclear matter. This feature makes us possible to study the behavior of nucleons and a hyperon in the nuclear interior with less deformation. The (pi^+,K^+) reactions, which are same kinematical condition as the (gamma,K^+) reaction, have established shell structure of the LAMBDA hypernuclei. These achievements done by the hadronic probes must be joined with the electromagnetic process of (gamma,K^+) and (e,e'K^+) reactions to get the unified recognition of the hypernuclei and the nuclear strangeness production. Kaon photoproduction on nuclei has not been done because of lack of the suitable facilities. The project of CEBAF will p … More ermit the detailed exploration of the nuclear strangeness in a few years.The measurement of the ^<12>C(gamma,K^+) reaction was carried out by using the tagged photon facilities at the 1.3 GeV electron synchrotron of the Institute for Nuclear Study, University of Tokyo. The 1.2 GeV electron beam extracted from the synchrotron was introduced to the photon tagging system with a duty factor of 10%. Photon beam in an energy range of 0.7-1.1 GeV were tagged by a set of 32ch tagging hodoscopes with an energy resolution of 10 MeV.The events of charged particles were measured by the magnetic spectrometer based on the TAGX system. Fast trigger counters and a TOF wall were installed to identify the leading particles. Two sets of the Silicon Strip Detectors (SSD) with lmm spatial resolution were used to improve the tracking resolution. The K^+ events were selected by the mass spectrum. Contaminations of the pion and proton events are estimated less than 5%. Obtained ^<12>C(gamma,K^+) cross section (preliminary). was compared with the p(gamma,K^+) cross sections. Since the E_<gamma> dependence of the K^+ yield behaves similar to that of the elementary process, the quasi-free process is considered to be dominant. The effective proton number N_<eff>=3.5, which is the number of the participating protons in the ^<12>C(gamma,K^+) reaction have been deduced. Comparing with the N_<eff>=1.7, which was obtained by ^<12>C(pi,K^+) reaction, the photo-Kaon process can be recognized a good probe to investigate the inside of nuclei. Less