Study of K^+ meson production with electromagnetic probes

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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