International Conference on Exotic Atoms and Related Topics - EXA2014

Recently, a study of kaonic nuclei have been actively conducted because they have a rich information, such as the sub-threshold Kbar-N interaction and the behavior of $\Lambda$(1405) in many body systems. While the existence of kaonic nuclei has been intensively studied both theoretically and experimentally, there is no conclusive result establishing its existence. Here, we have carried out a experiment as a first phase to search for a $K^-pp$ bound state, which is considered the simplest kaonic nucleus, by using the $d(\pi^+, K^+)X$ reaction at 1.69 GeV/$c$ (J-PARC E27 experiment). Since the production cross section of the $K^-pp$ would not be so large in this reaction, coincidence of high-momentum ($>$ 250MeV/c) proton(s) in large emission angle ($39^{o}$-$122^{o}$) was required to enhance the signal to background ratio. We have obtained an inclusive $d(\pi^+, K^+)X$ spectrum in a wide missing-mass range from $\Lambda$, $\Sigma$ to $\Lambda$(1405)/$\Sigma$(1385), in high statics and high energy resolution for the first time. A broad enhancement in the proton coincidence spectra are observed around the missing-mass of 2.27 GeV/$c^2$, which might be attributed to the $K^-pp$ production. We will report the results both on inclusive and coincidence analyses.