Speaker
Description
In recent years, the possible existence of deeply-bound $\bar K$ nuclear bound states has been widely discussed as a consequence of the strongly attractive $\bar KN$ interaction in I = 0 channels. Very recently, J-PARC E15 experiment reported an observation of the simplest kaonic nuclei, $\bar KNN$, in the $\Lambda p$ invariant-spectrum of the in-flight $K^-$ reaction on helium-3 [PLB789(2019)620, PRC102(2020)044002]. If the observed structure is truly the kaonic nuclear state, we can expect other kaonic nuclei can be produced in the same $K^-$ induced reaction. Observation of other kaonic nuclei would provide a further support for the existence of such exotic states. Furthermore, mass-number dependence of kaonic nuclear systems would be of great importance to study interplay between the $\bar KN$ attraction and the $NN$ repulsion at short distances.
Here, we focus on the second simplest system, $\bar KNNN$ with I = 0. This state could be populated by simply replacing the helium-3 target in J-PARC E15 with helium-4. Although the branching ratio is not known, one of the expected decay modes is $\Lambda d$, whose final particles are charged ones only. By detecting the $\Lambda d$ pair and by identifying neutron via the missing-mass method, we can exclusively study the $\Lambda dn$ final states in the same manner of $\Lambda pn$ in E15.
We already had a chance to collect $K^-$-induced data on helium-4 as a feasibility test of a lifetime measurement of light hypernuclei (J-PARC T77). Approximately 6$\times 10^9$ $K^-$ particles at 1 GeV/$c$ are delivered to the helium-4 target in $\sim$3-day beam time in June 2020. This number corresponds to $\sim$1/7 of that in E15. The decay particles are detected with the same cylindrical detector system as E15. In a preliminary analysis, we successfully reconstructed several hundreds of $\Lambda dn$ events.
In this contribution, we would like to present the latest results of the $\Lambda dn$ analysis described above, and discuss future prospects towards more comprehensive investigation of the $\bar KNNN$ system.
