Speaker
Description
The existence of a quasi-bound state of antikaon and nucleus, kaonic nucleus, has been discussed ever since the $\bar{K}N$ interaction in $I=0$ channel was confirmed to be strong attractive.
The $\bar{K}NN$ quasi-bound state is the lightest kaonic nucleus which is considered to be $I=1/2$ and $J^\pi = 0^-$.
To search for the $I_{z}=+1/2~\bar{K}NN$ state we conducted the J-PARC E15 experiment using the in-flight $K^-$-beam at J-PARC. Because the $K^-$-beam momentum of $1~{\rm GeV}/c$ used in the experiment maximizes the elementary cross section of nucleon knocked-out reactions, $(K^-,~N)$, the $I_z=+1/2~\bar{K}NN$ state is expected to be produced by sequential reaction of the primary $(K^-,~n)$ reaction followed by an absorption process of intermediate $\bar{K}$ to residual nucleons. Production of the $I_z=+1/2~\bar{K}NN$ state was examined by an exclusive analysis for the simplest non-mesonic reaction, $K^- \, ^{3}{\rm He} \to \Lambda pn$, in which $\Lambda p$ pair is expected to be decay products of the $\bar{K}NN$.In the $\Lambda p$ invariant-mass spectrum, we observed a distinct peak at the energy region below the $\bar{K}NN$ mass threshold. Because its peak position does not depend on the momentum transfer to the $\Lambda p$ system, the peak is produced by a resonance. Although the spectral decomposition was performed using the simple Breite-Wigner formula, whole distribution is reproduced well. The evaluated mass position and decay width are consistent with theoretical predictions, thus we concluded that the observed peak is a signal of the $I_z=+1/2~\bar{K}NN$ state.
As future prospects, there are two approaches to establish the kaonic nuclei more robustly. One is to search for heavier kaonic nuclei, and another is to study the observed $\bar{K}NN$ state more precisely. Thus, we have planed to perform series of experiments to study of kaonic nuclei using in-fight $K^-$ reactions at J-PARC.
As an analogy to $\bar{K}NN$ production with the $(K^-,~n)$ reaction, heavier kaonic nuclei could be produced similarly by replacing $^{3}{\rm He}$-target with heavier targets. As the first step to search for heavier kaonic nuclei, $\bar{K}NNN$ state will be searched for with the $^{4}{\rm He}(K^-,~n)$ reaction. For the $\bar{K}NN$ state, determination of spin-parity is the most important to confirm the observed state is a quantum state as well as to clarify its internal structure. $J^\pi$ of the $\bar{K}NN$ can be determine from the spin-spin correlation of the $\Lambda p$-pair from the $\bar{K}NN$ decay with a model independent manner.
As another measurement for the $\bar{K}NN$, we will measure the $^{3}{\rm He}(K^-,~p)\Lambda n$ reaction to search for the $I_z=-1/2~\bar{K}NN$ state.
To perform these measurements, we will construct a new solenoid spectrometer system to have neutron detection capability and a proton polarimeter system.
I would like to present the summary of J-PARC~E15 experiment and an overview of our future plan.
