Search for a bound state of kaon and pion

We have searched for a bound state of kaon and pion denoted by $X$. The $X$ was conjectured to explain the so-called ${\mathrm{\Theta }}^{+}$ resonance as a bound state of kaon, pion and nucleon. This model explains almost all properties of the ${\mathrm{\Theta }}^{+}$, however, the model works only if the $K\pi$ interaction is strongly attractive. It is so strong that it could make a bound state $X$. Here we report a result of the search for the $X$ by using the $K^+ + N \rightarrow X^+ + N$reactionatP$_K\sim$1.2GeV\mathrm{/}c\mathrm{.}The$X^+ \rightarrow K^+ \gamma \gamma$decayproduces$K^+$ in momentum region where other processes cannot fill. We observed signature of the $X^{+}$ with statistical significance of 2 $\sigma$. Production cross section of $X$ with respect to that of ${\pi }^0$ is 1$\pm$0.5% if we take it as an evidence and 1.5% if we set an upper limit.