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 $Θ^+$ resonance as a bound state of kaon, pion and nucleon. This model explains almost all properties of the $Θ^+$, however, the model works only if the $K π$ 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 $K^+ + N \rightarrow X^+ + N$ reaction at P$_K\sim$ 1.2 GeV/c. The $X^+ \rightarrow K^+ γγ$ decay produces $K^+$ in momentum region where other processes cannot fill. We observed signature of the $X^+$ with statistical significance of 2 $σ$. Production cross section of $X$ with respect to that of $π^0$ is 1$\pm$0.5% if we take it as an evidence and 1.5% if we set an upper limit.

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