Search for a bound state of kaon and pion

TL;DR

This study searched for a hypothesized bound state of kaon and pion related to the $ heta^+$ resonance, finding a tentative signal with limited statistical significance and setting an upper limit on its production cross section.

Contribution

First experimental search for the kaon-pion bound state $X$ associated with the $ heta^+$, providing initial evidence and constraints on its production.

Findings

Observed a 2σ signature of the $X$ state.

Estimated production cross section of $X$ as 1±0.5% of $\pi^0$ production.

Set an upper limit of 1.5% on the $X$ production cross section.

Abstract

We have searched for a bound state of kaon and pion denoted by $X$. The $X$ was conjectured to explain the so-called $\Theta^+$ resonance as a bound state of kaon, pion and nucleon. This model explains almost all properties of the $\Theta^+$, 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 $K^+ + N \rightarrow X^+ + N$ reaction at P$_K\sim$ 1.2 GeV/c. The $X^+ \rightarrow K^+ \gamma \gamma$ decay produces $K^+$ 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.