Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 $\Theta^+$ 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.

37
The categories of this paper are: supr-con
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
3
The categories of this paper are: physics
19
The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
10
The categories of this paper are: q-bio
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat