Search for a bound kaon and pion state

Abstract

We have searched for a bound kaon and pion state denoted by X. X was conjectured to explain the so-called + resonance as a bound kaon, pion, and nucleon state. This model explains almost all the properties of +; however, the model only works if the Kπ interaction is attractive and sufficiently strong to make a bound state X. Here we report a result of the search for X by using the K+ + N → X+ + N reaction at PK ∼ 1200 MeV/c. X+ → K+ γγ decay produces K+ in a momentum region where other processes cannot occur. The observed events cannot be explained by other known physical processes. If we assume that these events are due to the production of X, we obtain 1±0.5% as the production of X with respect to that of π0. Since we have no definitive evidence that they indeed correspond to production of X, we give 1.5% as the upper limit. © 2013 The Author(s).