Search for Bound States of Neutrons and Negative Pions

An experimental search has been performed for bound systems of neutrons and negative pions, denoted by ${\left({\pi }^{-}\right)}^Z{n}^N$. Such negatively charged nuclear fragments might be emitted in energetic proton-nucleus collisions. Upper limits for production cross sections of $Z=1\mathrm{}$, $N=2-6\mathrm{}$ and $Z=2\mathrm{}$, $N=3\mathrm{}$ systems have been derived by assuming that the lifetime of the bound pion(s) is of the order of that of the free pion or longer.