Searches for supermassiveX−particles in iron

Two searches have been made for negatively charged massive elementary particles ($X^{\mathrm{-}}$) in iron nuclei by utilizing the fact that such systems would have the nuclear properties of iron but the chemical properties of manganese. We have looked for ${}_{}{}^{56}{}_{}{}^{}$,58Fe$X^{\mathrm{-}}$ by searching for γ rays emitted following the β decays of ${}_{}{}^{56}{}_{}{}^{}{X}^{\mathrm{-}}$ and ${}_{}{}^{59}{}_{}{}^{}{X}^{\mathrm{-}}$ produced by (p,n) and (n,γ) reactions, respectively. No evidence of such particles was observed in either experiment, and a limit has been established on the possible concentration of $X^{\mathrm{-}}$ particles in iron of <1.2×${10}^{\mathrm{-}12}$ per nucleon.