Search for massive penetrating particles produced by 300-GeV protons

We have searched for negative long-lived penetrating particles of mass between 1.0 and 6.8 GeV/${\mathit{c}}^2$ produced by 300-GeV protons incident on Cu. At 2.38-GeV/c transverse momentum no event was found as compared to a yield of 1.28 × ${10}^9$ ${\pi }^{-}$. The 90%-confidence-level upper limit for the invariant production cross section per nucleon is $\frac{\mathrm{Ed}\sigma }{d^{3}p}=5.4\mathrm{}\times {10}^{-39\mathrm{}}$ ${\mathrm{cm}}^2$ Ge${\mathrm{V}}^{-2\mathrm{}}$.