Direct Electron-Pair Production in ${\pi }^{\pm }p$ Interactions at 18 GeV/c

With use of the Stanford Linear Accelerator Center hybrid facility 1-m bubble chamber fitted with tantalum plates, a measurement was made of direct unexplained $e^{+}{e}^{-}$ pair production in 18-GeV/c ${\pi }^{\pm }p$ interactions. Limits are set in ${\pi }^{+}p$ processes. In ${\pi }^{-}p$ a signal is observed which cannot be caused by $\eta$, $\omega$, or $\rho$ decay, and for masses $>m_{{\pi }^0}$, $\frac{e^{\pm }}{{\pi }^{\pm }}=(0.87\mathrm{}\pm 0.25)\times {10}^{-4\mathrm{}}$. Some properties of the events are discussed.