First Observation of Dielectron Production in Proton-Nucleus Collisions below 10 GeV

Abstract

We have begun a program to measure dielectron production in $p$-nucleus and nucleus-nucleus collisions at the LBL Bevalac. Results are presented for the reaction $p+\mathrm{Be}$ at 4.9 GeV. For the first time, direct dilepton production is observed below 10 GeV incident energy. The cross sections are discussed and compared to previous data at higher energies. The observation of a structure at a mass of about 275 MeV suggests that pion annihilation may be the dominant production mechanism in this mass range.