a Search for Low-Mass Dielectron Production in Proton-Beryllium Collisions at 2.1 GEV.

We have measured inclusive dilepton (e ^{+}e^{-} ) production in the reaction p + ^9 Betoe^{+ }e^{-} + X at an incident beam kinetic energy of 2.1 GeV. The purpose was to establish the existence of direct lepton pairs in the bombarding energy region below 10 GeV. Previous dilepton studies had found a low-mass (M < 1 GeV/c ^2) electron pair yield (above 10 GeV bombarding energy) in excess of explainable sources. The dileptons were characterized according to their invariant mass (M), transverse momentum (p_{ rm t}) and laboratory rapidity (y _{rm lab}). We used a moveable two-arm spectrometer with adequate electron efficiency and hadron rejection. The acceptance of our spectrometer was suitable for studying low-mass pairs with theta_{rm c.m.} = 90 ^circ. Direct dilepton production (low-mass lepton pair yield in excess of explainable sources, mainly pi^0 Dalitz decay) was observed in the pair mass range of 0.05 <= M <= 0.70 GeV/c^2 . The mass cross section distribution above 0.25 GeV/c^2 was found to be comparable to results found from higher energy experiments, but with a lower yield. The p_{rm t} ^2 yield was observed to be peaked at zero and follow an approximate exp(-6p _{rm t}^2) dependence. Evidence is presented for a structure in the invariant mass cross section centered at approximately 2m_pi with an average p_{rm t} of 0.30 GeV/c. This structure is suggestive of a two real pion annihilation mechanism as a source of low-mass dileptons. The total inclusive e^ {+}e^{-} cross section (for M > GeV/c^2 ) was found to parallel the behavior of total inclusive pi^{+}pi^{ -} cross section in pA collisions, but with a yield of approximately (alpha/2) ^2 lower. The existence of a direct dilepton signal at the lower energies makes possible the use of e^{+}e ^{-} pairs as a probe in AA collisions at the Bevalac and SIS. Further study of low-mass low -p_{rm t} direct dileptons may clarify the elementary production process(es).