A Search for Low-Mass
Dielectron Production in Proton-Berylium Collisions at 2.1 GeV
Jeffrey Gordon
We have measured inclusive dilepton (e+e-)
production in the reacton p + 9Be --> e+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 (pt) and laboratory rapidity (y_lab).
We used a movable two-arm spectrometer with adequate electron efficiency
and hadron rejection. The acceptance of our spectrometer was
suitable for studying low-mass pairs with THETA=90. Direct dilepton
production (low-mass lepton pair yield in excess of explainable
sources, mainly Pi0 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 pt^2 yield was observed to be peaked at zero and
follow an approximate exp(-6pt^2) dependence. Evidence is presented for
a structure in the invariant mass cross section centered at approximately 2m
with an average pt 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>0.20 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-pt direct dileptons
may clarify the elementary production process(es).
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