Production of positrons with low transverse momentum and low-mass electron-positron pairs in proton-proton collisions at a center-of-mass energy of 63 GeV

The production of inclusive prompt positrons in pp collisions at √s = 63 GeV and y=0 has been measured over the PT interval 0.08 < PT < 1.0 GeV/c. The e+/π ratio rises with decreasing PT to ≅ 1.6 x 10-3 at PT = 0.1 GeV/c. A measurement of low-mass e+e- pairs (0.05 < mee < 0.60 GeV/c2) has also been performed. It is found that the observed rise of the e+/π ratio can be explained by the low-mass continuum. The origin of this pair continuum, which has been observed by several previous experiments, has so far been unclear. In order to obtain some new insight into the possible production mechanism a measurement of the production of single positrons and electron-positron pairs as a function of the associated charge multiplicity has been carried out. The results indicate that the production of low-PT positrons (<0.4 GeV/c) and low-mass pairs is proportional to the square of the mean multiplicity in the central region (vertical strokeyvertical stroke<1). Such a quadratic dependence is not expected from final-state sources such as hadronic bremsstrahlung or hadronic decays but is predicted by models where low-mass electron pairs are produced by interactions of constituents created during the collision. (orig.)