We present measured dielectron production cross sections for Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A·GeV. Statistical uncertainties and systematic effects are smaller than in previous DLS nucleus-nucleus data. For pair mass M ≤ 0.35 GeV/c 2 : 1) the Ca+Ca cross section is larger than the previous DLS measurement and current model results, 2) the mass spectra suggest large contributions from π 0 and η Dalitz decays, and 3) dσ/dM ∝ A P ·A T . For M > 0.5 GeV/c 2 the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of A P ·A T values.
1Dielectrons produced in heavy-ion collisions are attractive probes for studying dynamical properties of nucleus-nucleus interactions. The e + e − pairs do not undergo significant rescattering in the reaction, thus the kinematics of the pairs retains information about their production. This is of particular interest if the e + e − pairs are produced by processes, such as pion annihilation, that must occur during in the hot, dense phase of the collisions. Use of this probe has produced interesting results at both Bevalac [1] and SPS [2] energies. We present in this letter the latest measurements of dielectron production from the Dilepton Spectrometer (DLS) Collaboration in nucleus-nucleus reactions at a beam kinetic energy of 1.0 A·GeV.The DLS collaboration has previously reported on dielectron production in several colliding systems [1,3,4]. The first generation DLS data from p+Be, Ca+Ca, and Nb+Nb [1,3] reactions provided the first observations of dielectrons produced at Bevalac energies. Early calculations suggested that such data could be dominated by contributions from π + π − annihilation [5,6]. Subsequent models of AA collisions in this energy regime [7][8][9] calculated that e + e − pairs of invariant mass below about 0.4 GeV/c 2 are produced primarily from conventional hadronic sources, such as pn bremsstrahlung and Dalitz decay processes (π 0 , ∆, and η), but that contributions from π + π − annihilation were needed to explain the Ca+Ca data at higher pair masses. Models that focus on density induced changes in the ρ-meson mass provide alternative descriptions of the pair yield at the higher masses [10,11]. Within the limited statistics of the first generation DLS data, it was not possible to distinguish among the models that provided results for specific DLS measurements.After improvements to the DLS apparatus [13][14][15], a second generation of measurements was obtained: first from p+p and p+d reactions at a number of energies [4], and then from the Ca+Ca, C+C, He+Ca, and d+Ca reactions presented in this letter. Each of these data sets contain significantly more pairs than earlier DLS data. To increase our sensitivity to the effects of multiple hadronic interactions (e.g. π + π − annihilation and multi-step resonance excitation), the nucleus-nucleus reactions were chosen to have different numbers of participant nucleons, but identical isospin and similar internal nuclear motion.
2A description of the DLS apparatus has been published [13...
