Dielectron Cross Section Measurements in Nucleus-Nucleus Reactions at1.0AGeV

We present measured dielectron production cross sections for $\mathrm{Ca}+\mathrm{Ca}$, $\mathrm{C}+\mathrm{C}$, $\mathrm{He}+\mathrm{Ca}$, and $d+\mathrm{Ca}$ reactions at $1.0A\mathrm{GeV}$. Statistical uncertainties and systematic effects are smaller than in previous dilepton spectrometer (DLS) nucleus-nucleus data. For pair mass $M\le 0.35\mathrm{GeV}{/c}^2$ we obtain (1) the $\mathrm{Ca}+\mathrm{Ca}$ cross section is larger than the previous DLS measurement and current model results, (2) the mass spectra suggest large contributions from ${\pi }^0$ and $\eta$ Dalitz decays, and (3) $d\sigma /\mathrm{dM}\propto A_P{A}_T$. For $M>\mathrm{}0.5\mathrm{GeV}{/c}^2$ the $\mathrm{Ca}+\mathrm{Ca}$ to $\mathrm{C}+\mathrm{C}$ cross section ratio is significantly larger than the ratio of $A_P{A}_T$ values.