Dielectron Production inC12+C12Collisions at2A GeVwith the HADES Spectrometer

Abstract

The invariant-mass spectrum of $e^{+}{e}^{-}$ pairs produced in ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ collisions at an incident energy of 2 GeV per nucleon has been measured for the first time. The measured pair production probabilities span over 5 orders of magnitude from the ${\pi }^0$-Dalitz to the $\rho /\omega$ invariant-mass region. Dalitz decays of ${\pi }^0$ and $\eta$ account for all the yield up to $0.15\mathrm{GeV}/c^2$, but for only about 50% above this mass. A comparison with model calculations shows that the excess pair yield is likely due to baryon-resonance and vector-meson decays. Transport calculations based on vacuum spectral functions fail, however, to describe the entire mass region.