Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

Abstract We report measurements of electron pair production in elementary p + p and d + p reactions at 1.25 GeV / u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n + p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass M e + e − > 0.15 GeV / c 2 is about an order of magnitude larger in n + p reactions as compared to p + p . A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C + C reactions are compatible with a superposition of elementary n + p and p + p collisions, leaving little room for additional electron pair sources in such light collision systems.