On a possible explanation of the DLS puzzle

The enhancement in the dilepton spectrum observed in heavy-ion collisions for invariant electron-positron masses in the range 0.15 GeV < Me+e- < 0.6 GeV has recently been traced back to a corresponding enhancement in np collisions relative to pp collisions. Whereas the dilepton spectra in the latter are understood quantitatively, theoretical descriptions fail to describe the much higher dilepton rate in np collisions at beam energies below 2 GeV, in particular regarding the region Me+e- < 0.3 GeV. We show that the missing strength can be attributed to ρ0-channel π+π- production, which is dominated by the t-channel ΔΔ excitation and the recently found isoscalar dibaryonic resonance structure at 2.37 GeV. These processes do not contribute in pp collisions, but only in np collisions. (orig.)