On a Possible Explanation of the DLS-Puzzle

Abstract

The enhancement in the dilepton spectrum observed in heavy-ion collisions for invariant electron-positron masses in the range 0.15 GeV/c$^2 < M_{e^+e^-} <$ 0.6 GeV/c$^2$ has recently been traced back to a corresponding enhancement in $pn$ 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 $pn$ collisions, in particular regarding the region $M_{e^+e^-} >$ 0.3 GeV/c$^2$ at beam energies below 2 GeV. We show that the missing strength can be attributed to the $\rho$-channel $\pi^+\pi^-$-production, which is dominated by the $t$-channel $\Delta\Delta$ excitation and the recently found isoscalar dibaryonic resonance structure at 2.37 GeV.