Excess dileptons in high-energy nucleus-nucleus collisions

It has been observed recently that while continuum dilepton production and open charm production in high-energy pA collisions can be described in terms of the superposition of pp collisions, dilepton yields in S+U collisions are in excess of similar extrapolations. This feature can be explained as arising from the interaction of gluons produced in different soft baryon-baryon collisions, leading to additional open-charm pairs in nucleus-nucleus collisions but not in pA collisions.