From the figure above, we see that hadrons would be scattered by strong interaction when other hadrons are around. In our experiment, the collisions of heavy ions produce a hadronic enviroment and thus all the hadrons comming out of the center of collision would be heavily scattered until they leave the hadronic enviroment. By the time they reach the detector, they no longer contain enough information about the initial hot, dense nucleus produced by the collision. On the other hand, dilepton pairs does not have strong interaction, saving them from losing the information the have as they merge toward the detector. That's why we use dilepton as the probe.

Wei Yuan Yang
National Taiwan University
Physics Department