Hadron formation time and dilepton mass spectra in heavy ion collisions

We point out that formation time of pions produced in heavy ion collisions modifies the mass spectrum of dileptons produced via π+π− → e+e−. Increasing formation time enhances the production of dileptons with lower masses. The effect offers an explanation of a part of the enhanced production of dileptons below the ϱ-meson mass as observed by the CERES and HELIOS Collaborations at the CERN SPS.