Fig. 1: A schematic view of e⁺e⁻ pair production and the QCD phase diagram.

The diagram illustrates matter properties with baryon chemical potential (equivalent to net baryon number density) and temperature, with landmarks of normal nuclei, neutron stars, and the phase transition to QGP. A heavy-ion collision creates a QGP at high baryon chemical potential and high temperature shortly after the initial impact, and the system evolves along the red arrow toward the phase boundary and hadronization. The insert depicts the dilepton spectrum with low-mass and intermediate-mass ranges corresponding to the dominant emission contribution from the transition and the QGP phases, respectively.