Speaker
Description
Lattice QCD predicts a phase transition from hadronic matter to the Quark-Gluon Plasma (QGP) at high temperature and low baryon chemical potential. Thermal dileptons can be produced throughout the entire evolution of relativistic heavy-ion collisions and do not involve strong interactions. As a result, they can carry original information about their emission source, and are therefore suggested as the ideal probes of the hot and dense medium. In particular, the invariant mass distribution of thermal dielectrons is not subjected to blue-shift effects, which enables the extraction of the average temperature of the hot QCD medium at different stages of the evolution. In this presentation, we report the measurements of thermal dielectron invariant mass spectra in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV with the STAR experiment. The average temperature extracted from thermal dielectrons in the low-mass and intermediate-mass regions will be presented as a function of baryon chemical potential and compared with those from other collision energies and systems. These measurements will shed new light on the understanding of the QGP evolution and the in-medium properties of the $\rho$ meson.
