Seminar in Hadronic Physics

Dimuon transverse momentum spectra as a tool to characterize the emission region in heavy-ion collisions

Thorsten Renk

University of Jyväskylä & University of Helsinki

Previous dilepton measurements in heavy-ion collisions have mainly focused on invariant mass spectra to clarify in-medium changes of vector meson properties. However, a dimuon is characterized by two scales — invariant mass M and transverse momentum p_T. Like transverse momentum spectra of hadrons, p_T spectra of dileptons arise from an interplay between emission temperature and collective transverse flow, whereas the invariant mass is insensitive to flow. Having two control parameters of which only one is sensitive to flow allows at given M to characterize the emission region in terms of average temperature and flow. Thus, one is able to study what phases of the fireball evolution radiate into a given mass window. I demonstrate this technique using the dimuon transverse momentum spectra measured in InIn collisions by the NA60 collaboration and present strong arguments that a thermalized evolution phase with T > 170 MeV leaves an imprint in the spectra.