What does the $rho$ do? Lessons from NA60's di-muon measurement.

Joerg Ruppert

The NA60 experiment has studied low-mass muon pair production in In- In collisions at 158 AGeV with unprecedented precision. We present a systematic study of different theoretically proposed models of in-medium changes of the $\rho$-meson compared to the data. We especially show that the NA60 measurements are a proof of considerable in-medium broadening effects of the $\rho$-meson. No indications are found that the $\phi$-meson changes considerably in the medium. We also studied what the data tell about possible scaling laws of the $\rho$-meson mass with the chiral condensate as e. g. Brown-Rho scaling. We argue that - differently from what was presented at QM 2005 - the correlation of changes in the mass and the width of the $\rho$-meson have to be taken into account in order to test those scaling laws.