What the NA60 dilepton data can tell

Abstract

The NA60 experiment has studied low-mass muon pair production in In-In collisions at 158 AGeV with unprecedented precision. Thus, there is hope that the in-medium modifications of the vector meson spectral function can be constrained more thoroughly than from previous experiments. Towards this goal, we present a study comparing different models for the in-medium modification of vector mesons. In particular, we investigate what can be learned about collisional broadening by a medium at finite temperature and baryon density and what model independent constraints can be obtained from the data. Of special relevance is a comparison to $p_T$-cuts of the dilepton spectra. Those not only provide independent tests of the evolution model and $p_T$-dependence of the spectral function but also unambiguously decide if the observed yield in the higher mass spectrum $M > 1 {\rm GeV}$ is mainly due to the hot phase of the evolution or four pion annihilation processes.