In-medium Modifications of Hadrons and the NA60 dimuon measurements

The theoretical understanding of dimuon spectra as measured in 158~$A\,$GeV In-In collisions by the NA60 collaboration at the CERN SPS is summarized. The low-mass region, $M \leq 0.9 \; \mathrm{GeV}$, is well described by in-medium modifications of the $\rho$-meson spectral function within a hadronic many-body approach. To account for the yield in the intermediate-mass region, $0.9 \; \mathrm{GeV} \leq M \leq 1.5 \; \mathrm{GeV}$, four-pion contributions in the electromagnetic emission function have to be taken into account. The data are consistent with the assumption of chiral mixing of isovector-vector and -axialvector currents, which could be indicative for the onset of chiral-symmetry restoration in heavy-ion collisions. Our calculation also includes the contribution from the quark-gluon plasma phase which turns out to be small compared to that of the hadronic source. Predictions for the modifications of $\omega$- and $\phi$-meson spectral shape may be experimentally checked in future experiments.