Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum $dN\mathrm{/}(d{M}_{\perp }^{2}d{M}^2)$ at fixed transverse mass $M_{\perp }$ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of $dN\mathrm{/}(d{M}_{\perp }^{2}d{M}^2)$ against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum $dN\mathrm{/}d{M}^2$, in the spectrum $dN\mathrm{/}d{M}^{2}d{M}_{\perp }^2$ with fixed $M_{\perp }$ the $\omega$ signal is clearly seen as bump riding on the $\rho$ background even in case of strong in-medium modifications.