We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions within two independent transport approaches that are based on quark, diquark, string, and hadronic degrees of freedom. Both transport models show their reliability for elementary $pp$ as well as light-ion ($\mathrm{C}+\mathrm{C}$, $\mathrm{S}\mathrm{i}+\mathrm{S}\mathrm{i}$) reactions. However, for central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ ($\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$) collisions above $\sim 5A\mathrm{G}\mathrm{e}\mathrm{V}$ the measured $K^{\pm }$ transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus, the pressure generated by hadronic interactions in the transport models above $\sim 5A\mathrm{G}\mathrm{e}\mathrm{V}$ is lower than observed in the experimental data. This finding shows that the additional pressure—as expected from lattice QCD calculations at finite quark chemical potential and temperature—is generated by strong partonic interactions in the early phase of central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ ($\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$) collisions.

• Received 25 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.032302