Ratios of hadronic abundances are analyzed for $\mathrm{pp}$ and nucleus-nucleus collisions at $\sqrt{s}\approx 20\mathrm{GeV}$ using the microscopic ultrarelativistic quantum molecular dynamics transport model. Secondary interactions significantly change the primordial hadronic composition of the system. A strong dependence on rapidity is predicted. Without assuming thermal and chemical equilibrium, predicted hadron yields and ratios agree with many of the data ( $\pi /p$, $d/p$, $\overline{p}/p$, $\overline{\Lambda }/\Lambda$, $\overline{\Xi }/\overline{\Lambda }$, etc.).

• Received 19 November 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.4092