Protoninduced Pion-, Eta- and Kaon-Production at Threshold
  We present some details of our recent theoretical investigations on the total cross section of the reactions $pp\to pp \pi^0$, $pp\to pp \eta$ and $pp\to p\Lambda K^+$ at threshold. Applying standard nonrelativistic perturbation theory within the framework of meson exchange models (MEC) to all of the investigated processes we observe that theory is still far--off from a quantitative understanding of the existing data, although the qualitative behaviour of the threshold cross sections can be reproduced. The often quoted $\eta^2$--dependence of the $pp\to pp \pi^0$ cross section at threshold is due to a calculational mistake of Koltun and Reitan (1966) and not based on theoretical grounds, where threebody phasespace behaviour would yield $\eta^4$, which can be affected by final state and short range effects. To respect the high momentum transfers for $pp\to pp \eta$ and $pp\to p\Lambda K^+$ relativistic models were developed. Hereby the $\eta$--production still is based on MEC, while the $K^+$ production was investigated in the quark-gluon-picture based on the Bethe--Salpeter formalism. 
F. Kleefeld, M. Dillig