Results on ,¯ , − , and¯ + production in central Pb+Pb reactions at 20A, 30A, 40A, 80A, and 158A GeV are presented. The energy dependence of transverse mass spectra, rapidity spectra, and multiplicities is discussed. Comparisons to string hadronic models (UrQMD and HSD) and statistical hadron gas models are shown. Although the latter provide a reasonable description of all particle yields, the first class of models fails to match the − and¯ + multiplicities.

Kaons and protons carry large parts of two conserved quantities, strangeness and baryon number.It is argued that their correlation and thus also fluctuations are sensitive to conditions prevailing at the anticipated parton-hadron phase boundary. Fluctuations of the (K + +K − )/(p+ p) and K + /p ratios have been measured for the first time by NA49 in central Pb+Pb collisions at 5 SPS energies between √ s NN = 6.3 GeV and 17.3 GeV. Both ratios exhibit a change of sign in σ dyn , a measure of non-statistical fluctuations, around √ s NN = 8 GeV. Below this energy, σ dyn is positive, indicating higher fluctuation compared to a mixed event background sample, while for higher energies, σ dyn is negative, indicating correlated emission of kaons and protons. The results are compared to hadronic transport model calculations which fail to reproduce the energy dependence.