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The research programme of the NA61 collaboration covers a wide range of hadronic physics in the CERN SPS energy range. It encompasses measurements of hadron-hadron, hadron-nucleus as well as nucleus-nucleus collisions. The latter are analyzed to better understand the properties of hot and dense nuclear matter. In this contribution we present results on production in proton-proton interactions at beam energies of 40 and 158 GeV/c.

The ratios of particle multiplicities determined in central collisions of heavy nuclei fit well into the concept of statistical particle production. In fact, statistical model caculations using heavy ion data on particle multiplicities converge on an approximate universal line in the T-μB plane for the resulting values of temperature and baryochemical potential. We discuss whether variations of system size can lead to significant deviations from this universal interrelation. Previous analyses of data from C+C and Si+Si collisions have shown that small systems produce a higher chemical freeze-out temperature than central collisions of heavy projectiles, whereas the baryochemical potential seems to be independent of system size. These findings suggest that in small systems a departure from the universal freeze-out line towards higher temperatures is possible. Based on new data on the centrality dependence of proton and antiproton spectra from NA49, which await a dedicated statistical model analysis, we argue that in peripheral collisions both the freeze-out temperature and the baryochemical potential are lower than in central collisions, suggesting a departure from the universal freeze-out line towards the origin of the phase diagram.