Energy dependence of hyperon production in nucleus-nucleus collisions at SPS

A measurement of strange baryon and antibaryon production in Pb-Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 A GeV/c beam momentum. Results on $\Lambda$, $\Xi$ and $\Omega$ hyperon yields at mid-rapidity in the most central 53% of Pb-Pb collisions at 40 A GeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range. The $\Lambda$ and $\Xi^-$ yields per unit rapidity stay roughly constant while those of $\Omega^-$, $\bar\Lambda$, $\bar\Xi^+$ and $\bar\Omega^+$ increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in $\sqrt{s_{NN}}$ = 130 GeV Au-Au collisions at RHIC.