Transverse mass distributions of strange particles produced in Pb-Pb collisions at 158 A GeV/c

Experiment NA57 has collected high statistics, high purity samples of $K^0_s$, $\Lambda$, $\Xi$ and $\Omega$ produced in Pb-Pb collisions at 158 $A$ GeV/$c$. In this paper we present a study of the transverse mass spectra of these particles for a sample of events corresponding to about the most central 55% of the inelastic Pb-Pb cross section. We analyse the transverse mass distributions in the framework of the blast-wave model for the full sample under consideration and, for the first time at the SPS, as a function of the event centrality.