In ultrarelativistic heavy ion collisions the bulk of the particles is produced in the transverse momentum region below 2 GeV/c. Nevertheless, important information about the produced matter comes from the rare particles in the higher p_t domain. The nuclear modification factor and baryon/meson ratios observed at RHIC indicate the presence of a dense colour-charged medium and suggest hadronisation via quark coalescence. A study of these observables at the SPS can shed additional light on the nature of the matter created at lower energies, complementing the picture obtained from the multitude of other observables.
New results on the p_t dependence of pion, proton, charged kaon, K^0_s and Lambda production at midrapidity in Pb+Pb interactions at 158A GeV beam energy are presented. The most relevant ingredients of the data analysis are discussed. These extend reliable track reconstruction to above p_t=2.5GeV/c for charged particles and allow identification of neutral strange particles via their decay topology in the high track density environment of Pb+Pb collisions.
The nuclear modification factor R_CP is extracted for pions, protons and charged kaons. Particle yield ratios are presented, exhibiting high (>>1) values of the baryon to meson ratios at high transverse momentum. These results are compared to RHIC data and theoretical calculations.