l - ex / 0 40 10 07 v 1 8 J an 2 00 4 Λ and φ Production in Heavy Ion Collisions

Recent NA49 results on Λ and φ production in central Pb-Pb collisions at beam energies from 40 to 158 A·GeV are presented. The measurement of strange baryons like Λ(uds), which contain between 30 and 60% of the total strangeness produced, allows to study simultaneously strangeness production and the effect of baryon density in A-A collisions. Essentially half of the ¯ s quarks are contained in K +. The φ meson consists of a s¯ s pair, and should therefore be more sensitive than kaons and lambdas to the production mechanism in the early stage of the collision. Since 1994 the NA49 collaboration has investigated hadron production in central Pb-Pb collisions at 158 A·GeV over a large range of rapidity and transverse momentum. Within the framework of the NA49 energy scan programme , started five years later, large data sets at lower energies (20, 30, 40, 80 A·GeV) have been recorded, for details see Ref. [1]. In the following, results on Λ hyperon and φ meson production in central Pb-Pb collisions at beam energies 40-158 A·GeV are presented. Starting from the reconstruction method we will focus on the energy dependence of the inverse slope parameter and the particle multiplicities. In NA49, neutral strange and multi-strange particles are identified by the topology of the charged decay products, which are measured with the TPCs. The invariant mass distributions of Λ and ¯ Λ hyperons are given in Ref. [2]. The mass resolution (σ m) of 2 MeV/c 2 is remarkably good. After corrections for acceptance and reconstruction efficiency the m T spectra and rapidity 1