Measurement of kaons in the Helios experiment

Minimum Bias production cross sections of η mesons have been measured in 200 AGeV/cS+Au and S+S collisions at the CERN SPS by reconstructing the η → γγ decay. The measurements have been made over the rapidity range 2.1 ≤ y ≤ 2.9 using the leadglass spectrometer of WA80. Within the statistical and systematical uncertainties the spectral shapes of π⁰ and η mesons yields are identical when their invariant differential cross section is plotted as a function of the transverse mass. The relative normalization of the η to π⁰ transverse mass spectra is found to be 0.53±0.07 for S+Au and 0.43±0.15 for S+S reactions. Extrapolation to full phase space leads to an integrated cross section ratio of η to π⁰ mesons of 0.147±0.017(stat.)±0.015(syst.), and 0.120±0.034(stat.)±0.022(syst.) for S+Au and S+S collisions, respectively.

It is shown that in ultra-relativistic nucleus-nucleus collisions, as a consequence of the geometry of the collision, there is a dependence of the total and differential cross sections on effective atomic mass numbers of the target and beam nuclei. The general expressions of the effective atomic mass numbers are deduced and they are computed for 0+Cu, O+U, S+U, Si+Au and U+U collisions. This particular A-dependence of the cross sections can have a strong effect on the production ratio of two particles of different species. As a consequence it might simulate a signature of quark-gluon plasma which is expected to be given by the yields ratio of two different particles. It is shown that this effect can explain quantitatively the K/π ratios measured in Si+Au and S+W collisions as well as the decrease of the J/ψ production relative to Drell-Yan lepton pairs measured in O+Cu, O+U and S+U collisions. The need of precise measurements of the powers α and α(x_F, P_T) in inclusive nuleon-nuleus interactions for the understanding of particle production in nucleus-nucleus collisions is stressed. Expectations for future experiments are presented.

Low mass muon pair production at high P_T and low X_F studied in pU, OU and SU 200 GeV per nucleon react ions. When energy density or projectile mass are increased, φ production is enhanced as compared with the yield of muon pairs in the mass continuum (1.7<M_μμ< 2.4 GeV/c²), whereas the production of ω and ϱ, experimentally unresolved, remains approximately constant. This φ enhancement is in agreement with predictions based on quark-gluon plasma formation and, together with the previously reported J/Ψ suppression, puts severe constraints on a purely hadronic description of nucleus-nucleus collisions.