Minimum Bias production cross sections of $\eta$ mesons have been measured in 200 AGeV/c S+Au and S+S collisions at the CERN SPS by reconstructing the $\eta \to \gamma\gamma$ decay. The measurements have been made over the rapidity range $2.1 \leq y \leq 2.9$ using the leadglass spectrometer of WA80. Within the statistical and systematical uncertainties the spectral shapes of $\pi^0$ and $\eta$ mesons yields are identical when their invariant differential cross section is plotted as a function of the transverse mass. The relative normalization of the $\eta$ to $\pi^0$ transverse mass spectra is found to be $0.53 \pm 0.07$ for S+Au and $0.43 \pm 0.15$ for S+S reactions. Extrapolation to full phase space leads to an integrated cross section ratio of $\eta$ to $\pi^0$ mesons of $0.147 \pm 0.017 { \rm (stat.)} \pm 0.015 { \rm (syst.)}$, and $0.120 \pm 0.034 { \rm (stat.)} \pm 0.022 { \rm (syst.)}$ for S+Au and S+S collisions, respectively.
Categories: hep-ex