Hard $\pi^0$ and $\eta$ production in S + Au and Pb + Pb nuclear collisions at SPS energies and the possible signature of quark–gluon plasma

Abstract

Hard $\pi^0$ and $\eta$ production is investigated in Landau and Bjorken hydrodynamical models, taking a great number of hadronic resonances (16 and 42) in the hadronic phase into account. Central and peripheral S + Au and Pb + Pb collisions are considered. We consider two different scenarios: one with quark–gluon plasma (QGP) formation and one with only hadronic gas without QGP. The Cronin effect and hard direct meson emission are taken into account. It is shown that these two scenarios give similar $p_{\perp}$ pion spectra, which approximately agree with experimental data for S + Au collisions obtained by the WA80 Collaboration. Therefore we conclude that it is difficult to extract proof of QGP formation from hadronic spectra. However, we also calculate the $\eta/\pi^0$ ratio. We show that this value agrees with experimental data only for the scenario with plasma and a phase transition for hadrons. The scenario of hadronic gas without plasma disagrees with experimental data.