Hadron production in S+Ag and S+Au collisions at 200 $\text{GeV}\text{nucleon}$

In the vicinity of the quark-hadron critical point, in the phase diagram, simple power-law relations constrain the mid-rapidity net-baryon density, for different heavy-ion processes, in a unifying scheme. These relations reveal a characteristic trend of the SPS experiments (S + S, S + Ag, S + Au, Pb + Pb) supporting the hypothesis of a critical point of second order, at nonzero baryon density. However, the central events in these experiments stay outside the critical area, especially in Pb + Pb collisions, suggesting the absence of strong critical fluctuations in these processes. The role of noncentral events and the possibility to reach the QCD critical point at RHIC and LHC, is also discussed.

Theoretical and experimental studies of in-medium properties of hadrons in hot and/or dense matter is one of the most active frontiers in nuclear physics. High-energy heavy-ion collisions provide the possibility to create in laboratory strongly interacting matter at finite density and temperature. Electromagnetic signals, namely photon and dileptons spectra, provide a way to detect and study directly the properties of hadrons in the early stage of high-energy heavy-ion collisions. In this review, we first present an overview of current experimental efforts towards the measurement of dilepton and photon spectra in heavy-ion collisions at CERN SPS energies. We then review in detail the various theoretical investigations of and explanations for the low-mass dilepton enhancement, intermediate-mass dilepton enhancement, and direct photon production observed in heavy-ion collisions at CERN SPS. We emphasize the possibility of learning about the in-medium properties of hadrons, especially those of vector mesons, through these experimental and theoretical efforts.

Strange and multistrange baryon production is expected to be enhanced in heavy ion interactions if a phase transition from hadronic matter to a Quark-Gluon Plasma takes place. The production yields of Λs, $\text{Λ}$s, Ξ⁻s, and $\text{Ξ}{}^{\mathrm{+}}$s relative to the production of negative particles are presented for sulphur-tungsten interactions at 200 GeV/c per nucleon. These production yields are compared to those produced in proton-tungsten interactions and the enhancements of strange and multistrange baryons and antibaryons are presented.

We study systematically the production of strangeness in nuclear reactions from SIS to SPS energies within the covariant hadronic transport approach HSD. Whereas the proton and pion rapidity distributions as well as pion transverse momentum spectra are well described in the hadronic transport model from 2–200 A GeV, the K⁺ and K⁻ spectra are noticeably underestimated at AGS energies while the K⁺ spectra match well at SIS and SPS energies with the experimental data. We conclude that the failure of the hadronic model at AGS energies points towards a non-hadronic phase during the collision of heavy systems around 10 A GeV.