Energy dependence of pion and kaon production in central Pb+Pb collisions

TLDR

The study discusses alternative interpretations of the observed particle production patterns. The analysis compares the measured yields with lower‑ and higher‑energy data and with proton–proton interaction results. Charged pion and kaon production in central Pb+Pb collisions shows a linear rise of mean pion multiplicity per wounded nucleon with √sNN^1/4, a change of slope around 15–40 A GeV, a transition from pion suppression to enhancement near 40 A GeV, and a nonmonotonic K⁺/π⁺ ratio peaking near 40 A GeV, suggesting increased entropy production and a reduced strangeness‑to‑entropy ratio consistent with deconfined matter above these energies.

Abstract

Measurements of charged pion and kaon production in central Pb+Pb collisions at 40, 80, and 158 A GeV are presented. These are compared with data at lower and higher energies as well as with results from $p+p$ interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with ${s}_{\mathrm{NN}}^{1/4}$ with a change of slope starting in the region 15--40 A GeV. The change from pion suppression with respect to $p+p$ interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40A GeV. A nonmonotonic energy dependence of the ratio of ${K}^{+}$ to ${\ensuremath{\pi}}^{+}$ yields is observed, with a maximum close to 40A GeV and an indication of a nearly constant value at higher energies. The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central Pb+Pb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.