Energy Dependance of Azimuthal Correlations (and more) in Nucleus-Nucleus Collisions at the CERN SPS

TL;DR

This study investigates how azimuthal correlations in heavy-ion collisions at CERN SPS energies vary with collision energy, revealing energy-independent away-side peaks and energy-dependent near-side peaks, providing insights into the medium's properties and the onset of deconfinement.

Contribution

First to perform detailed azimuthal correlation scans at SPS energies, comparing results with UrQMD simulations to explore medium effects and the onset of deconfinement in heavy-ion collisions.

Findings

Away-side peak is energy-independent, suggesting non-jet origins.

Near-side peak amplitude decreases with energy, disappearing below 8.8 GeV.

Hints of ridge structure observed at 17.3 GeV.

Abstract

Analysing two-particle azimuthal and pseudorapidity-azimuth correlations of high-pT hadrons, experiments at the RHIC have obtained results - "the double hump", "the ridge" etc. - which provide interesting information about properties of the hot, dense medium produced in central high-energy heavy-ion collisions as well as of its interaction with products of hard scattering. However, not all of the observed effects have been fully explained by theory or models yet; such explanations are hampered in part by the lack of good reference from lower collision energies. In an attempt to further our understanding of angular dihadron correlations NA49 has, as one of the first experiments at the SPS, performed a number of scans of these correlations in Pb+Pb, Si+Si and p+p collisions at sqrt(sNN) = 17.3 GeV, as well as central Pb+Pb collisions at sqrt(sNN) = 12.3, 8.8, 7.6 and 6.3 GeV. Results of these scans have been compared to UrQMD simulations. Our experimental results from two-particle azimuthal correlations show a flattened away-side peak in central Pb+Pb (Au+Au) collisions, only weakly depends on collision energy over the whole investigated energy range - suggesting this effect originates from sources other than jet-medium interactions. This hypothesis is reinforced by good agreement with UrQMD regardless of whether jet production was enabled in the model or not. On the other hand, the amplitude of the near-side peak in central Pb+Pb (Au+Au) collisions drops visibly with decreasing collision energy, flattening out around sqrt(sNN) = 8.8 GeV and turning into a depletion below that energy - possibly an effect related to the onset of deconfinement. Finally, hints of the ridge structure can be observed in pseudorapidity-azimuth correlation functions from central Pb+Pb collisions at sqrt(sNN) = 17.3 GeV.