Investigating Sources of Angular Correlations at High pT in Nucleon-Nucleon and Nucleus-Nucleus Collisions at the CERN SPS

TL;DR

This study investigates the sources of angular correlations at high transverse momentum in various collision systems at CERN SPS energies, revealing persistent away-side correlations and energy-dependent near-side depletion, challenging standard parton energy loss models.

Contribution

It provides the first comprehensive energy and system-size scan of two-particle azimuthal correlations at SPS energies, comparing experimental data with UrQMD simulations to identify different correlation sources.

Findings

Away-side correlations are weakly dependent on collision energy.

Near-side peak diminishes and turns into depletion below 8.8 GeV.

UrQMD reproduces away-side but not near-side correlations.

Abstract

Angular correlations of high-pT hadrons can serve as a probe of interactions of partons with the dense medium produced in high-energy heavy-ion collisions but other effects may also be important at SPS energies. To study the various contributions, NA49 has performed an energy and system-size scan of two-particle azimuthal correlations in Pb+Pb, Si+Si and p+p collisions at sqrt(sNN) = 17.3 GeV, as well as central Pb+Pb collisions at 6.3, 7.6, 8.8 and 12.3 GeV. These results were compared to UrQMD simulations. NA49 results show a flattened away side of C2(dphi) in central Pb+Pb (Au+Au) collisions which depends weakly on collision energy even for low SPS energies. This is at odds with the standard scenario of parton energy loss. On the other hand, the near-side peak amplitude drops visibly with decreasing collision energy, turning into a depletion below sqrt(sNN) = 8.8 GeV. UrQMD describes the correlation functions on the away side but disagrees on the near side.