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

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 , 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(Δϕ) 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 . UrQMD describes the correlation functions on the away side but disagrees on the near side.