  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.
