Transition in yield and azimuthal shape modification in dihadron correlations in relativistic heavy ion collisions

Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements of π0 trigger particles at transverse momenta pTt=4-12GeV/c and associated charged hadrons (pTa=0.5-7GeV/c) vs relative azimuthal angle Δφ in Au+Au and p+p collisions at √sNN=200GeV. The Au+Au distribution at low pTa, whose shape has been interpreted as a medium effect, is modified for pTt<7GeV/c. At higher pTt, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured pTa, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in Au+Au relative to p+p (IAA) is suppressed at high pT (IAA ≈ 0.35-0.5), but less than for inclusive suppression (RAA ≈ 0.2).