Jet-like correlations with neutral pion triggers in pp and central Pb-Pb collisions at 2.76 TeV

Abstract We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta 8 p T trig 16 GeV / c and associated charged particles of 0.5 p T assoc 10 GeV / c versus the azimuthal angle difference Δ φ at midrapidity in pp and central Pb–Pb collisions at s NN = 2.76  TeV with ALICE. The new measurements exploit associated charged hadrons down to 0.5 GeV / c , which significantly extends our previous measurement that only used charged hadrons above 3 GeV / c . After subtracting the contributions of the flow background, v 2 to v 5 , the per-trigger yields are extracted for | Δ φ | 0.7 on the near and for | Δ φ − π | 1.1 on the away side. The ratio of per-trigger yields in Pb–Pb to those in pp collisions, I AA , is measured on the near and away side for the 0 – 10 % most central Pb–Pb collisions. On the away side, the per-trigger yields in Pb–Pb are strongly suppressed to the level of I AA ≈ 0.6 for p T assoc > 3 GeV / c , while with decreasing momenta an enhancement develops reaching about 5 at low p T assoc . On the near side, an enhancement of I AA between 1.2 at the highest to 1.8 at the lowest p T assoc is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high p T assoc . Only AMPT captures the enhancement at low p T assoc , both on the near and away side. However, it also underpredicts I AA above 5  GeV/ c , in particular on the near-side.