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 Δφ 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, v₂ to v₅ , 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.