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 < ptrig T < 16 GeV/c and associated charged particles of 0.5 < passoc T < 10 GeV/c versus the azimuthal angle difference ϕ at midrapidity in pp and central Pb–Pb collisions at √sNN = 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 |ϕ| < 0.7 on the near and for |ϕ − π| < 1.1 on the away side. The ratio of per-trigger yields in Pb–Pb to those in pp collisions, IAA, 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 IAA ≈ 0.6 for passoc T > 3 GeV/c, while with decreasing momenta an enhancement develops reaching about 5 at low passoc T . On the near side, an enhancement of IAA between 1.2 at the highest to 1.8 at the lowest passoc T 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 passoc T . Only AMPT captures the enhancement at low passoc T , both on the near and away side. However, it also underpredicts IAA above 5 GeV/c, in particular on the near-side.