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 <ptrigT<16GeV/cand associated charged particles of 0.5 <passocT<10GeV/cversus the azimuthal angle difference ϕat midrapidity in pp and central Pb–Pb collisions at √sNN=2.76TeVwith ALICE. The new measurements exploit associated charged hadrons down to 0.5GeV/c, which significantly extends our previous measurement that only used charged hadrons above 3GeV/c. After subtracting the contributions of the flow background, v2to v5, the per-trigger yields are extracted for | ϕ| <0.7on the near and for | ϕ−π| <1.1on 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≈0.6for passocT>3GeV/c, while with decreasing momenta an enhancement develops reaching about 5at low passocT. On the near side, an enhancement of IAAbetween 1.2at the highest to 1.8at the lowest passocTis 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 passocT. Only AMPT captures the enhancement at low passocT, both on the near and away side. However, it also underpredicts IAAabove 5GeV/c, in particular on the near-side.