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 \(8 < p_{\mathrm{T}}^{\rm trig} < 16 \mathrm{GeV}/c\) and associated charged particles of \(0.5 < p_{\mathrm{T}}^{\rm assoc} < 10 \mathrm{GeV}/c\) versus the azimuthal angle difference \(\Delta\varphi\) at midrapidity in pp and central Pb-Pb collisions at \(\sqrt{s_{\mathrm{NN}}}=2.76\) TeV with ALICE. The new measurements exploit associated charged hadrons down to \(0.5 \mathrm{GeV}/c\), which significantly extends our previous measurement that only used charged hadrons above \(3 \mathrm{GeV}/c\), After subtracting the contributions of the flow background, \(v_2\) to \(v_5\), the per-trigger yields are extracted for \(|\Delta\varphi|<0.7\) on the near and for \(|\Delta\varphi-\pi| < 1.1\) on the away side. The ratio of per-trigger yields in Pb--Pb to those in pp collisions, \(I_{\mathrm{AA}}\), 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 \(I_{\mathrm{AA}} \approx 0.6\) for \(p_{\mathrm{T}}^{\rm assoc} > 3 \mathrm{GeV}/c\), while with decreasing momenta an enhancement develops reaching about \(5\) at low \(p_{\mathrm{T}}^{\rm assoc}\), On the near side, an enhancement of \(I_{\mathrm{AA}}\) between \(1.2\) at the highest to \(1.8\) at the lowest \(p_{\mathrm{T}}^{\rm assoc}\) 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 \(p_{\mathrm{T}}^{\rm assoc}\), Only AMPT captures the enhancement at low \(p_{\mathrm{T}}^{\rm assoc}\), both on the near and away side. However, it also underpredicts \(I_{\mathrm{AA}}\) above \(5\) GeV/\(c\), in particular on the near-side.