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 $8<p_{\mathrm{T}}^{\mathrm{trig}}<16\mathrm{GeV}/c$ and associated charged particles of $0.5<p_{\mathrm{T}}^{\mathrm{assoc}}<10\mathrm{GeV}/c$ versus the azimuthal angle difference $\Delta \phi$ 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 $|\Delta \phi |<0.7$ on the near and for $|\Delta \phi -\pi |<1.1$ 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 $p_{\mathrm{T}}^{\mathrm{assoc}}>3\mathrm{GeV}/c$, while with decreasing momenta an enhancement develops reaching about $5$ at low $p_{\mathrm{T}}^{\mathrm{assoc}}$. On the near side, an enhancement of $I_{\mathrm{AA}}$ between $1.2$ at the highest to $1.8$ at the lowest $p_{\mathrm{T}}^{\mathrm{assoc}}$ 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 $p_{\mathrm{T}}^{\mathrm{assoc}}$. Only AMPT captures the enhancement at low $p_{\mathrm{T}}^{\mathrm{assoc}}$, both on the near and away side. However, it also underpredicts $I_{\mathrm{AA}}$ above $5$ GeV/$c$, in particular on the near-side.

26 pages, 5 captioned figures, 1 table, authors from page 20, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/3021

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