Abstract
Invariant yields of neutral pions at midrapidity in the transverse momentum range $0.6 < p_{T} < 12 GeV/c$ measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV are presented for six centrality classes. The pp reference spectrum was measured in the range $0.4 < p_{T} < 10 GeV/c$ at the same center-of-mass energy. The nuclear modification factor, $R_{\rm AA}$, shows a suppression of neutral pions in central Pb-Pb collisions by a factor of up to about $8-10$ for $5 \lesssim p_{T} \lesssim 7 GeV/c$. The presented measurements are compared with results at lower center-of-mass energies and with theoretical calculations.
Highlights
Quantum chromodynamics (QCD) predicts a transition from hadronic matter to a state of deconfined quarks and gluons, i.e., to the quark-gluon plasma (QGP), at a temperature of Tc ≈ 150−160 MeV at vanishing net baryon number [1,2]
Two independent methods of photon detection were employed: with the photon spectrometer (PHOS) which is an electromagnetic calorimeter [41], and with photon conversions measured in the central tracking system using the inner tracking system (ITS) [42] and the time projection chamber (TPC) [43]
PHOS and the central tracking detectors used in the photon conversion method (PCM) were in different readout partitions of the ALICE experiment which resulted in tThheediPfbfe–rPebntdinattaegartat√edsNluNm=ino2s.i7ti6esT.eV were recorded in the 2010 LHC run
Summary
Quantum chromodynamics (QCD) predicts a transition from hadronic matter to a state of deconfined quarks and gluons, i.e., to the quark-gluon plasma (QGP), at a temperature of Tc ≈ 150−160 MeV at vanishing net baryon number [1,2]. Hard-scattered quarks and gluons, produced in the initial stage of the heavy-ion collision, must traverse the QGP that is produced around them and lose energy in the process through interactions with that medium. This phenomenon (“jet quenching”) leads to a modification of hadron yields at high pT [6,7]. In the factorization approach of a perturbative QCD calculation of particle production from hard scattering, the overlap function TAA can be interpreted as the increase of the parton flux in going from pp to A–A collisions. The data are well suited to test models aiming at a description of particle production over the full transverse momentum range, including the potentially complicated interplay between jets and the evolving medium
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