Abstract
Transverse momentum ($p_{\rm{T}}$) spectra of pions, kaons, and protons up to $p_{\rm{T}} = 20$ GeV/$c$ have been measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV using the ALICE detector for six different centrality classes covering 0-80%. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at $p_{\rm{T}} \approx 3$ GeV/$c$ in central Pb-Pb collisions that decreases towards more peripheral collisions. For $p_{\rm{T}} > 10$ GeV/$c$, the nuclear modification factor is found to be the same for all three particle species in each centrality interval within systematic uncertainties of 10-20%. This suggests there is no direct interplay between the energy loss in the medium and the particle species composition in the hard core of the quenched jet. For $p_{\rm{T}} < 10$ GeV/$c$, the data provide important constraints for models aimed at describing the transition from soft to hard physics.
Highlights
In ultrarelativistic heavy-ion collisions, a strongly interacting deconfined medium of quarks and gluons is created.Experimental evidence for this state of matter has been found at both the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4] and the CERN Large Hadron Collider (LHC) [5,6,7,8,9]
Ordering by pT, from lowest to highest, the results are obtained using the specific energy loss, dE/dx, in the silicon inner tracking system (ITS), the dE/dx in the time projection chamber (TPC), the time of flight measured by the time-of-flight (TOF) detector, the Cherenkov angle measured by the high-momentum particle identification detector (HMPID), and the TPC dE/dx in the relativistic rise region
In the centrality intervals where the HMPID measurements are available, they improve the systematic uncertainty of the kaon and proton yields by approximately a factor of two in the pT region where it is later observed that the peaks of the kaon-to-pion and the proton-to-pion ratios are located
Summary
In ultrarelativistic heavy-ion collisions, a strongly interacting deconfined medium of quarks and gluons is created. In the intermediate transverse momentum regime, the baryon-to-meson ratios, e.g., the proton yield divided by the pion yield, measured by experiments at the RHIC revealed a, so far, not well understood enhancement [28,29,30]. This so-called “baryon anomaly” could indicate the presence of new hadronization mechanisms such as parton recombination [31,32,33] that could be significantly enhanced and/or extended out to higher pT at the LHC owing to larger minijet production [34]. III, the final spectra are presented, and the particle ratios and nuclear modification factors are discussed and compared with theoretical calculations and results from previous experiments at lower center-of-mass energies
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