Abstract
Transverse momentum spectra of π±, K± and p(p¯) up to pT=20 GeV/c at mid-rapidity in pp, peripheral (60–80%) and central (0–5%) Pb–Pb collisions at sNN=2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at pT≈3 GeV/c in central Pb–Pb collisions. Below the peak, pT<3 GeV/c, both ratios are in good agreement with hydrodynamical calculations, suggesting that the peak itself is dominantly the result of radial flow rather than anomalous hadronization processes. For pT>10 GeV/c particle ratios in pp and Pb–Pb collisions are in agreement and the nuclear modification factors for π±, K± and p(p¯) indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.
Highlights
Heavy-ion collisions at ultra relativistic energies produce a new form of QCD matter characterized by the deconfined state of quarks and gluons
The additional information provided by particle identification (PID) is of fundamental interest to study the differences in the dynamics of fragmentation between quarks and gluons to baryons and mesons [15], and to study the differences in their interaction with the medium considering that, due to the color Casimir factor, gluons lose a factor of two more energy than quarks [16,17]
The results presented in this Letter address three open experimental questions: Are there indications that the kaons are affected by radial flow at intermediate pT? Does the baryonto-meson ratio return to the pp value for high pT (>10 GeV/c) as suggested by the recent publication of the Λ/K0S ratio [18]? Are there large particle species dependent jet quenching effects as predicted in several models [19,20,21], where measurements at RHIC, in particular for baryons, are inconclusive due to the limited pT-range and the large systematic and statistical uncertainties [22,23,24]?
Summary
Heavy-ion collisions at ultra relativistic energies produce a new form of QCD matter characterized by the deconfined state of quarks and gluons (partons). In particular there is an intermediate transverse momentum regime, 2 < pT < 8 GeV/c, where the baryon-to-meson ratios, e.g. the proton yield divided by the pion yield, measured by experiments at RHIC revealed a, so far, not well understood enhancement [1,2,3] This so-called “baryon anomaly” could indicate the presence of new hadronization mechanisms such as parton recombination [4,5,6] that could be significantly enhanced and/or extended out to higher pT at LHC due to larger mini-jet production [7]. For transverse momenta above 10 GeV/c one expects to be able to study the pure energy loss (jet quenching) of high pT scattered partons traversing the medium [8,9,10].
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