Centrality dependence of $\rm \mathbf π$, K, p production in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

ALICE Collaboration

In this paper measurements are presented of $\rm \pi$$^+$, $\rm \pi$$^-$, K$^+$, K$^-$, p and $\overline{\rm p}$ production at mid-rapidity < 0.5, in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV as a function of centrality. The measurement covers the transverse momentum ($p_{\rm T}$) range from 100, 200, 300 MeV/$c$ up to 3, 3, 4.6 GeV/$c$, for $\rm\pi$, K, and p respectively. The measured $p_{\rm T}$ distributions and yields are compared to expectations based on hydrodynamic, thermal and recombination models. The spectral shapes of central collisions show a stronger radial flow than measured at lower energies, which can be described in hydrodynamic models. In peripheral collisions, the $p_{\rm T}$ distributions are not well reproduced by hydrodynamic models. Ratios of integrated particle yields are found to be nearly independent of centrality. The yield of protons normalized to pions is a factor ~1.5 lower than the expectation from thermal models.

Figures

Figure 1

ITS standalone tracking efficiency for (a) pions, (b) kaons, (c) protons and global tracking efficiency for (c) pions, (d) kaons, (e) protons in central and peripheral collisions.
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Figure 2

Performance of the PID detectors:
(a) ${\rm d}E/{\rm d}x$ distribution measured in the ITS, the continuous curves represent the Bethe-Bloch parametrization, the dashed curves the asymmetric bands used in the PID procedure;
(b) ${\rm d}E/{\rm d}x$ measured in the TPC with global tracks (see text for the definition of global tracks), the continuous curves represent the Bethe-Bloch parametrization;
(c) fit of the TOF time distribution with the expected contributions for negative tracks and for the kaon mass hypothesis, in the bin $2.4 < \pt < 2.5$ GeV/$c$.
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Figure 3

${\rm DCA}_{xy}$ of (a) protons and (b) antiprotons in the $\pt$-range between 0.6 GeV/$c$ and 0.65 GeV/$c$ together with the Monte Carlo templates which are fitted to the data (0-5% most central collisions). The dashed areas represent the individual templates, the continuous curve the combined fit.
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Figure 4

Transverse momentum ($\pt$) distribution of (a) $\pi$, (b) K, and (c) p as a function of centrality, for positive (circles) and negative (squares) hadrons. Each panel shows central to peripheral data; spectra scaled by factors $2^n$ (peripheral data not scaled). Dashed curves: blast-wave fits to individual particles; dotted curves: combined blast-wave fits (see text for details). Statistical (error bars) and systematic (boxes) uncertainties plotted. An additional normalization uncertainty (see table 3) has to be added in quadrature.
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Figure 5

(a) $\pi^-/\pi^+$ (b) $K^-/K^+$ (c) $\bar{\rm p}/{\rm p}$ ratios as a function of $\pt$ for central, semi-central and peripheral events.
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Figure 6

Local slopes of the $\pt$ distributions of (a) $\pi$, (b) K, (c) p (summed charge states) as a function of $\pt$ and centrality. See text for details.
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Figure 7

Mean transverse momentum $\avpT$ as a function of $\dNchdeta$ for (a) $\pi$, (b) K, (c) p, compared to RHIC results at $\snn = 200$ GeV. Negative charge results displaced for better readability. Boxes: systematic uncertainties.
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Figure 8

(a) $\pi^-/\pi^+$, (b) $K^-/K^+$ (c) $\bar{\rm p}/{\rm p}$ ratios as a function of $\dNchdeta$, compared to previous results at $\snn = 200$ GeV.
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Figure 9

(a) $\rm{p}/\pi = (\mathrm{p} +\mathrm{\bar{p}})/(\pi^++\pi^-)$ and (b) $\rm{K}/\pi = (\mathrm{K^+} + \mathrm{K^-}))/(\pi^++\pi^-)$ ratios as a function of $\dNchdeta$ compared to previous results at $\snn = 200$ GeV.
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Figure 10

(a) Results of blast-wave fits, compared to similar fits at RHIC energies. The uncertainty contours include the effect of the bin-by-bin systematic uncertainties, the dashed error bars represents the full systematic uncertainty (see text for details), the STAR contours include only statistical uncertainties.
(b) Comparison of fit results for different fit ranges; the error bars include only the effect of the bin-by-bin systematics (see text for details).
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Figure 11

Blast-wave parameters (a) $\langle\beta_{\rm T}\rangle$ and (b) $T_{\rm kin}$ as a function of $\dNchdeta$, compared to previous results at $\snn = 200$ GeV (full systematic uncertainties for both experiments).
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Figure 12

Ratio of the measured spectra to the combined blast-wave fit as a function of $\pt$ for (a) $\pi$, (b) K, (c) p.
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Figure 13

Spectra of particles for summed charge states in the centrality bin 0-5%, compared to hydrodynamical models and results from RHIC at $\snn=200$ GeV. See text for details. Systematic uncertainties plotted (boxes); statistical uncertainties are smaller than the symbol size.
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Figure 14

Spectra of particles for summed charge states in the centrality bin 20-30% compared to hydrodynamical models and results from RHIC at $\snn=200$ GeV. See text for details. Systematic uncertainties plotted (boxes); statistical uncertainties are smaller than the symbol size.
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Figure 15

Spectra of particles for summed charge states in the centrality bin 70-80% compared to hydrodynamical models and results from RHIC at $\snn=200$ GeV. See text for details. Systematic uncertainties plotted (boxes); statistical uncertainties are smaller than the symbol size.
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Figure 16

(a) $\rm{p}/\pi = (\mathrm{p} +\mathrm{\bar{p}})/(\pi^++\pi^-)$ as a function of $\pt$ for different centrality bins compared to ratios from the Krak'ow and HKM hydrodynamic models and to a recombination model;
(b) $\rm{K}/\pi = (\mathrm{K^+} + \mathrm{K^-}))/(\pi^++\pi^-)$ ratio as a function of $\pt$ for different centrality bins compared to ratios from the Krak'ow and HKM hydrodynamic models.
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Figure 17

Transverse momentum $\pt$ distribution of total protons based on tracks reconstructed using only TPC information (full circles) compared to primary protons (open circles) and corresponding total spectra measured by the STAR collaboration in Au-Au collisions at $\snn=200$ GeV (full stars). 0-5% most central events. Box: systematic uncertainty; Statistical uncertainties smaller than the symbols; curve: individual blast wave fit.
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