Production of charged pions, kaons, and (anti-)protons in Pb-Pb and inelastic $pp$ collisions at $\sqrt{s_{NN}}=5.02$ TeV

Midrapidity production of ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p$ measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic $pp$ collisions at $\sqrt{s_{NN}}=5.02$ TeV, is presented. The invariant yields are measured over a wide transverse momentum ($p_{\mathrm{T}}$) range from hundreds of $\mathrm{MeV}/c$ up to 20 $\mathrm{GeV}/c$. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range $0–90\%$. The comparison of the $p_{\mathrm{T}}$-integrated particle ratios, i.e., proton-to-pion ($p/\pi$) and kaon-to-pion ($K/\pi$) ratios, with similar measurements in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV show no significant energy dependence. Blast-wave fits of the $p_{\mathrm{T}}$ spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios ($p/\pi , K/\pi$) as a function of $p_{\mathrm{T}}$ show pronounced maxima at $p_{\mathrm{T}}\approx 3\mathrm{GeV}/c$ in central Pb-Pb collisions. At high $p_{\mathrm{T}}$, particle ratios at 5.02 TeV are similar to those measured in $pp$ collisions at the same energy and in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high $p_{\mathrm{T}}$ and compatible with measurements at $\sqrt{s_{NN}}=2.76$ TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.

Figure 1

Separation power of hadron identification in the ITS (red), TPC (magenta), TOF (blue), and HMPID (green) as a function of $p_{\mathrm{T}}$ at midrapidity for inelastic pp and $0–90\%$ Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. The left (right) panel shows the separation of pions and kaons (kaons and protons), expressed as the distance between the expected average PID signal divided by the resolution for the pion (kaon) [see Eq. (3)], averaged over $\left|\eta \right|<0.5$. The lower panels show the range in which the ITS, TPC, TOF, and HMPID provide a separation power $\approx 2\sigma$ or larger.

Figure 2

Invariant mass distribution of identified charged kaons from their decay products in $pp$ (a) and Pb-Pb collisions (b) at $\sqrt{s_{NN}}=5.02$ TeV. The red circles and blue lines represent the experimental data and Monte Carlo simulation, respectively, before (upper) and after (lower) the topological selection. The peak centered at $M_{\mu \nu }=0.49$ GeV/${\mathit{c}}^2$ is for the decay channel $K \to \mu +{\nu }_{\mu }$ (B.R. $=63.55\%$), whereas the peak centered at $M_{\mu \nu }=0.43$ GeV/${\mathit{c}}^2$ is for the decay channel $K \to \pi +{\pi }^0$ (B.R. = 20.66%), whose invariant mass is calculated with the wrong mass hypothesis.

Figure 3

Transverse momentum spectra of pions (left), kaons (middle), and (anti-)protons (right) measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV for different centrality classes. Scale factors are applied for better visibility. The results are compared with the spectra measured in inelastic $pp$ collisions at $\sqrt{s}=5.02$ TeV. Statistical and systematic uncertainties are displayed as error bars and boxes around the data points, respectively.

Figure 4

Ratios of centrality-dependent $p_{\mathrm{T}}$ spectra to model (blast-wave parametrization) predictions in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV for pions (top), kaons (middle) and protons (bottom). The fit ranges are indicated as gray shaded areas.

Figure 5

Average expansion velocity ($\langle {\beta }_{\mathrm{T}}\rangle$) and kinetic freeze-out temperature ($T_{\mathrm{kin}}$) progression from the simultaneous Boltzmann-Gibbs blast-wave fit to ${\pi }^{\pm }, K^{\pm }$, and p($\overline{p}$) spectra measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ and 2.76 TeV [14]. The correlated uncertainties from the global fit are shown as ellipses. The elliptic contours correspond to $1\sigma$ uncertainties, with statistical and systematic uncertainties being added in quadrature.

Figure 6

Mean transverse momentum as a function of $\langle d{\mathit{N}}_{\mathrm{ch}}/d\eta \rangle$ for ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p$ in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ (full color markers) and 2.76 TeV [14] (full black markers) and in inelastic $pp$ collisions at $\sqrt{s}=5.02$ and 2.76 TeV (empty color markers) [59]. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across centrality bins (not estimated in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV). Continuous lines represent the Bayesian analysis predictions.

Figure 7

Transverse momentum integrated $K/\pi$ (top) and p/$\pi$ (bottom) ratios as a function of $\langle d{\mathit{N}}_{\mathrm{ch}}/d\eta \rangle$ in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, compared to Pb-Pb at 2.76 TeV [14]. The values in $pp$ collisions at $\sqrt{s}=5.02$ and 2.76 TeV are also shown. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across centrality bins (not estimated in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV). Continuous lines represent the Bayesian analysis predictions.

Figure 8

Centrality dependence of the $K/\pi$ (top) and $p/\pi$ (bottom) ratios as a function of transverse momentum, measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ and 2.76 TeV [28]. The ratios in $pp$ collisions at $\sqrt{s}=5.02$ TeV is also shown. The statistical and systematic uncertainties are shown as error bars and boxes around the data points, respectively.

Figure 9

Centrality dependence of the nuclear modification factor of charged ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p$ as a function of transverse momentum, measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. The statistical and systematic uncertainties are shown as error bars and boxes around the data points. The total normalization uncertainty ($pp$ and Pb-Pb) is indicated in each panel by the vertical scale of the box centered at $p_{\mathrm{T}}$ = 1 $\mathrm{GeV}/c$ and $R_{AA}$ = 1.

Figure 10

Centrality dependence of the nuclear modification factor of charged ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p$ as a function of transverse momentum, measured in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ [28] and 5.02 TeV, for $0–5\%$ and $60–80\%$ centrality classes. The statistical and systematic uncertainties are shown as error bars and boxes around the data points. The total normalization uncertainty ($pp$ and Pb-Pb) is indicated in each panel by the vertical scale of the box centered at $p_{\mathrm{T}}$ = 1 $\mathrm{GeV}/c$ and $R_{AA}=1$.

Figure 11

Ratios of data to iEBE-VISHNU and McGill models (see text for details), for ${\pi }^{\pm }, K^{\pm }$ and $\left(\overline{p}\right)p p_{\mathrm{T}}$ spectra in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV for centrality classes 0-5%, 30-40% and 70-80%. The statistical and systematic uncertainties are shown as error bars and bands around the data points, respectively.

Figure 12

Ratios of data to EPOS3 model (see text for details), for ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p p_{\mathrm{T}}$ spectra in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV for centrality classes 0–5%, 30–40%, and 70–80%. The statistical and systematic uncertainties are shown as error bars and bands around the data points, respectively.

Figure 13

(Top) $K/\pi$ and $p/\pi$ ratios in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV in 0–5% centrality class, compared to iEBE-VISHNU, McGill, and EPOS3 model predictions; see text for details. The statistical and systematic uncertainties are shown as error bars and boxes around the data points, respectively. For model predictions the statistical uncertainties are represented by the band width. (Bottom) Data-to-model ratio, the statistical and systematic uncertainties are shown as error bars and bands around the data points, respectively.

Figure 14

(Top) $K/\pi$ and $p/\pi$ ratios in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV in 30–40% centrality class, compared to iEBE-VISHNU, McGill, and EPOS3 model predictions; see text for details. The statistical and systematic uncertainties are shown as error bars and boxes around the data points, respectively. For model predictions the statistical uncertainties are represented by the band width. (Bottom) Data-to-model ratio, the statistical and systematic uncertainties are shown as error bars and bands around the data points, respectively.

Figure 15

(Top) $K/\pi$ and $p/\pi$ ratios in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV in 70–80% centrality class, compared to iEBE-VISHNU, McGill, and EPOS3 model predictions; see text for details. The statistical and systematic uncertainties are shown as error bars and boxes around the data points, respectively. For model predictions the statistical uncertainties are represented by the band width. (Bottom) Data-to-model ratio, the statistical and systematic uncertainties are shown as error bars and bands around the data points, respectively.

Figure 16

${\pi }^{\pm }, K^{\pm }$ and $\left(\overline{p}\right)p p_{\mathrm{T}}$ spectra in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV divided for the same spectra measured in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV [28] for centrality classes 0–5%, 30–40%, and 70–80%, compared to iEBE-VISHNU, McGill, and EPOS3 model predictions; see text for details. The statistical and systematic uncertainties are shown as error bars and boxes around the data points, respectively. For model predictions the statistical uncertainties are represented by the band width.