Production of charged pions, kaons and protons at large transverse momenta in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

Transverse momentum spectra of $\pi^{\pm}$, $\rm K^{\pm}$ and p($\bar{\rm p}$) up to $p_{\rm T}$ = 20 GeV/$c$ at mid-rapidity in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=$ 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 $p_{\rm T} \approx 3 GeV/c$ in central Pb-Pb collisions. Below the peak, $p_{\rm T}$<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 $p_{\rm T}$>10 GeV/$c$ particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for $\pi^{\pm}$, $\rm K^{\pm}$ and $\rm p$($\bar{\rm p}$) 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.

Heavy ion collisions at ultra relativistic energies produce a new form of QCD matter characterized by the deconfined state of quarks and gluons (partons).Measurements of the production of identified particles provide information about the dynamics of this dense matter.In particular at intermediate transverse momentum, 2 < p T < 8 GeV/c, 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 effect [1][2][3][4], known as the "baryon anomaly", that may indicate the presence of new hadronization mechanisms.For larger transverse momenta (> 10 GeV/c) it is possible to study the energy loss (jet quenching) of high p T scattered partons when they traverse the medium [5].This affects the inclusive charged particle p T spectrum as has been seen at RHIC [6][7][8] and over an extended p T range, up to 100 GeV/c, at the LHC [9,10].The additional information provided by particle identification (PID) allows two open experimental questions to be addressed: Does the baryon-to-meson ratio return to the pp value for very high p T (> 10 GeV/c) as hinted at in the recent publication of the Λ/K 0 S ratio [11]?Are there particle species dependent jet quenching effects as predicted in several models [12][13][14] where measurements at RHIC are inconclusive due to the limited p T -range [15,16]?
Counts (arb.units.)Pb-Pb (pp) collision events are used in this analysis.Data were taken during 2010 (2011) and reconstructed using the ALICE apparatus [17].The Inner Tracking System (ITS) and the Time Projection Chamber (TPC) are used for vertex finding and tracking.The TPC also provides PID through the measurement of the specific energy loss, dE/dx.Minimum bias interactions are triggered based on the signals from forward scintillators (VZERO) and, in pp collisions, the two innermost silicon pixel layers of the ITS (SPD).The collision's centrality is determined from the measured amplitude in the VZERO detector [18] which is related to the number of participating nucleons and the nuclear overlap function (T AA ) through simulations based on a Glauber model [19].The same event and track selection is used as in the inclusive charged particle analysis [20].Track cuts are optimized in order to select primary charged particles in the pseudorapidity range |η| < 0.8.Since each track is required to have at least one reconstructed point in the SPD (closer than 10 cm to the primary vertex) the correction for feed-down from hyperon decays is small.For p T = 2 (3) GeV/c it is of order 0.3 % (4%) for the pion (proton) yield and decreasing with increasing p T .Nevertheless, all results presented in this paper are corrected for feed-down from weak decays.
The dE/dx is obtained as the truncated mean of the 0-60% lowest charge samples associated with the track in the TPC [21].Particle identification is performed in the relativistic rise regime of the Bethe-Bloch (BB) curve where the dE/dx separation between particles with different masses is nearly constant [22].
The dE/dx response depends on the track length so the analysis is done in four equally sized |η|-intervals, and a geometrical cut to remove tracks entering the gap in between the TPC readout chambers is applied to select tracks with the best dE/dx resolution.The separation in number of standard deviations (σ ) between pions and kaons (pions and protons) in pp and peripheral Pb-Pb collision is around 3.2 (4.6) at p ≈ 6 GeV/c for 0.6 < |η| < 0.8 where the separation is largest.In central Pb-Pb collisions one finds a separation of 2.4 (3.5) σ .In the worst case, |η| < 0.2, the separation is 11-15 % smaller.
Figure 1 shows an example of the dE/dx spectrum obtained for pp and Pb-Pb (central and peripheral) collisions for two momentum, p, intervals and |η| < 0.2 where p ≈ p T .The pion, kaon, and proton yields are extracted by fitting a sum of four Gaussian functions (including electrons) to the dE/dx spectra1 .
To reduce the degrees of freedom in the fits from 12 to 4, parameterizations of the BB ( dE/dx ) and resolution (σ ) curves as a function of β γ are extracted first using tracks from identified particles.Samples of secondary pions (30 < β γ < 50) and protons (3 < β γ < 7) are determined through the reconstruction of the weak decay topology of K 0 S and Λ, respectively; a similar algorithm is used to identify electrons resulting from photon conversions (fixing the dE/dx plateau: β γ > 1000) .Finally, using information from the time-of-flight detector the relative pion content can be enhanced for sub-samples of the full datasets (16 < β γ < 50).
The dE/dx separation between kaons and protons in the high p T analysis is smallest for p ≈ 3 GeV/c and increases with p until both species are on the relativistic rise [22].The K 0 S yields [11] are used to constrain further the BB curve in the region 3 < β γ < 17 for Pb-Pb data.The effect of the K 0 S condition is only relevant in central collisions at low p T (< 4 GeV/c).At 3 GeV/c the effect on the extracted kaon yield is 10% (< 1%) for 0-5% (60-80%) collision centrality.
With the above information the BB and the resolution curves are determined for kaons and protons in the full momentum interval reported here and for pions with p < 7 GeV/c.For p > 7 GeV/c the pion dE/dx is restricted by the logarithmic rise until the dE/dx starts to approach the plateau.This lack of additional constraint currently limits the p T reach of the analysis to ∼ 20 GeV/c.From the fits in Fig. 1 the particle fractions, f π/K/p (p) are extracted.To obtain the fractions as a function of p T , f π/K/p (p T ), a weighting procedure is implemented using a matrix which relates the measured p to p T .The p T -dependent fractions are found to be independent of η and so all four η regions are averaged.
Finally, the invariant yields are obtained using the p T spectrum for inclusive charged particles [20], d 2 N ch dp T dη , in the following way: where (ε ch ) ε π/K/p is the efficiency for (un)identified particles and J π/K/p is the Jacobian correction (from η to y).Normalizing to the p T spectrum of inclusive charged particles guarantees that only the systematic uncertainty due to PID is relevant when comparing the modification of the p T spectra of π/K/p to those    The systematic uncertainty on the invariant yields has three main components: event and track selection, efficiency correction of the fractions, and the fraction extraction.Contributions from the event and track selection are taken directly from the inclusive charged particle analysis [20].Efficiency ratios (ε ch /ε π/K/p ) are found to be nearly independent of p T (a small dependence is only observed for kaons), similar for all systems, and model independent within 3 %.The largest systematic uncertainty in the extraction of the fractions comes from the uncertainty in the constrained parameters: the means ( dE/dx ) and the widths (σ ) used in the fits.The uncertainty on these parameters are estimated as the difference between the final parameterizations and the actual values from the clean samples.In addition, the statistical uncertainty on the extracted BB parameterization in peripheral Pb-Pb collisions is found to be of a similar magnitude and also taken into account for the variations.The dE/dx spectra are then refitted, varying the mean and sigma within the estimated uncertainties, and the variation of the fractions are assigned as systematic errors.A summary of the PID systematic uncertainties is shown in Table 1. Figure 2 shows the invariant yields measured in Pb-Pb collisions compared to those in pp collisions scaled to the number of binary collisions, N coll [19].For Pb-Pb the results of the lower p T analysis of bulk production are also plotted for p T below 2 (3) GeV/c for pions (kaons and protons) [23].For peripheral Pb-Pb collisions the shapes of the invariant yields are similar to those observed in pp collisions.The spectra exhibit a reduction in the production of high-p T particles with respect to the reference which is characteristic of jet quenching.
Figure 3 shows the proton-to-pion ratio, (p + p)/(π + + π − ), as a function of p T .For central (peripheral) Pb-Pb collisions it reaches ∼ 0.80 (∼ 0.38) at the maximum around 3 GeV/c and then decrease with increasing p T .Within the systematic and statistical uncertainties the magnitudes of these ratios are equal to those measured by PHENIX [16] and STAR [15], when p/π + and p/π − ratios are averaged and data are corrected for feed-down.At LHC energies the mini-jet activity is expected to be larger than at RHIC energies, which motivated certain ratio predictions in the framework of recombination models to be an order of magnitude larger than the measurements reported here [24].Other predictions including both recombination and fragmentation are more consistent with the data for high p T [2].It is interesting to note that in central Pb-Pb collisions the (K + + K − )/(π + + π − ) ratio also exhibits a small bump at p T ∼ 3 GeV/c.This behavior is not predicted by coalescence models and, even though the large systematic error in this region does not permit any strong conclusions, this feature is qualitatively well described by EPOS [25] where the interaction between bulk matter (which thermalizes and flows) and jets is considered [26].Finally, for higher p T (> 10 GeV/c) both particle ratios behave like in pp, suggesting that fragmentation dominates the hadron production.In this p T regime, the particle ratios in pp are not well described by the pQCD calculations in [27], hence the spectra themselves can be used to constrain identified fragmentation functions.Figure 4 shows the nuclear modification factor R AA as a function of p T defined as the ratio of the Pb-Pb spectra to the N coll scaled pp spectra shown in Fig. 2. The R AA for the sum of kaons and protons is also shown as this gives the best precision for testing whether pions are suppressed similarly to other light hadrons.For p T < 10 GeV/c protons appear to be less suppressed than kaons and pions consistent with the particle ratios shown in Fig. 2. At larger p T (> 10 GeV/c) all particle species are equally suppressed; so despite the strong jet quenching effects observed in the most central heavy-ion collisions, the particle composition (and ratios) at high p T is similar to those in vacuum.This observation disfavors significant modifications of hadro-chemistry within the hard core of jets [12] and contradicts predictions based on formation time arguments [14].
In summary, we have measured the production of high-p T pions, kaons and protons in central and peripheral Pb-Pb collisions.From the invariant yields we derived the particle ratios and the R AA as a function of p T .At moderate p T (< 10 GeV/c) we observe that the proton-to-pion ratio reaches a maximum of ∼ 0.80 (∼ 0.38) at p T = 3 GeV/c in central (peripheral) Pb-Pb collisions.In central collisions, the kaonto-pion ratio exhibits a small enhancement at the same p T .At higher-p T , the ratios are compatible to those in pp collisions.From the nuclear modification factor R AA , we conclude that for p T > 10 GeV/c within the systematic and statistical uncertainties, pions, kaons and protons are suppressed equally.

1 Fig. 1 :
Fig. 1: (Color online).dE/dx distributions measured for |η| < 0.2 and normalized to the integrated yields.The signals are fitted to a sum of four Gaussian functions (solid line).Two p intervals are shown for central (left) and peripheral (center) Pb-Pb; and pp (right) collisions.In all momentum intervals the electron fraction is below 1 %.

Fig. 2 :
Fig.2:(Color online).Invariant yields of identified particles in central and peripheral Pb-Pb collisions.Black markers show the pp reference yields scaled by the average number of binary collisions[19].The statistical and systematic uncertainties are shown as vertical error bars and boxes, respectively.For Pb-Pb the published data below 2 (3) GeV/c for pions (kaons and protons) are also shown[23].

Fig. 3 :
Fig. 3: (Color online).Particle ratios as a function of p T measured in pp and Pb-Pb collisions.Statistical and systematic uncertainties are displayed as vertical error bars and boxes, respectively.For Pb-Pb the published data below 3 GeV/c are also shown [23].

Fig. 4 :
Fig. 4: (Color online).The nuclear modification factor R AA as a function of p T for different particle species.Results for 0-5% (left) and 60-80% (right) collision centralities are shown.Statistical and PID systematic uncertainties are plotted as vertical error bars and boxes around the points, respectively.The p T dependent systematic uncertainty that is common between the charged and the PID analyzes is plotted as a grey band at R AA = 1 in the top plot.The normalization uncertainty is indicated by the black boxes inside this band[20].
for the unidentified particles.

Table 1 :
Systematic uncertainties, separated into the N ch , PID, and efficiency part, on the invariant yields from 3 < p T < 4 GeV/c to 10 < p T < 20 GeV/c.