Light-flavour hadron production in pp and Pb Pb collisions in the ALICE experiment

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1 EPJ Web of Conferences 60, 302 (203 DOI: 0.05/ejconf/ Owned by the authors, ublished by EDP Sciences, 203 Light-flavour hadron roduction in and Pb Pb collisions in the exeriment at the LHC Francesco Barile,2,a for the Collaboration Università degli Studi Di Bari 2 Sezione INFN, Bari, Italy Abstract. Nuclear matter under extreme conditions of temerature and density can be investigated in ultrarelativistic heavy-ion collisions. he measurement of transverse momentum ( distributions and yields of identified articles is a fundamental ste in understanding collective and thermal roerties of the matter roduced in such collisions. At intermediate transverse momentum, it allows for testing the recombination models where hadrons could be formed by the coalescence of quarks from a deconfined quark-gluon lasma (QGP. At higher transverse momenta, article sectra allow one to investigate the mechanism of arton energy loss in the hot and dense hadronic medium. he measurement of sectra in collisions not only rovides the baseline for the heavy-ion data but also allows for the tuning and otimization of QCD-insired models. he latest results on identified and inclusive light-flavour charged articles in collisions at s = 0.9, 2.76 and 7 ev and Pb-Pb collisions at s NN = 2.76 ev is reviewed. sectra, yields and ratios in as a function of the collision energy is shown and comared to revious exeriments and Monte Carlo redictions. Recent Pb Pb results in different centrality intervals is resented and comared to s NN = 200 GeV Au Au collisions at RHIC. Comarison with redictions from thermal and hydrodynamic models is also discussed. Figure. Schematic layout of the detector with its main subsystems. Introduction (A Large Ion Collider Exeriment [] [2] [3] is a general-urose heavy-ion detector at the CERN LHC (Large Hadron Collider devoted to study the hysics of strongly-interacting matter and the quark-gluon lasma. It has unique article identification (PID caability among the LHC exeriments, allowing for the identification of articles in a wide range in. he exeria francesco.barile@cern.ch ment, shown in figure, consists of a central-barrel detector and several forward detector systems. he central system covers the mid-raidity region η < 0.9 over the full azimuthal angle and it is located inside a solenoidal magnet (B =. It includes a six-layer highresolution inner-tracking system (IS, a large-volume time-rojection chamber (PC and electron and chargedhadron identification detectors which exloit transitionradiation (RD and time-of flight (OF techniques, resectively. A small-area detectors for high- articleidentification (HMPID, hoton and neutral-meson measurement (PHOS and jet reconstruction (EMCal comlement the central barrel. he large-raidity system includes a single-arm muon sectrometer covering the seudoraidity range -4.0 η -2.4 and several smaller detectors (VZERO, ZERO, FMD, ZDC, and PMD for triggering, multilicity measurements and centrality determination. Since November 2009 the detector has collected roton-roton data at several centre-of-mass energy ( s = 0.9, 2.76, 7 and 8 ev and during the two LHC heavyion runs (200 and 20 it recorded Pb Pb collisions at centre-of-mass energy er nucleon air of s NN = 2.76 ev. In this aer, recent results on the light-flavour hadron roduction in and in Pb Pb will be discussed. 2 Particle identification in In this section the main article-identification detector relevant to the analysis resented here are briefly discussed. his is an Oen Access article distributed under the terms of the Creative Commons Attribution License 2.0, which ermits unrestricted use, distribution, and reroduction in any medium, rovided the original work is roerly cited. Article available at htt:// or htt://dx.doi.org/0.05/ejconf/

2 EPJ Web of Conferences K / (π π (K s=7 ev ALI PREL π ( / (π ALI PREL s =2.76 ev 7 ev Pythia, Perugia20 NLO (Phys. Rev. D 82, 0740 ( (GeV/c s=7 ev s =2.76 ev 7 ev Pythia, Perugia20 NLO (Phys. Rev. D 82, 0740 ( (GeV/c Figure 2. K/π and /π roduction ratios in collisions at s = 2.76 ( > 3 GeV/c and 7 ev comared to PYHIA Monte Carlo rediction and NLO calculations. ALI-PUB Figure 4. Midraidity articles ratios, comared to RHIC results and rediction from thermal models for central Pb Pb collisions at the LHC (combined statistical and systematic errors. A detailed review of the exeriment and of its PID caabilities are discussed in detail in [] [2] [3]. he Inner racking System (IS is a six-layer silicon detector with radii between 3.9 cm and 43 cm roviding vertexing and tracking informations. Four of the 6 layers also measure the secific energy loss er unit length (de/dx of the articles and are used for article identification in the non-relativistic region. he ime Projection Chamber (PC [] [3] is the main central-barrel tracking detector of. It is a large volume high granularity, cylindrical detector with an outer radius of 2.78 m and a length of 5. m. It rovides threedimensional hit information and de/dx measurement with u to 59 samles. Particle identification is based on the secific energy deosit of each article in the drift gas of the PC. It identifies articles in the region of the relativistic rise momenta u to 50 GeV/c. he ime Of Flight (OF [] [3] detector is a large area array devoted to article ID in the intermediate momentum range u to 2.5 GeV/c for ions and kaons, u to 4 GeV/c for rotons. OF measures the time of flight of the articles coming from the interaction oint with a very good resolution (about 20 s in and 85 s in Pb Pb. he High Momentum Particle Identification Detector (HMPID [] [3] [5] enhances the article identification caabilities of at higher momenta (ion and kaons u to 3 GeV/c and roton u to 6 GeV/c. It is designed as a single-arm roximity-focusing Ring Imaging Cherenkov (RICH detector and consists of seven identical counters. 3 Light flavour hadron roduction ALI-PUB-4533 Figure 3. ransverse momentum sectra comared with 3 hydrodynamic models and with the blast wave fit alied searately for each sectrum. has measured the yields of roduced charged ions, kaons and rotons in a wide momentum range and in several colliding systems. he measurements have been erformed in collisions at several centre-of-mass energies ( s = 0.9 [4], 2.76 and 7 ev and in Pb Pb collisions at s NN = 2.76 ev as a function of collision centrality. In this section, we resent article ratios and sectra for rimary articles, defined as romt articles roduced in

3 LHCP 203 R AA 05% PbPb π π K K s NN = 2.76 ev R AA 2040% PbPb π π K K s NN = 2.76 ev ALI PREL (GeV/c ALI PREL (GeV/c Figure 5. Charged ion, kaon and (anti-roton nuclear modification factor R AA as a function of for Pb Pb collisions at s = 2.76 ev for two centrality bins (0-5% and 20-40%. Statistical and systematic uncertainties are dislayed as error bars and boxes, resectively. 05% PbPb s NN = 2.76 ev 50% PbPb s NN = 2.76 ev 020% PbPb s NN = 2.76 ev π ( /(π 2040% PbPb s NN = 2.76 ev 4060% PbPb s NN = 2.76 ev 6080% PbPb s NN = 2.76 ev ALI PREL (GeV/c Figure 6. Proton-to-ion ratio in Pb Pb collisions comared with the results at 2.76 ev in several centrality bins. the collision and all decay roducts (excet roducts from weak decays of strange articles. In figure 2, the ratios K/π and /π as a function of in collisions at 2.76 and 7 ev and different Monte Carlo are shown. hey are comared with theoretical model redictions. As it can be seen, both ratios do not show evident energy deendence (within the systematic uncertainties. he ratios are not reroduced by NLO calculations. PHYIA Monte Carlo generator under estimates the roton-to-ion ratio at intermediate. In Pb-Pb collisions the distribution of ositive and negative articles have been measured and they were found to be comatible within systematic errors. In figure 3 the results are shown for summed charge sectra. he sectra are comared to RHIC results in Au Au collisions at s NN =200 GeV [7] [8] and to hydrodynamic models: they show a harder distribution resect to RHIC energies, indicating a significantly stronger radial flow. he sectra have been comared with three different models. A viscous hydrodynamic model VISH2 [9], that reroduces well the ion and kaon distributions in <.5 GeV/c but it misses the rotons, both in shae and absolute abundance. he difference is ossibly due to the lack of an exlicit descrition of the hadronic hase. his interretation is suorted by the comarison with HKM [0]. Kraków [4] introduces non-equilibrium corrections due to the viscosity at the transition from the hydrodynamic descrition to articles, which change the effective ch leading to a good agreement with the data [7] [8]. In order to quantify the kinetic freeze-out arameters, a combined fit with blast-wave [9] function has been erformed, with the freeze-out temerature kin, the average transverse velocity β and the exonent of the velocity rofile as free arameters. he data are well described by the combined blast wave fit with a collective radial flow velocity β = 5 ± 0.02 and a kinetic freeze-out of kin = 95 ± 0 MeV. As comared to fits to central Au Au collisions at s NN = 200 GeV/c, in similar range, β at the LHC is 0% higher while kin is comarable within errors [7]. he article ratios are comared in figure 4 to results at lower energy ( s NN = 200 GeV/c and to the redic

4 tion of the thermal model, using µ B = MeV and a ch of 64 MeV or 70 MeV. he antiarticle/article (π /π, K /K, / ratios are all unity within errors, consistent with a vanishing baryochemical otential µ B. he same ratio ( / of the yields has been measured by in collisions at s= 0.9 and 7 ev. he result is consistent with the standard model of baryon-number transort [20]. In figure 4 the K/π and /π is also shown. he kaon-toion ratio is similar to the lower energy values and agrees with the exectations from the thermal model [2]. he roton-to-ion ratio is significantly lower than exected. In figure 5 [23], the nuclear modification factor R AA for identified charged ions, kaons and (anti-rotons as a function of for two centrality classes (0-20% and 20-40% is shown. his seems to suggest that, at higher ( > 8-0 GeV/c, the dense medium formed in Pb Pb collisions does not affect the fragmentation [23]. In figure 6 and figure 7, the roton-to-ion ratio in Pb Pb collisions in several centrality bins with results at 7 ev is shown. For intermediate it exhibits a relatively strong enhancement, a factor 3 higher than in case of collisions at = 3 GeV/c. his is reminiscent of the increase in the baryon-to-meson ratio observed at RHIC in the intermediate. he ratio is comatible (within the errors to the value at higher momenta ( above 8-0 GeV/c as shown in figure 6. In figure 7 (to roton-to-ion ratio in Pb Pb collisions in several centrality bins comared with result at 7 ev. It is interesting that even in the data it exhibits a maximum in the same region as observed in Pb Pb collisions. In the same figure (figure 7, bottom roton-to-ion ratio in central Pb Pb collisions comared with different models. In articular EPOS model [22] shows a good agreement with the data. 4 Summary and conclusions he most recent results of exeriment on light flavour hadron roduction both in and in Pb Pb collisions have been reorted. he transverse momentum sectra for ions, kaons and rotons have been measured with in several colliding systems and energies at the LHC showing the excellent PID erformance caabilities of the exeriment and of its sub-systems. In collisions, the results show no evident s deendence in hadron roduction ratios. In Pb Pb collisions, the transverse momentum sectra indicate a 0% stronger radial flow than at RHIC exeriment. he roton-to-ion ratio is significantly lower than statistical model redictions with a chemical freeze-out temerature ch = MeV. At intermediate transverse momentum region, an enhancement of the baryon-to-meson ratio is observed. he maximum of the ratio is shifted to higher with resect to RHIC measurement. he nuclear modification factor for ion, kaon and roton has been resented. At higher, this measurement indicates that the medium does not significantly affect the fragmentation. π ( /( π ALI PREL EPJ Web of Conferences ( /(π π PbPb s NN =2.76 ev 05% 2040% 6080% (GeV/c s=7 ev.2 05% PbPb s NN = 2.76 ev EPOS 2.7 v3 AMP Recombination ALI PREL (GeV/c Figure 7. o: roton-to-ion ratio in Pb Pb collisions in several centrality bins comared with result at 7 ev. Bottom: rotonto-ion ratio in central Pb Pb collisions comared with models. References [] Collaboration, Physics Performance Reort, Volume I, J.Phys.G: Nucl. Part. Phys. 30 ( [2] Collaboration, Physics Performance Reort, Volume II, J.Phys.G: Nucl. Part. Phys. 32 ( [3] Collaboration, J.Instrum.3,SO8002 (2008. [4] Collaboration, Production of ions, kaons and rotons in collisions at s=900 GeV with at the LHC, Eur.Phys.J.C.7(6; 655, 20. [5] F. Barile, Inclusive charged hadrons roduction in collisions with the -HMPID detector at the LHC, htt://cds.cern.ch/record/5744?ln=it [6] C. sallis, J. Stat. Phys. 52, 479 (988. [7] B. Abelev et al. (SAR Collaboration, Phys. Rev. C 79, (2009. [8] S. S. Adler et al. (PHENIX Collaboration, Phys. Rev. C 69, (2004. [9] C. Shen, U.W. Heinz, P. Huovinen, and H. Song, Phys. Rev. C 84, (20. [0] Y. Karenko and Y. Sinyukov, J. Phys. G 38, (20. [] Y. Karenko, Y. Sinyukov, and K.Werner, arxiv: [2] S. Bass et al., Prog. Part. Nucl. Phys. 4, 255 (998. [3] M. Bleicher et al., J. Phys. G 25, 859 (999. [4] P. Bozek, Phys. Rev. C 85, (202. [5] P. Bozek, Acta Phys. Pol. B 43, 689 (

5 LHCP 203 [6] I. Arsene et al. (BRAHMS Collaboration, Phys. Rev. C 72, (2005. [7] Pion, Kaon and Proton Production in Central Pb Pb Collisions at s NN = 2.76 ev. B.Abelev et all. (AL- ICE Collaboration. 09, (202. [8] Centrality Deendence of π, K, Production in Pb Pb Collisions at s NN = 2.76 ev. Collaboration. arxiv: v. [9] E. Schnedermann, J. Sollfrank, and U.W. Heinz, Phys. Rev. C 48, 2462 (993. [20] Midraidity Antiroton-to-Proton Ratio in Collisions at s = 0.9 and 7 ev Measured by the AL- ICE Exeriment. K.Aamodt et al. ( collaboration DOI: 0.03 / Phys. Rev. Lett PRL 05. [2] A. Andronic, P. Braun-Munzinger, and J. Stachel, Phys. Lett. B 673, 42 (2009. [22] K. Werner, I. Karenko, M. Bleicher,. Pierog, and S. Porteboeuf-Houssais, Phys. Rev. C 85, (202, arxiv: [nucl-th]. [23] Nuclear Physics A ( c 766c htt://dx.doi.org/0.06/j.bbr