Identified charged hadron production in Pb-Pb collisions at the LHC with the ALICE Experiment
IIdentified charged hadron production in Pb–Pb collisions at theLHC with the ALICE Experiment
Leonardo Milano (for the ALICE Collaboration) Dipartimento di Fisica dell’Universit´a di Torino & Istituto Nazionale di Fisica Nucleare - INFN
Abstract
Identified particle spectra represent a crucial tool to understand the behavior of the matter cre-ated in high-energy heavy-ion collisions. The transverse momentum p T distributions of iden-tified hadrons contain informations about the transverse expansion of the system and constrainthe freezeout properties of the matter created. The ALICE experiment has good particle iden-tification performance over a broad p T range. In this contribution the results for identified pi-ons, kaons and protons in heavy-ion collisions at 2.76 TeV center-of-mass energy are presented.These results are compared with other identified particle measurements obtained by previousexperiments, and discussed in terms of the thermal and hydrodynamic pictures. The status ofextensions of this analysis, with the study of identified particles as a function of event-by-eventflow in Pb–Pb collisions, is also discussed.The ALICE experiment has unique particle identification (PID) capabilities. The combina-tion of di ff erent detectors which use di ff erent PID techniques allows identification over a broad p T range [1]. The results presented here are obtained using the Pb–Pb data at √ s NN = | η | ≤ E / d x signal in the silicon Inner Tracking System (ITS) and in the Time Projection Chamber(TPC) and the information from the Time Of Flight (TOF) detector. A pair of forward scintilla-tor hodoscopes, the VZERO detectors (2.8 < η < < η < -1.7), is used for triggering[2]. The centrality of the collision can be estimated using the signal in the VZERO detector, thereconstructed multiplicity in the central barrel, or other forward detectors [2]. p T distribution of identified hadrons The p T distribution of hadrons contains the information about the collective expansion ofthe fireball (radial flow) and the temperature of kinetic freezeout ( T kin ). The ALICE measure-ment of identified particle spectra in central (0-5%) Pb–Pb collisions at √ s NN = p T distributions of positive andnegative particles are found to be compatible within errors, for this reason results for summedcharge states are presented. Hadron spectra are reported for primary particles, defined as promptparticles produced in the collision, including decay products, except those from weak decays of A list of members of the ALICE Collaboration and acknowledgements can be found at the end of this issue.
Preprint submitted to Nuclear Physics A November 6, 2018 a r X i v : . [ h e p - e x ] F e b a t a / M ode l ) c ) ( G e V / y d T p / ( d N d T p π / e v N / ) c (GeV/ T p × ( π + + π × ( + K + K 1) × (pp + = 2.76 TeV NN sALICE, Pb Pb = 200 GeV NN sSTAR, Au Au, = 200 GeV NN sPHENIX, Au Au, VISH2+1HKMkowKraMUSIC + UrQMD π + + π + K + K pp + ALI−DER−45229 s e l e c t i on N o q % l o w e s t q s e l e c t i on N o q % h i ghe s t q ) c (GeV/ T p All Charged π + + π +K + K pp + =2.76 TeV NN sPb Pb centrality 30 40% Systematic errorStatistical errorNormalization error
ALI−PREL−32211
Figure 1: Left: transverse momentum distributions of the sum of positive and negative particles(box: systematic errors; statistical errors smaller than the symbol for most data points) comparedto RHIC data and hydrodynamic models. Right: ratio between the raw spectra in the sample with10% highest (lowest) q and the unbiased sample.strange particles. The fraction of primaries in the track sample at a given p T bin was estimatedfrom data by fitting the DCA xy distribution with three Monte Carlo templates: primary particles,secondaries from material and secondaries from weak decays [3]. A detailed description of thesystematic error and PID procedure can be found in [4]. Spectra measured at the LHC are com-pared with RHIC results for Au–Au collisions at √ s NN =
200 GeV (black-empty markers). Thespectral shape is significantly harder at the LHC with respect to RHIC. The kinetic freezeoutparameters (cid:104) β T (cid:105) and T kin can be extracted from a simultaneous blast wave fit to the π , K and pspectra. The p T ranges used in the fit are 0.5-1 GeV / c , 0.2-1.5 GeV / c , 0.3-3 GeV / c for π , K andp. Data are well described by the blast wave function with (cid:104) β T (cid:105) = ± T kin = ±
10 MeV. It should be noted that T kin is sensitive to the pion fit range (due to large contributionfrom resonances) while (cid:104) β T (cid:105) does not strongly depend on the p T range used in the fit. A similarfit to central Au–Au collisions at √ s NN =
200 GeV was performed in [5]: the (cid:104) β T (cid:105) is ∼ T kin is compatible within errors. The comparisonwith predictions (see [4] for details) seems to suggest that hydrodynamic models with a refinedlate fireball description are able to reproduce the measured p T spectra at the LHC.In order to further investigate the hydrodynamic behaviour of the hadron production twoof the main features of hydrodynamics were correlated: p T distribution of hadrons and ellipticflow. For a given centrality the eccentricity of the collision (related with the initial geometry)fluctuates. A strategy to select events based on the geometry of the overlapping region (so called“event shape engineering”) was presented for the first time during this conference in [6, 7]. Oneway to do this is using the VZERO detector to calculate the flow vector (cid:126) Q on an event-by-eventbasis, a 2D vector with components Q , x = (cid:80) i w i cos(2 φ i ) and Q , y = (cid:80) i w i sin(2 φ i ), where the2um i runs over all the channels of the VZERO detector, w i is the multiplicity of channel i and φ i is the angle of channel i . The module of the (cid:126) Q vector is normalized by the multiplicity M inthe VZERO: q = | Q | / √ M . In [6, 7] it has been shown that the sample with large (small) q shows a significantly larger (smaller) v with respect to the unbiased sample and that the non-flow contributions are negligible. Systematic checks show that this selection does not introducetrivial biases related with multiplicity shift or jet contribution. A modification of the p T spectrumin semi-central (30-40%) Pb–Pb collisions at √ s NN = p T region(from ∼ ∼ / c ) is observed, when events are selected according to the event shapeengineering (Figure 1, right): higher (lower) v means harder (softer) spectra. This modificationvanishes at high p T supporting a correlation related with hydrodynamics rather than with hardprocesses. A hint of mass ordering can be observed in the region between ∼ ∼ / c . Amore detailed study (including comparison with models and hydrodynamic fit of particle spectra)will allow to better understand the observed correlation between v and radial flow.
2. Thermal production of hadrons at the LHC
The thermal description of hadron production was found to be successful over a broad rangeof energies (from √ s NN = √ s NN =
200 GeV [8, 9]). There are only these threeparameters which govern the thermal model: the chemical freezeout temperature T ch , the baryon-chemical potential µ B and the volume V . In order to extract the parameters T ch , µ B and V athermal fit [8] of integrated yields at midrapidity d N / d y in central (0-20%) Pb–Pb collisionswas performed. It is reported in Figure 2 (left). Results from strange and multi-strange particleanalyses [10] are also included in the fit. The antibaryon over baryon ratios suggest a vanishingbaryon-chemical potential at the LHC: for this reason µ B is fixed to 1 MeV in the fit. φ and K ∗ are not included in the fit. d N / d y Data: ALICE, 0 20% (preliminary)=39.6/ 9 df /N χ Thermal model fit, = 1 MeV fixed) b µ ( T=152 MeV, V=5300 fm =2.76 TeV NN sPb Pb + π π + K K p p Λ Ξ + Ξ Ω + Ω φ K* ALI−PREL−32248 Y i e l d r e l a t i v e t o p i on s =2.76 TeV (ALICE, 0 20%, preliminary) NN s =0.2 TeV (STAR) NN s =0.2 TeV (BRAHMS) NN s =0.2 TeV (PHENIX) NN sModel, T=164 MeVModel, T=152 MeV particle+anti particle K 3 × p 4 ×Λ × Ξ × Ω ×φ × K* ALI−PREL−32253
Figure 2: Left: integrated yields at mid-rapidity d N / d y in central (0-20%) Pb–Pb collisions at √ s NN = N / d y incentral (0-20%) Pb–Pb collisions relative to pions with RHIC comparison and thermal modelpredictions.The temperature extracted from the fit T ch = ± T ch constant above SPS energies (164 MeV). From3igure 2 (left) it is possible to see some tension between the data and the fit especially for strangeand multi-strange particles. This is reflected also in the large value of the χ / N d . f . = /
9. Thecomparison of integrated yields relative to pions (Figure 2, right) with RHIC hints to decreasingratios at the LHC, especially for what concerns p / π and Λ/ π . The prediction from the thermalmodel is reported with two di ff erent values of the freezeout temperature: T ch =
164 MeV (valueobtained from fit to RHIC data) and T ch =
152 MeV (from the fit described above). T ch = / π and Λ/ π . On the other hand the model prediction using T ch =
152 MeV obtained fromthe fit is closer to the measured p / π and Λ/ π ratios but it misses ratios involving multi-strangehadrons. It has already been pointed out that interactions in the hadronic phase, in particular viathe large cross section channel for antibaryon-baryon annihilation, could explain the significantdeviation from the usual thermal ratios [11, 12].
3. Conclusions p T distributions of π , K, p in central (0-5%) Pb–Pb collisions at the LHC are harder thanpreviously measured at RHIC. They are well described by hydrodynamic models including arefined description of the late fireball stages. Fitting the spectra with a hydrodynamic-inspiredblast wave model results in the highest radial flow parameter ever measured, (cid:104) β T (cid:105) = ± ff erent values of the ellipticflow based on the magnitude of the flow vector (cid:126) Q . The p T distributions of π , K, p in semi-central(30-40%) Pb–Pb collisions show a correlation between radial flow and elliptic flow: high (low)elliptic flow events mean harder (softer) spectra.In central (0-20%) Pb–Pb collisions at √ s NN = Λ . The temperature obtained fromthermal fit to integrated yields at mid-rapidity d N / d y is lower than expected from previous exper-iments. This discrepancy can be explained in terms of interaction in the hadronic phase whichcan modify the relative hadron abundances. It should be noted that a refined description of thehadronic phase is also needed to reproduce femtoscopy correlations at the LHC [13].Identified hadron results at the LHC cast a new light upon the hydrodynamic and thermalbehavior of the hadron production in heavy-ion collisions. The p-A run expected at the LHCat the beginning of 2013, together with the continuously improving experimental precision anddescription from the theory, will provide further insights (and model constraints) on the heavy-ion puzzle. ReferencesReferences [1] K. Safarik for the ALICE Collaboration, these proceedings[2] ALICE Collaboration, Phys. Rev. Lett. 106, 032301 (2011)[3] A. Kalweit for the ALICE Collaboration, J. Phys. G: Nucl. Part. Phys. 38 124073 (2011)[4] ALICE Collaboration, arXiv:1208.1974 [hep-ex] (2012)[5] STAR Collaboration, Phys.Rev.C79:034909 (2009)[6] S. Voloshin for the ALICE Collaboration, these proceedings[7] A. F. Dobrin for the ALICE Collaboration, these proceedings[8] A. Andronic, P. Braun-Munzinger and J. Stachel, Phys.Lett.B678:516 (2009)[9] J. Cleymans and K. Redlich, Phys.Rev.Lett. 81,5284-5286 (1998)[10] S. Singha for the ALICE Collaboration, these proceedings
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