Multi-strange baryon production in Pb-Pb and pp collisions at s NN − − − − √ = 2.76 TeV with the ALICE experiment at the LHC
MMulti-strange baryon production in Pb–Pb and ppcollisions at √ s NN = 2.76 TeV with the ALICEexperiment at the LHC Domenico Colella (for the ALICE Collaboration)
Dipartimento Interateneo di Fisica ”M. Merlin” and Sezione INFN, Via Orabona 4, 70126Bari, ItalyE-mail: [email protected]
Abstract.
The production of Ξ − and Ω − baryons and their anti-particles in Pb–Pb and ppcollisions at √ s NN = 2.76 TeV has been measured by the ALICE collaboration. The transversemomentum spectra at mid-rapidity ( | y | < .
5) in pp and in Pb–Pb collisions for five centralityintervals have been compared with model predictions. Hyperon yields and spectra in Pb–Pbcollisions, normalized to the corresponding measurements in pp at the same centre-of-massenergy, allow the study of the strangeness enhancement and the nuclear modification factor asa function of the transverse momentum ( p T ) and collision centrality.
1. Introduction
The study of strange and multi-strange particle production in relativistic heavy-ion (A–A)interactions is an important tool to investigate the properties of the strongly interacting systemcreated in the collision, as there is no net strangeness content in the colliding nuclei. In particular,baryons with more than one unit of strangeness are very useful probes of the early partonic stagesof the collision due to their small hadronic cross-section.The enhancement of strangeness in heavy-ion collisions was one of the earliest proposedsignals for the Quark-Gluon Plasma [1]. It rests on the expectation that in a deconfined state theabundances of parton species should quickly reach their equilibrium values, resulting in a higherabundance of strangeness per participant than that seen in pp interactions. This enhancement,first observed at SPS [2] and confirmed afterwards by data at RHIC [3], is more pronouncedfor particles with larger strangeness content and decreases as the centre-of-mass energy of thecollision increases. Over the past 15 years, it has been found that the canonical suppressioneffect, for pp collisions, is important [4] and contributes to the overall hyperon enhancement inA–A. This interpretation can now be re-examined at the much higher LHC energy.A suppression of high-momentum unidentified particle production in Pb–Pb compared to ppcollisions has been observed by the ALICE collaboration and used as evidence for strong partonenergy loss and large medium density at the LHC [5]. Measurements of the nuclear modificationfactors for identified particles could provide insight into particle production and energy lossmechanisms.The ALICE experiment was specifically designed to study heavy-ion collisions at the LHC,namely the properties of strongly interacting matter at high energy density. The first LHC a r X i v : . [ h e p - e x ] N ov eavy-ion run took place at the end of 2010 with Pb–Pb ions accelerated at a centre-of-massenergy √ s NN = 2.76 TeV. Almost 30 × minimum bias nuclear interaction triggers wererecorded. The pp reference data were collected during March 2011. Almost 80 × minimumbias nucleon interaction triggers were recorded. The tracking and vertexing are performed withthe full tracking system: the Inner Tracking System (ITS, six layers of silicon detectors) andthe Time Projection Chamber (TPC), which is also used for particle identification via specificionization. The Silicon Pixel Detector (SPD, the two innermost layers of the ITS) and theVZERO detector (scintillation hodoscopes placed on either side of the interaction point) wereused for triggering. The VZERO was also crucial for the collision centrality determination. Acomplete description of the ALICE sub-detectors can be found in [6].
2. Analysis method
Multi-strange baryons are reconstructed through their weak decay topologies, namely Ξ − → π − + Λ, Ω − → K − + Λ (with Λ → π − + p), and the corresponding charge conjugate decays forthe anti-particles. The resulting branching ratios are 63.9% and 43.3%, for the Ξ and the Ωrespectively. The Ξ and the Ω candidates are found by combining reconstructed charged tracks:cuts on geometry and kinematics are applied to select first the Λ candidate and then to matchit with all the remaining secondary tracks (bachelor candidates). In addition, cuts on particleidentification via specific energy loss in the TPC for the three daughter tracks are used.To extract the signal in p T intervals, a symmetric region around the invariant mass peak( ± σ ) is defined by fitting the distribution with the sum of a Gaussian and a polynomial. Thebackground is sampled in two regions on both sides of the peak and fitted with a polynomialof first or second degree (depending on the colliding system and the p T interval). In each p T interval the signal in the peak region is obtained by subtracting the integral of the backgroundfit function from the peak population. More details can be found in [7].
3. Results and conclusions
Transverse momentum spectra, corrected for acceptance and efficiency, in pp and Pb–Pbcollisions are shown in Figure 1 and 2 respectively. In pp collisions, the comparison withpredictions obtained using PYTHIA 6 (Perugia-2011 tune [8][9]) shows that this modelunderestimates the yields, both for Ξ and Ω, and does not reproduce the shape of the spectra.Spectra in Pb–Pb collisions, reconstructed in five centrality classes (0-10%, 10-20%, 20-40%,40-60% and 60-80%), are compared with predictions obtained using four hydrodynamic models:VISH2+1 [10] , HKM [11] , Krak´ow [12] and EPOS [13] . Krak´ow provides a good descriptionfor both yields and shapes in the p T range up to 3 GeV/ c , and EPOS gives the most successfuldescription of spectral shape in a wider p T range. The overall description for the Ω is lesssuccessful while for both particles types it degrades going from central to more peripheralcollisions.The strangeness enhancements, calculated as the ratio between the yields in Pb–Pb collisionsand those in pp interactions at the same energy, both normalized to the number of participants( N part ), are shown in Figure 3(a-b). The pp reference values were obtained by interpolatingALICE and STAR data at different energies [7] and checked with the preliminary measurementsof the yields in pp collisions at the reference energy. The enhancements as a function of the meannumber of participants increase with centrality and with the strangeness content of the particleas already observed at lower energies, and decrease as the centre-of-mass energy increases,continuing the trend established at lower energies (between SPS and RHIC energies).The hyperon-to-pion ratios Ξ /π ≡ (Ξ − + Ξ + ) / ( π − + π + ) and Ω /π ≡ (Ω − + Ω + ) / ( π − + π + ),for A–A and pp collisions, both at LHC and RHIC energies, are shown in Figure 3(c) as afunction of (cid:104) N part (cid:105) . The relative production of strangeness in pp collisions at the LHC is largerthan at RHIC energy while it is almost the same at the two energies for A–A collisions, giving an igure 1. Transverse momentum spectra normalized to the number of inelastic collisions forΞ − , Ξ + and Ω in pp collisions, compared with predictions obtained with PYTHIA 6 (Perugia-2011 tune [8][9]). Ratios of data to models are also shown. The error bars represent the statisticaluncertainties and the boxes the systematic ones. Figure 2.
Transverse momentum spectra for Ξ (a) and Ω (b) hyperons (average of particleand anti-particle) in five different centrality classes, compared to hydrodynamic models. Ratiosof models to data are also shown.explanation of the decrease of enhancement with increasing energy. The measurements of theseratios in Pb–Pb collisions are in agreement with predictions from the thermal model, based ona grand canonical approach [14][15]. The hyperon-to-pion ratio increases when going from pp toA–A, showing a relative enhancement of strangeness production in A–A collisions (normalizedto the pion yield) which is about half of that seen when normalizing to the mean number ofparticipants. The enhancement rises with centrality up to about (cid:104) N part (cid:105) ∼
150 and apparentlysaturates thereafter. igure 3. (a,b) Enhancements in the rapidity range | y | < . (cid:104) N part (cid:105) . Boxes on the dashed line at unity indicate statistical and systematicuncertainties on the pp or p–Be reference. (c) Hyperon-to-pion ratios as a function of (cid:104) N part (cid:105) , forA–A and pp collisions at LHC and RHIC energies. The lines mark the thermal model predictionsfrom [14] (full line) and [15] (dashed line).The nuclear modification factor ( R AA ) is defined as the ratio between the differential yieldin A–A collisions (d N AA / d p T dy ) and the differential cross-section of particle production in ppcollisions (d σ NN / d p T dy ), normalized by the geometric nuclear overlap function T AA [16]. It hasbeen measured by ALICE in different centrality intervals and both for multi-strange baryonsand lighter particles [17] [18]. The R AA for Ξ follows the same trend as the proton at high p T ( > c ), where the suppression does not depend on the particle mass. At intermediate p T there are indications of mass-ordering among the baryons. The R AA for the Ω seems to bestrongly affected by the strangeness enhancement and the effect becomes weaker when goingfrom the most central to the most peripheral class. References [1] J. Rafelski, B. M¨uller,
Physics Review Letters , (1982) 1066[2] NA57 Collaboration F. Antinori et al. , Journal of Physics G , (2010) 045105[3] STAR Collaboration B. I. Abelev et al. , Physical Review C , (2008) 044908[4] J. S. Hamieh, K. Redlich and A. Tounsi, Physics Letters B , (2000) 61[5] ALICE Collaboration K. Aamodt et al. , Physics Letters B , (2011) 30[6] ALICE Collaboration K. Aamodt et al. , JINST , (2008) S08002[7] ALICE Collaboration K. Aamodt et al. , arXiv:1307.5543 [nucl-ex] (2013)[8] P. Z. Skands, Physical Review D , (2010) 074018[9] P. Z. Skands, arXiv:1005.3457 [hep-ph] (2011)[10] C. Shen, U. Heinz, P. Huovinen and H. Song, Physical Review C , (2011) 044903[11] Y. Karpenko, Y. Senyukov and K. Werner, arXiv:1204.5351 [nucl-th] (2012)[12] P. Bozek, Acta Physica Polonica B , Physical Review C , (2012) 064907[14] A. Andronic, P. Braun-Munzinger and J. Stachel, Physics Letters B , (2009) 142; Erratum Physics LettersB , (2009) 561[15] J. Cleymans, I. Kraus, H. Oeschler, K. Redlich and S. Wheaton, Physical Review C , (2006) 034903[16] ALICE Collaboration K. Aamodt et al. , arXiv:1301.4361arXiv:1301.4361