The ALICE Collaboration at the CERN Large Hadron Collider has measured the production of multi-strange baryons in Pb-Pb collisions at 2.76TeV. Preliminary results on mid-rapidity transverse momentum spectra of the charged Ξ and Ω baryons, in different centrality classes, are presented

Colella, D.

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DOI 10.1393/ncc/i2013-11414-9Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Multi-strange baryon production at mid-rapidity in Pb-Pbcollisions at√sNN = 2.76TeV with ALICED. Colella on behalf of the ALICE CollaborationDipartimento Interateneo di Fisica “M. Merlin” and INFN, Sezione di Bari - Bari, Italyricevuto il 31 Agosto 2012Summary. — The ALICE Collaboration at the CERN Large Hadron Collider hasmeasured the production of multi-strange baryons in Pb-Pb collisions at 2.76TeV.Preliminary results on mid-rapidity transverse momentum spectra of the charged Ξand Ω baryons, in different centrality classes, are presented.PACS 25.75.-q – Relativistic heavy-ion collisions.The study of strange and multi-strange particle production in relativistic heavy-ioninteractions is an important tool to investigate the properties of the strongly interactingsystem created in the collision, as there is no net strangeness content in the collidingnuclei. In particular, baryons with more than one unit of strangeness are very usefulprobes of the early partonic stages of the collision due to their small hadronic cross-section. The enhancement of strangeness production in relativistic heavy-ion collisionsrelative to proton-induced reactions was one of the predicted signatures of the Quark-Gluon Plasma formation [1]. It was first observed at SPS, confirmed afterwards by dataat RHIC [2], and the original assumptions, on which the explanation of strangenessenhancement was based, have gone through further theoretical developments [3]. TheALICE data can provide additional test for these theories.The ALICE experiment was specifically designed to study heavy-ion physics at theLHC, namely the properties of strongly interacting matter at high-energy density. Thefirst LHC heavy-ion run took place at the end of 2010, using Pb ions, accelerated sothat the centre-of-mass energy of the collision is√sNN = 2.76TeV; and the ALICEdetector collected almost 30×106 nuclear interaction minimum bias triggers. The analysisdescribed in this proceedings is mainly based on the sub-detectors mentioned in thefollowing. The tracking and vertexing are performed with the full tracking system: theInner Tracking System (ITS, six layers of silicon detectors) and the Time ProjectionChamber (TPC), which is also used for particle identification via specific ionization.The Silicon Pixel Detector (SPD, the two innermost layers of the ITS) and the VZEROdetector (scintillation hodoscopes placed on either side of the interaction point) wereused for triggering. The VZERO was crucial for the centrality determination as well.c© CERN on behalf of the ALICE Collaboration under CC BY-NC 3.0 8990 D. COLELLA on behalf of the ALICE COLLABORATION (GeV/c)Tp0 1 2 3 4 5 6 7 8 9-1 (GeV/c)|y|<0.5 |T dN/dpevts1/N-610-510-410-310-210-110110 = 2.76 TeVNNsPb-Pb at  2 + sys2staterror =  -Ξ0-20%20-40%40-60%60-90%Preliminary (GeV/c)Tp0 1 2 3 4 5 6 7 8-1 (GeV/c)|y|<0.5 |T dN/dpevts1/N-610-510-410-310-210-1101 = 2.76 TeVNNsPb-Pb at  2 + sys2staterror =  -Ω0-20%20-40%40-60%60-90%PreliminaryFig. 1. – Transverse-momentum spectra of the Ξ− and the Ω− in four centrality classes.Multi-strange baryons are measured through the reconstruction of the topology ofthe following weak decays: Ξ− → Λ + π−, Ω− → Λ + K− (with Λ → π− + p) plus thecharge conjugates for the anti-particle decays. The resulting branching ratios are 63.9%and 43.3% for the Ξ and the Ω, respectively. The Ξ and the Ω candidates are found bycombining reconstructed charged tracks: cuts on geometry and kinematics, e.g. relatedto the impact parameter of the tracks and the invariant masses, are applied to select firstthe Λ candidate and then to match it with all the remaining secondary tracks (bachelorcandidates). In order to reduce the combinatorial background while keeping reasonablethe signal loss, the selection cuts are tightened and optimized at the analysis level. Inaddition, cuts on particle identification via specific ionization in the TPC for the threedaughter tracks are used.The raw counts in pT bins are obtained extracting the signal from the invariantmass distributions; here a symmetric region around the peak (±3σ) is defined fittingthe distribution with the sum of a gaussian and a polynomial. In each pT bin, thebackground is sampled on both sides of the peak in two regions 6σ wide and 6σ awayfrom the peak and fitted with a polynomial of second degree, and the signal in the peakregion is obtained subtracting the integral of the fit function of the background from thepeak population. About 3×106 HIJING events were generated and used to compute theacceptance-efficiency correction. In order to extract particle yields integrated over the fullpT range, the corrected spectra (fig. 1) are fitted using the blast-wave parametrization [4].Yields are then calculated integrating the corrected spectra in the measured pT rangeand extrapolating with the fit function outside.The enhancements have been calculated as the ratio between the yields in Pb-Pbcollisions and those in pp interactions at the same energy, both normalized to the numberof participants. The pp reference values were obtained interpolating ALICE data at twoenergies (√sNN = 0.9 and 7TeV [5,6]) for the Ξ and STAR data at 200GeV and ALICEdata at 7TeV for the Ω. For both particles, the excitation function of PYTHIA yields(Perugia 2011 tune 88 S350 [7]) is assumed: despite a constant underestimation of theyields, its energy dependence in s0.13 (which is slightly higher than s0.11, the one forcharged-particle pseudorapidity density [8]) was found to be satisfactory. In fig. 2 theenhancements as a function of the mean number of participants are shown (full symbols):they increase with centrality and with the strangeness content of the particle as alreadyobserved at lower energies. Comparing the ALICE measurements with those at SPSand RHIC (hollow symbols), the enhancements are found to decrease as the centre-of-mass energy increases, continuing the same trend established at lower energies and firstobserved at SPS [2].MULTI-STRANGE BARYON PRODUCTION IN Pb-Pb COLLISIONS AT√sNN = 2.76TeV 91>part<N1 10 210> relative to pp/BepartYield / <N110Preliminary = 2.76 TeVNNsPb-Pb at  -ΞΛNA57 (17.2 GeV):STAR (200 GeV):    >part<N1 10 210> relative to pp/BepartYield / <N110+Ω+-Ω+ΞΛ      NA57 (17.2 GeV):STAR (200 GeV):Fig. 2. – Enhancements in |y| < 0.5 as a function of the mean number of participants Npartmeasured by ALICE and compared to SPS and RHIC data. The bars on the dotted lineindicate the systematic uncertainties on the pp reference.ConclusionsALICE has measured multi-strange baryon spectra in Pb-Pb interactions at√sNN =2.76TeV, up to pT of 9 and 8GeV/c for the most central collisions, for Ξ and Ω, respec-tively. The enhancements follow the hierarchy based on the strangeness content of theparticle and are found to increase with collision centrality. The comparison with lowerenergy data shows a decrease of the strangeness enhancement as the energy increases,following the same trend observed at SPS and between SPS and RHIC, in qualitativeagreement with the removal of canonical suppression.∗ ∗ ∗Copyright CERN for the benefit of the ALICE Collaboration.REFERENCES[1] Koch P., Mu¨ller B. and Rafelski J., Phys. Rep., 142 (1986) 167.[2] Antinori F. et al. (NA57 Collaboration), J. Phys. G, 32 (2006) 427; Abelev B. I. etal. (STAR Collaboration), Phys. Rev. C, 77 (2008) 044908.[3] Schaffner-Bielich J., J. Phys. G, 30 (2004) R245.[4] Schnedermann E., Sollfrank J. and Heinz U., Phys. Rev. C, 48 (1993) 2462.[5] Aamodt K. et al. (ALICE Collaboration), Eur. Phys. J. C, 71 (2011) 1594.[6] Abelev B. I. et al. (ALICE Collaboration), arXiv:1204.0282.[7] Skands P. Z., Phys. Rev. D, 82 (2010) 074018.[8] Aamodt K. et al. (ALICE Collaboration), Phys. Rev. Lett., 105 (2010) 252301.

Multi-strange baryon production at mid-rapidity in Pb-Pb

collisions at √sNN = 2.76TeV with ALICE

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Multi-strange baryon production at mid-rapidity in Pb-Pb collisions at √sNN = 2.76TeV with ALICE

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