doi:10.1016/j.nuclphysa.2018.09.022

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XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2018)Balance functions of (un)identified hadrons in Pb–Pb, p–Pb,and pp collisions at the LHCJinjin Pan (on behalf of the ALICE Collaboration)Wayne State University, 666 W. Hancock St, Detroit, MI 48201, USAAbstractIn ultrarelativistic heavy-ion collisions, correlations of particles with opposite quantum numbers provide insight intoquark production mechanisms and time scales, collective motion, and re-scattering in the hadronic phase. The longitu-dinal and azimuthal widths of balance functions for charged kaons and charged pions are used to examine the two-wavequark production model proposed to explain quark-antiquark production within the QGP, which predicts a large increasein up and down quark pairs relative to strange quark pairs around the time of hadronization. Balance functions are alsoanalyzed in small collision systems such as p–Pb and pp to study fragmentation effects and possible collective effectsin high-multiplicity events. A comprehensive set of balance functions has been measured using the ALICE detector,including results for unidentified hadrons in Pb–Pb and p–Pb collisions at√sNN=5.02 TeV, for charged pions in Pb–Pbcollisions at√sNN=2.76 TeV, p–Pb collisions at√sNN=5.02 TeV and pp collisions at√s=7 TeV, and for chargedkaons in Pb–Pb collisions at√sNN=2.76 TeV. The first balance function yield results are also presented.Keywords:LHC, ALICE, balance function, unidentified hadron, charged-pion, charged-kaon, Pb–Pb, p–Pb, pp1. IntroductionIn ultrarelativistic heavy-ion collisions, due to conservation of quantum numbers, a negative balancingcharge is produced at approximately the same space–time for each positive general charge. The balancefunction (BF) is defined asB(Δy,Δφ)=12{〈N+−〉(Δy,Δφ)−〈N++〉(Δy,Δφ)〈N+〉+〈N−+〉(Δy,Δφ)−〈N−−〉(Δy,Δφ)〈N−〉},(1)where〈Nab〉(Δy,Δφ) denotes the average number of pairs per event with charge combinationab(a,b=+,−)as a function of two particle rapidity differenceΔyand azimuthal angle differenceΔφ, while〈Na〉denotesthe average number of single particles per event with chargea. Thus, the BF locates general balancingcharges in the final state on a statistical basis [1]. The BF is sensitive to multiple phenomena that determinethe production of balancing charges and their transport, including two-wave quark production [2], radialflow [3], diffusion [4], quantum statistics [5], and the Coulomb effect.Available online at www.sciencedirect.comNuclear Physics A 982 (2019) 315–3180375-9474/© 2018 The Authors. Published by Elsevier B.V.www.elsevier.com/locate/nuclphysahttps://doi.org/10.1016/j.nuclphysa.2018.09.022This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Jinjin Pan

2. Analysis detailsThe results presented in this paper are based on the data sets acquired at the LHC using the ALICEdetector [6] during the Pb–Pb runs at√sNN=2.76 TeV in 2010 and at√sNN=5.02 TeV in 2015, the p–Pbruns at√sNN=5.02 TeV in 2013 and 2016, and the pp runs at√s=7 TeV in 2010. The acquisition ofevents is triggered with a minimum bias trigger using the V0 detector, which requires a coincidence of hitsin both V0A and V0C detectors in Pb–Pb and p–Pb collisions, while in pp collisions it requires at least onehit in the Silicon Pixel Detector (SPD) or in one of V0A and V0C. Background events, such as generatedby beam–gas interactions, are eliminated to a negligible level (<0.1%) by comparing the signal arrival timebetween V0A and V0C with 1 ns resolution. In total, with all the selection criteria applied, about 14×106,82×106, 100×106, 620×106and 240×106events are analyzed for Pb–Pb at 2.76 TeV and 5.02 TeV, p–Pbin 2013 and 2016, and pp collisions, respectively. The particle identification is achieved by using combinedinformation from the Time Projection Chamber (TPC) and the Time-Of-Flight (TOF) detector, with purityof both charged kaons (K±) and charged pions (π±) better than 96%. For unidentified hadrons (h±), thepseudorapidity selection is|η|≤0.8, while for K±andπ±, the rapidity selection is|y|≤0.8. Detectionefficiency and acceptance are fully corrected using a weight technique [7].3. ResultsyΔ1−0.5−00.51y)ΔB(00.10.20.3 0-10% 30-40% 50-90% = 2.76 TeVNNsPb-Pb ALICE Preliminaryc 2 GeV/≤ Tp ≤0.2 π ≤| φΔ|±KALI−PREL−159000yΔ1−01y)ΔB(00.20.40.6 0-5% 30-40% 70-90% = 2.76 TeVNNsPb-Pb ALICE Preliminaryc 2 GeV/≤ Tp ≤0.2 π ≤| φΔ|±πALI−PREL−158908Centrality (%)0 20406080yΔσ0.40.60.81ALICE Preliminary = 2.76 TeVNNsPb-Pb c 2 GeV/≤ Tp ≤0.2 π ≤| φΔ 1.6, |≤y| Δ: |±π π ≤| φΔ 1.1, |≤y| Δ: |±KALI−PREL−159008Fig. 1. The BF as a function ofΔyfor K±(left) andπ±(middle) in selected centralities, and their RMS widths as a function of centrality(right) in Pb–Pb collisions at√sNN=2.76 TeV. (rad)φΔ024)-1) (radφΔB(00.050.10.15 0-10% 30-40% 50-90% = 2.76 TeVNNsPb-Pb ALICE Preliminaryc 2 GeV/≤ Tp ≤0.2 1.1≤y| Δ|±KALI−PREL−159004 (rad)φΔ024)-1) (radφΔB(00.10.20.30.4 0-5% 30-40% 70-90% = 2.76 TeVNNsPb-Pb ALICE Preliminaryc 2 GeV/≤ Tp ≤0.2 1.6≤y| Δ|±πALI−PREL−158912Centrality (%)0 20406080 (rad)φΔσ0.511.52ALICE Preliminary = 2.76 TeVNNsPb-Pb c 2 GeV/≤ Tp ≤0.2 π ≤| φΔ 1.6, |≤y| Δ: |±π π ≤| φΔ 1.1, |≤y| Δ: |±KALI−PREL−159012Fig. 2. The BF as a function ofΔφfor K±(left) andπ±(middle) in selected centralities, and their RMS widths as a function of centrality(right) in Pb–Pb collisions at√sNN=2.76 TeV.In this paper, only one-dimensional projections of B(Δy,Δφ) ontoΔyandΔφaxis along with their RMSwidths are presented. Figures 1 and 2 present B(Δy) and B(Δφ) for K±andπ±in selected centralities, andJ. Pan / Nuclear Physics A 982 (2019) 315–318316

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