(cid:0) N d’ordre : D.U. 2406 PCCF T 1306 Universit´e Blaise Pascal de Clermont-Ferrand U.F.R. Sciences et Technologies (cid:0) E´cole Doctorale des Sciences Fondamentales N 774 Universit`a degli Studi di Torino Scuola di Dottorato in Scienza ed Alta Tecnologia Indirizzo di Fisica ed Astrofisica – XXVI ciclo THESE pr´esent´ee pour obtenir le grade de Docteur d’Universit´e Sp´ecialit´e : Physique Corpusculaire par Massimiliano Marchisone Probing the Quark-Gluon Plasma from bottomonium production at forward rapidity with ALICE at the LHC Soutenue publiquement le 6 d´ecembre 2013 a` Clermont-Ferrand, devant la commission d’examen compos´ee de : Directeurs de th`ese : P. Dupieux LPC, Clermont-Ferrand E. Vercellin Universit`a di Torino e INFN Rapporteurs : G. Iaselli Universit`a di Bari, Politecnico di Bari e INFN I. Laktineh IPN, Lyon Examinateurs : R. Arnaldi INFN di Torino X. Lopez LPC, Clermont-Ferrand Universit`a degli Studi di Torino Scuola di Dottorato in Scienza ed Alta Tecnologia Indirizzo di Fisica ed Astrofisica – XXVI ciclo Universit´e Blaise Pascal de Clermont-Ferrand (cid:0) ´ Ecole Doctorale des Sciences Fondamentales N 774 Sp´ecialit´e : Physique Corpusculaire Tesi di Dottorato di Ricerca in Scienza ed Alta Tecnologia Probing the Quark-Gluon Plasma from bottomonium production at forward rapidity with ALICE at the LHC Clermont-Ferrand, 06/12/2013 Massimiliano Marchisone Tutors: P. Dupieux LPC, Clermont-Ferrand E. Vercellin Universit`a di Torino e INFN Controrelatori: I. Laktineh IPN, Lyon G. Iaselli Universita` di Bari, Politecnico di Bari e INFN Esaminatori: R. Arnaldi INFN di Torino X. Lopez LPC, Clermont-Ferrand Contents Introduction v 1 The Quark-Gluon Plasma in heavy-ion collisions 1 1.1 QCD and asymptotic freedom . . . . . . . . . . . . . . . . . . 1 1.2 The Quark-Gluon Plasma . . . . . . . . . . . . . . . . . . . . 3 1.3 Heavy-ion collisions. . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 QGP effects: experimental signatures . . . . . . . . . . . . . . 7 2 Quarkonium in medium 9 2.1 Discovery of quarkonium states . . . . . . . . . . . . . . . . . 9 2.2 Decay and feed-down of quarkonium . . . . . . . . . . . . . . 10 2.3 Quarkonium production mechanisms . . . . . . . . . . . . . . 11 2.3.1 Color Singlet Model . . . . . . . . . . . . . . . . . . . 12 2.3.2 Color Evaporation Model . . . . . . . . . . . . . . . . 12 2.3.3 Non-Relativistic QCD . . . . . . . . . . . . . . . . . . 13 2.4 Quarkonium as a probe of the QGP . . . . . . . . . . . . . . 14 2.5 Cold nuclear matter effects . . . . . . . . . . . . . . . . . . . 16 2.5.1 Initial-state effects . . . . . . . . . . . . . . . . . . . . 16 2.5.2 Final-state effects . . . . . . . . . . . . . . . . . . . . . 18 2.6 Quarkonium measurements in heavy-ion collisions. . . . . . . 18 2.6.1 Before the LHC era . . . . . . . . . . . . . . . . . . . 19 2.6.2 Charmonium measurements at the LHC . . . . . . . . 21 3 Υ production 23 3.1 The Υ family . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.2 Υ production in pp and pp¯ collisions . . . . . . . . . . . . . . 25 3.2.1 Introduction to the quarkonium polarization . . . . . 25 3.2.2 Experimental results . . . . . . . . . . . . . . . . . . . 26 3.3 Υ production in pA and dA collisions . . . . . . . . . . . . . 35 3.3.1 Experimental results . . . . . . . . . . . . . . . . . . . 35 3.4 Υ production in heavy-ion collisions . . . . . . . . . . . . . . 39 3.4.1 Introduction to theoretical models . . . . . . . . . . . 40 3.4.2 Experimental results . . . . . . . . . . . . . . . . . . . 41 i 3.5 Summary of the results . . . . . . . . . . . . . . . . . . . . . 43 4 ALICE at the LHC 45 4.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . 45 4.2 A Large Ion Collider Experiment . . . . . . . . . . . . . . . . 47 4.2.1 The ALICE coordinate system . . . . . . . . . . . . . 47 4.2.2 Central barrel detectors . . . . . . . . . . . . . . . . . 48 4.2.3 Forward detectors . . . . . . . . . . . . . . . . . . . . 55 4.2.4 The Muon Spectrometer . . . . . . . . . . . . . . . . . 58 4.3 Online Control System . . . . . . . . . . . . . . . . . . . . . . 59 4.3.1 Detector Control System. . . . . . . . . . . . . . . . . 59 4.3.2 Central Trigger Processor . . . . . . . . . . . . . . . . 59 4.3.3 Data AcQusistion System . . . . . . . . . . . . . . . . 60 4.3.4 High Level Trigger . . . . . . . . . . . . . . . . . . . . 60 4.3.5 Data Quality Monitoring . . . . . . . . . . . . . . . . 60 4.3.6 Detector Algorithms . . . . . . . . . . . . . . . . . . . 61 4.4 ALICE offline framework . . . . . . . . . . . . . . . . . . . . 61 4.4.1 AliRoot . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.4.2 The GRID . . . . . . . . . . . . . . . . . . . . . . . . 63 4.5 ALICE upgrade program . . . . . . . . . . . . . . . . . . . . 64 5 The forward Muon Spectrometer and its trigger system 66 5.1 Physics with the Muon Spectrometer . . . . . . . . . . . . . . 66 5.2 The Muon Spectrometer setup . . . . . . . . . . . . . . . . . 67 5.2.1 Front absorber . . . . . . . . . . . . . . . . . . . . . . 67 5.2.2 Muon filter . . . . . . . . . . . . . . . . . . . . . . . . 68 5.2.3 Dipole magnet . . . . . . . . . . . . . . . . . . . . . . 68 5.2.4 Beam shield . . . . . . . . . . . . . . . . . . . . . . . . 69 5.2.5 Muon Tracking . . . . . . . . . . . . . . . . . . . . . . 69 5.2.6 Trigger system . . . . . . . . . . . . . . . . . . . . . . 72 5.3 The Muon Trigger system . . . . . . . . . . . . . . . . . . . . 72 5.3.1 Detector geometry . . . . . . . . . . . . . . . . . . . . 73 5.3.2 Resistive Plate Chambers . . . . . . . . . . . . . . . . 74 5.3.3 Trigger principle . . . . . . . . . . . . . . . . . . . . . 76 5.3.4 Electronics . . . . . . . . . . . . . . . . . . . . . . . . 78 6 Cluster size of the Muon Trigger 82 6.1 Cluster size: definition and dependences . . . . . . . . . . . . 82 6.2 Estimate of the cluster size . . . . . . . . . . . . . . . . . . . 83 6.2.1 Analysis conditions . . . . . . . . . . . . . . . . . . . . 83 6.2.2 Cluster size distribution and average values . . . . . . 84 6.3 Cluster size in Pb–Pb collisions . . . . . . . . . . . . . . . . . 85 6.3.1 Dependence on the geometry of the detector . . . . . 85 6.3.2 Time dependence of the cluster size . . . . . . . . . . 87 ii 6.3.3 Comparison with results in pp collisions . . . . . . . . 87 6.4 Simulation of the cluster size . . . . . . . . . . . . . . . . . . 88 6.4.1 State of the art before 2012 . . . . . . . . . . . . . . . 88 6.4.2 New simulation procedure . . . . . . . . . . . . . . . . 89 6.4.3 Results with the new class . . . . . . . . . . . . . . . . 91 7 Muon Trigger performance in Pb–Pb collisions 93 7.1 Muon Trigger configurations . . . . . . . . . . . . . . . . . . . 93 7.2 Analysis conditions . . . . . . . . . . . . . . . . . . . . . . . . 94 7.3 Multiplicities . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 7.3.1 Muon multiplicity . . . . . . . . . . . . . . . . . . . . 95 7.3.2 Strip multiplicity . . . . . . . . . . . . . . . . . . . . . 96 7.4 Trigger response as a function of pT . . . . . . . . . . . . . . 97 7.4.1 Effects of the cluster size . . . . . . . . . . . . . . . . 97 7.5 Global triggers . . . . . . . . . . . . . . . . . . . . . . . . . . 100 7.6 Trigger-tracking matching probability . . . . . . . . . . . . . 101 8 Analysis of Υ production in Pb–Pb collisions 103 8.1 Υ analysis framework . . . . . . . . . . . . . . . . . . . . . . 103 8.1.1 The Correction Framework . . . . . . . . . . . . . . . 103 8.1.2 Dedicated classes for Υ analysis . . . . . . . . . . . . . 104 8.2 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . 106 8.2.1 Pass1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 8.2.2 Pass2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 8.2.3 Pass2 after refit . . . . . . . . . . . . . . . . . . . . . . 107 8.3 Data selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 8.3.1 Triggers . . . . . . . . . . . . . . . . . . . . . . . . . . 108 8.3.2 Centrality . . . . . . . . . . . . . . . . . . . . . . . . . 108 8.3.3 Number of minimum bias events . . . . . . . . . . . . 109 8.3.4 Track selection . . . . . . . . . . . . . . . . . . . . . . 110 8.4 Signal extraction . . . . . . . . . . . . . . . . . . . . . . . . . 111 8.4.1 Fit procedure . . . . . . . . . . . . . . . . . . . . . . . 111 8.4.2 Fit results . . . . . . . . . . . . . . . . . . . . . . . . . 113 8.4.3 Systematic uncertainties . . . . . . . . . . . . . . . . . 115 8.5 MC simulation . . . . . . . . . . . . . . . . . . . . . . . . . . 122 8.5.1 Embedding production . . . . . . . . . . . . . . . . . . 122 8.5.2 Systematic uncertainties on MC parametrizations . . . 126 8.6 Systematic uncertainties on detector response . . . . . . . . . 128 8.7 pp reference cross section . . . . . . . . . . . . . . . . . . . . 129 8.7.1 Interpolation of the Υ(1S) cross section at midrapidity 130 8.7.2 Extrapolation to forward rapidity . . . . . . . . . . . . 131 8.8 Summary of the uncertainties . . . . . . . . . . . . . . . . . . 135 8.9 Nuclear modification factor . . . . . . . . . . . . . . . . . . . 136 8.9.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . 136 iii 8.9.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 137 8.10 Central-to-peripheral ratio . . . . . . . . . . . . . . . . . . . . 139 9 Discussion and comparison of the results 140 9.1 Comparison to the J/ψ RAA measured at forward rapidity by ALICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 9.2 Comparison to the CMS results . . . . . . . . . . . . . . . . . 142 9.3 Comparison to theoretical predictions . . . . . . . . . . . . . 144 10 Conclusions and outlook 147 10.1 Muon Trigger cluster size . . . . . . . . . . . . . . . . . . . . 147 10.2 Muon Trigger performance. . . . . . . . . . . . . . . . . . . . 147 10.3 Υ production . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 A Fitting functions 149 A.1 Crystal Ball function . . . . . . . . . . . . . . . . . . . . . . . 149 A.2 Extended Crystal Ball function . . . . . . . . . . . . . . . . . 149 A.3 Double exponential function . . . . . . . . . . . . . . . . . . . 151 A.4 Double power-law function. . . . . . . . . . . . . . . . . . . . 151 R´esum´e 152 Abstract 154 List of Figures 156 List of Tables 164 Bibliography 166 Acknowledgements 179 iv Introduction Ultrarelativistic heavy-ion collisions are the unique tool to produce in laboratory nuclear matter at very high temperature and energy density. Undertheseextremeconditions, thecreatedsystemundergoesaphasetran- sition, from the ordinary hadronic phase constituted by uncoloured baryons and mesons, to a new state of deconfined quarks and gluons, called Quark- Gluon Plasma (QGP). Amongst the possible probes of the QGP, heavy quarks are of parti- cular interest since they are expected to be produced in the primary par- tonic scatterings and to coexist with the surrounding medium. Therefore, the measurement of the quarkonium production is expected to provide the essential information on the properties of the strongly-interacting system formed in the early stages of heavy-ion collisions. Furthermore, according to the colour screening model, measurement of the in-medium dissociation of the different quarkonium states should provide an estimate of the initial temperature of the system. A Large Ion Collider Experiment (ALICE) is specifically designed to study the characteristics of this matter created by colliding Pb nuclei ac- celerated in the Large Hadron Collider (LHC) of CERN at a center-of-mass energy of 2.76 TeV per nucleon. Beside the ALICE detectors, the Muon Spectrometer is dedicated to the study of quarkonium, open heavy-flavours and low-mass mesons via the (di)muon decay channel at forward rapidity (2.5 < y <4) and down to pT = 0. The work here presented was carried out from 2011 to 2013 during the first years of data taking of ALICE. It is mainly focused on the analysis of the Υ production in Pb–Pb collisions, but important detector performance 1 are also studied . This Ph.D. thesis is divided in the following chapters: (cid:0) in Chap. 1 a short introduction to the Quark-Gluon Plasma (QGP) in heavy-ion collisions is given and the most important experimental 1Warning: all figures obtained from my data analysis without the ALICE logo are not officially approved by the ALICE Collaboration (except Fig. 8.19, 8.20 and all those containedinChap. 9). Therefore,theymustbeconsideredasresultsofmypersonalwork under my own responsibility. v signatures are described; (cid:0) Chap. 2 contains an overview of the quarkonium production with a particular regard to the role played by the hot and dense medium; (cid:0) Chap. 3 is a review of the most important results obtained so far on the Υ production in different colliding systems with a comparison to theoretical models; (cid:0) in Chap. 4 the LHC facility and the ALICE apparatus are briefly described,focusingtheattentionontheperformanceofthosedetectors directly involved in the following analyses; (cid:0) the Muon Spectrometer is described in Chap. 5 with particular at- tention to the trigger system: the hardware and software components and their behaviour are summarized; (cid:0) Chap. 6 is dedicated to the analysis of the Muon Trigger cluster size: afteranintroductionofthetopic,itssimulationprocedureisdescribed and the results are compared to real data; (cid:0) Chap. 7 contains a description of some Muon Trigger performance in Pb–Pb collisions in order to demonstrate the good stability of the detector and its effectiveness in view of the analysis contained in next chapter; √ (cid:0) the analysis of Υ production in Pb–Pb at sNN = 2.76 TeV is pre- sented in Chap. 8 in order to extract the nuclear modification factor and the central-to-peripheral ratio used to quantify the QGP forma- tion; (cid:0) in Chap. 9 the results obtained in the previous chapter are compared to other experimental measurements and to the prediction of models described in Chap. 3; (cid:0) finally, Chap. 10 contains a summary of the most important results achieved in this thesis and the future perspectives. vi Chapter 1 The Quark-Gluon Plasma in heavy-ion collisions The main purpose of ultrarelativistic heavy-ion collisions is the investi- gation of a complex and evolving system in extreme conditions of energy density and temperature, called Quark-Gluon Plasma (QGP). In this chap- ter the QGP and its most important signatures will be described after a brief introduction to the quantum chromodynamics. 1.1 QCD and asymptotic freedom The Standard Model of particle physics was formulated in its present form in the 1970s after a variety of experimental discoveries and theoretical predictions. To the present knowledge, all matter is built up by 12 elemen- tary fermions (6 leptons and 6 quarks) and their interactions are mediated by 5 bosons, as depicted in Fig. 1.1. Quantum chromodynamics (QCD), the gauge field theory that describes the strong interactions of coloured quarks and gluons, is the SU(3) compo- nent of the SU(3)×SU(2)×U(1) unitary groups of the Standard Model. One of its fundamental properties is the running coupling constant, i.e. the de- pendenceofthecouplingconstantαs onthescaleofthemomentumtransfer Q, according to the following equation for 1-loop corrections [1]: 12π αs(Q) = (cid:2) (cid:3) (1.1) 2 Q (11nc−2nf)ln Λ2 QCD wherenc andnf arethenumberofcoloursandthenumberofquarkflavours respectively. When Q is high (i.e. the scale of the interaction is very small) the coupling constant is small as shown in Fig. 1.2. This is the asymptotic 1 Figure 1.1: The fundamental particles predicted by the Standard Model. Figure 1.2: Summary of αs measurements as a function of the energy scale Q [2]. 2