Universita(cid:18) degli Studi di Padova Dipartimento di Fisica e Astronomia \Galileo Galilei" Measurement of the D0 meson production in Pb–Pb and p–Pb collisions with the ALICE experiment at the LHC Andrea Vitturi Direttore della Scuola: Ch.mo Prof. Andrea Dainese Supervisore: Dott. Andrea Festanti Dottorando: Scuola di dottorato di ricerca in Fisica XXVII CICLO Abstract This thesis presents the measurement of the charmed D0 meson production relative to the reaction plane in Pb{Pb collisions at the centre-of-mass energy per nucleon{nucleon collision of ps = 2:76 TeV, and the measurement of the D0 production in p{Pb NN collisions at ps = 5:02 TeV with the ALICE detector at the CERN Large Hadron NN Collider. TheD0 azimuthalanisotropywithrespecttothereactionplaneissensitivetothein- teractionofthecharmquarkswiththehigh-densitystrongly-interactingmediumformed in ultra-relativistic heavy-ion collisions and, thus, to the properties of this state of mat- ter. In particular, this observable allows to establish whether low-momentum charm quarks participate in the collective expansion of the system and whether they can reach thermal equilibrium with the medium constituents. The azimuthal anisotropy is quan- ti(cid:12)ed in terms of the second coe(cid:14)cient v in a Fourier expansion of the D0 azimuthal 2 distribution and in terms of the nuclear modi(cid:12)cation factor R , measured in the direc- AA tion of the reaction plane and orthogonal to it. The measurement of the D0 production in p{Pb collisions is crucial to disentangle the e(cid:11)ects induced by cold nuclear matter from the (cid:12)nal state e(cid:11)ects induced by the hot medium formed in Pb{Pb collisions. The D0 production is measured in both systems by reconstructing the two-prong hadronic decay D0 K (cid:25)+ in the central rapidity region, exploiting the separation of (cid:0) ! the decay vertex from the primary vertex. The raw signal is obtained with an invariant mass analysis, and corrected for selection and reconstruction e(cid:14)ciency. A positive elliptic (cid:13)ow v is observed in Pb{Pb collisions in the centrality class 2 30{50%, with a mean value of 0:204+0:099 in the interval 2 < p < 6 GeV/c, which 0:036 T (cid:0) decreases towards more central collisions. Consequently, the nuclear modi(cid:12)cation factor shows a stronger suppression in the direction orthogonal to the reaction plane. The v and the R measured in two azimuthal regions with respect to the reaction plane 2 AA are compared to theoretical calculations of charm quark transport and energy loss in high-density strongly-interacting matter. The models that include substantial elastic interactions with an expanding medium provide a good description of the observed anisotropy. The D0 nuclear modi(cid:12)cation factor R in p{Pb collisions is compatible with unity pPb within uncertainties. The measured R is compared to theoretical models including pPb initial state e(cid:11)ects, as well as to the nuclear modi(cid:12)cation factor measured in central Pb{Pb collisions. The D0 R results are consistent with the modi(cid:12)cation of the nu- pPb cleon parton distribution functions induced by the nuclear environment, and provide experimental evidence that the modi(cid:12)cation of the D meson momentum spectrum ob- served in Pb{Pb with respect to pp collisions is due to strong (cid:12)nal state e(cid:11)ects induced by the hot medium. Riassunto La tesi presenta la misura della produzione di mesoni D0 rispetto al piano di reazione in collisioni Pb{Pb all’energia nel centro di massa di ps = 2:76 TeV per coppia di NN nucleoni e la misura della produzione di D0 in collisioni p{Pb all’energia di ps = NN 5:02 TeV con l’esperimento ALICE situato al Large Hadron Collider del CERN. L’anisotropiaazimutaledeimesoniD0 rispettoalpianodireazione(cid:18)esensibileallein- terazionidelquarkcharmconilmezzoadaltadensit(cid:18)aefortementeinteragenteprodotto incollisionitraionipesantiadenergiaultra-relativisticae,diconseguenza,allepropriet(cid:18)a diquestostatodellamateria. Inparticolare,permettedistabilireseiquarkcharmparte- cipano all’espansione collettiva del sistema e se raggiungono l’equilibrio termico con i costituenti del mezzo. L’anisotropia azimutale (cid:18)e quanti(cid:12)cata tramite il secondo coe(cid:14)- ciente v dello sviluppo in serie di Fourier della distribuzione azimutale dei mesoni D0 e 2 tramite la misura del fattore di modi(cid:12)ca nucleare R nel piano di reazione e nella di- AA rezioneortogonaleadesso. LamisuradellaproduzionediD0 incollisionip{Pbpermette distudiareglie(cid:11)ettiindottidallamaterianuclearefredda,inmododapoterlidistinguere da quelli indotti dal mezzo denso fortemente interagente prodotto in collisioni Pb{Pb. La produzione di mesoni D0 (cid:18)e stata misurata attraverso la ricostruzione dei decadi- menti adronici a due corpi D0 K (cid:25)+ nella regione centrale di rapidit(cid:18)a, sfruttando la (cid:0) ! separazionedeiverticisecondarididecadimentorispettoalverticeprimariod’interazione. Il segnale (cid:18)e stato ottenuto attraverso un’analisi della distribuzione di massa invariante e corretto per l’e(cid:14)cienza di ricostruzione e selezione dei decadimenti. Il coe(cid:14)ciente di (cid:13)usso ellittico v dei mesoni D0 misurato in collisioni Pb{Pb nella 2 classe di centrali(cid:18)a 30{50% (cid:18)e positivo, il valore medio nell’intervallo 2 < p < 6 GeV/c T (cid:18)e pari a 0:204+0:099. Di conseguenza, il fattore di modi(cid:12)ca nucleare (cid:18)e minore nella 0:036 (cid:0) direzione ortogonale al piano di reazione. Il v osservato decresce all’aumentare della 2 centralit(cid:18)a delle collisioni. Il v e l’R misurato in due regioni azimutali ortogonali 2 AA rispetto al piano di reazione sono stati confrontati con calcoli teorici per il trasporto e la perdita di energia dei quark charm nella materia densa fortemente interagente. L’anisotropia osservata (cid:18)e descritta dai modelli che includono le interazioni elastiche tra i quark all’interno di un mezzo in espansione. Il fattore di modi(cid:12)ca nucleare dei mesoni D0 R (cid:18)e compatibile con l’unit(cid:18)a entro le pPb incertezze. R (cid:18)e stato confrontato con predizioni teoriche che descrivono gli e(cid:11)etti di pPb stato iniziale e con il fattore di modi(cid:12)ca nucleare misurato in collisioni Pb{Pb centrali. I risultati sono consistenti con e(cid:11)etti dovuti alla modi(cid:12)ca delle funzioni di distribuzione partonicheall’internodeinucleonilegatiedimostranochelamodi(cid:12)cadelladistribuzione del momento trasverso dei mesoni D osservata in collisioni Pb{Pb rispetto a quella in collisionipp(cid:18)edovutaallaperditadienergiadeiquarkcharmnelmezzodensofortemente interagente. Contents Introduction 1 1 Physics of Ultra-Relativistic Heavy-Ion Collisions 5 1.1 Quantum Chromodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Asymptotic Freedom and Con(cid:12)nement . . . . . . . . . . . . . . . . . . . . 7 1.2.1 Running Coupling Constant . . . . . . . . . . . . . . . . . . . . . . 8 1.3 Lattice QCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4 The QCD Phase Diagram and the Quark-Gluon Plasma . . . . . . . . . . 10 1.5 Heavy-Ion Collisions and the QGP . . . . . . . . . . . . . . . . . . . . . . 14 1.5.1 Geometry of the Collision . . . . . . . . . . . . . . . . . . . . . . . 17 1.5.2 Global Event Properties . . . . . . . . . . . . . . . . . . . . . . . . 20 1.5.2.1 Particle Multiplicity . . . . . . . . . . . . . . . . . . . . . 20 1.5.2.2 Identical Boson Correlations . . . . . . . . . . . . . . . . 21 1.5.3 Photon Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.5.4 Hard Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.5.4.1 Quarkonium Suppression . . . . . . . . . . . . . . . . . . 23 2 Heavy-Flavour Probes in Heavy-Ion Collisions 27 2.1 Heavy-Flavour Production in pp Collisions. . . . . . . . . . . . . . . . . . 28 2.2 Heavy-Flavour Production in Pb{Pb and p{Pb Collisions . . . . . . . . . 29 2.3 Heavy Quarks as QGP Probes . . . . . . . . . . . . . . . . . . . . . . . . 31 2.3.1 In-Medium Energy Loss . . . . . . . . . . . . . . . . . . . . . . . . 32 2.3.2 Azimuthal Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.4 Initial-State E(cid:11)ects and the Role of pA Collisions . . . . . . . . . . . . . . 45 2.5 Objective of the Thesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3 The ALICE Experiment at the LHC 53 3.1 The LHC Accelerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.1.1 Acceleration Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.1.2 pp, p{Pb and Pb{Pb Collisions . . . . . . . . . . . . . . . . . . . . 55 3.2 The ALICE Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.2.1 General Detector Layout. . . . . . . . . . . . . . . . . . . . . . . . 57 3.2.2 ITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.2.3 TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.2.4 TOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.3 Central-Barrel Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.3.1 Interaction Vertex Reconstruction with SPD . . . . . . . . . . . . 62 vii Contents viii 3.3.2 Track Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 63 3.3.3 Final Reconstruction of the Interaction Vertex . . . . . . . . . . . 65 3.4 Particle Identi(cid:12)cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.4.1 PID in the TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.4.2 PID in the TOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 3.5 Impact Parameter Resolution . . . . . . . . . . . . . . . . . . . . . . . . . 69 4 Experimental Observables 73 4.1 Azimuthal Anisotropy of D0 Production in Pb{Pb Collisions . . . . . . . 73 4.1.1 Event-Plane De(cid:12)nition . . . . . . . . . . . . . . . . . . . . . . . . . 73 4.1.2 Elliptic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.1.3 R Azimuthal Dependence . . . . . . . . . . . . . . . . . . . . . 75 AA 4.2 D0 Production in p{Pb Collisions . . . . . . . . . . . . . . . . . . . . . . . 76 4.2.1 Production Cross Section and R . . . . . . . . . . . . . . . . . 77 pPb 4.2.2 Nuclear Modi(cid:12)cation Factor in Classes of Event Activity: Q . . 78 pPb 4.2.3 Self-normalized Yields . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.3 Proton{Proton Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5 D0 K (cid:25)+ Decay Reconstruction 83 (cid:0) ! 5.1 Data Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.2 Event-Plane Determination in Pb{Pb Collisions . . . . . . . . . . . . . . . 84 5.3 D0 Reconstruction and Selection . . . . . . . . . . . . . . . . . . . . . . . 86 5.3.1 Secondary Vertex Reconstruction . . . . . . . . . . . . . . . . . . . 87 5.3.2 Topological Selection Strategy . . . . . . . . . . . . . . . . . . . . 87 5.3.3 Particle Identi(cid:12)cation . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.4 Yield Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.4.1 Pb{Pb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 5.4.2 p{Pb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 5.5 Study of the \Re(cid:13)ected" Signal . . . . . . . . . . . . . . . . . . . . . . . . 104 5.6 E(cid:14)ciency Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.7 Correction for Feed-Down from B Decays . . . . . . . . . . . . . . . . . . 117 6 Systematic Uncertainties 121 6.1 Yield Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.2 Cut E(cid:14)ciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 6.3 PID E(cid:14)ciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 6.4 Monte Carlo p Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 T 6.5 Monte Carlo Multiplicity Distribution . . . . . . . . . . . . . . . . . . . . 137 6.6 Tracking E(cid:14)ciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 6.7 Feed-down Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 6.8 Proton{Proton Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 6.9 Summary of Uncertainties on v . . . . . . . . . . . . . . . . . . . . . . . 144 2 6.10 Summary of Uncertainties on R In- and Out-Of-Plane . . . . . . . . . 145 AA 6.11 Summary of Uncertainties on the Cross Section in p{Pb Collisions and on R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 pPb 6.12 Summary of Uncertainties on Q and Relative Yields . . . . . . . . . . 149 pPb 7 Azimuthal Anisotropy of D0 Production in Pb{Pb Collisions 151 Contents ix 7.1 Elliptic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 7.2 Comparison with Other Methods for v Measurement . . . . . . . . . . . 155 2 7.3 R In and Out of the Event Plane . . . . . . . . . . . . . . . . . . . . . 156 AA 7.4 Comparison with Model Calculations . . . . . . . . . . . . . . . . . . . . . 157 8 D0 Production in p{Pb Collisions 161 8.1 Production Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 8.2 Nuclear Modi(cid:12)cation Factor R . . . . . . . . . . . . . . . . . . . . . . 163 pPb 8.3 Q as a Function of Event Activity . . . . . . . . . . . . . . . . . . . . 166 pPb 8.4 Relative Yields as a Function of Multiplicity . . . . . . . . . . . . . . . . . 166 Conclusions 170 Bibliography 185 Acknowledgements 186