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Prompt D Meson Production in Pp, P-Pb and Pb-Pb Collisions at LHC

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Prompt D Meson Production in Pp, P-Pb and Pb-Pb Collisions at LHC Universita' degli Studi di Torino Scuola di Dottorato in Scienze della Natura e Tecnologie Innovative Indirizzo in Fisica ed Astrofisica + Prompt Ds meson production in pp, p-Pb and Pb-Pb collisions at LHC with ALICE Author: Supervisors: Anastasia Barbano Prof. Stefania Beol`e Dr. Francesco Prino Examiners: Dr. Ralf Averbeck Dr. Patrick Robbe December 11, 2017 iii Abstract + Prompt Ds meson production in pp, p-Pb and Pb-Pb collisions at LHC with ALICE The aim of this thesis is the study of the D+-meson production in pp collisions at the p s p centre-of-mass energy s = 7 TeV and in p-Pb and Pb-Pb collisions at sNN = 5:02 TeV, with the ALICE detector at the CERN Large Hadron Collider. Heavy quarks provide an excellent way to investigate the properties of the Quark-Gluon Plasma (QGP) created in high-energy nucleus-nucleus collisions via the measurements of the nuclear modification factor (RAA) and azimuthal anisotropy of hadrons originating from their hadronisation. At low and intermediate transverse momentum pT, the + study of the Ds meson should also reveal information about the charm-quark hadro- nisation mechanism. If charm quarks hadronise by recombining with lighter quarks + from the medium, the relative abundance of Ds mesons with respect to non-strange D mesons is expected to be larger in Pb-Pb than in pp collisions, at low and inter- mediate pT, due to the large abundance of strange quarks in the QGP. The analyses + + − + presented in this thesis are based on the full reconstruction of the Ds ! K K π decay, exploiting the displacement of the decay vertex from the interaction point. The + + pT-differential cross-section of Ds mesons and the relative yields of Ds to non-strange D mesons were measured in pp and Pb-Pb collisions. In particular, the nuclear mod- + ification factors and, for the first time at LHC, the elliptic flow of Ds mesons were measured in Pb-Pb collisions for central, semi-peripheral and peripheral collisions. The observed increase of strange particle yield relative to pion yield with increas- ing particle multiplicity in pp and p-Pb collisions suggested to investigate whether + a similar enhancement is observed in the relative yield of Ds mesons with respect to non-strange D mesons in high multiplicity p-Pb collisions. To this purpose, the + + relative ratios of Ds -meson and D -meson yields were measured in p-Pb collisions as a function of the event particle multiplicity in different pT intervals. In the first Chapter, the physics of heavy-ion collisions is introduced. The second Chapter is entirely dedicated to the role of heavy-flavour observables in the investi- gation of the QGP. A description of the main features of the ALICE apparatus and the detectors used in the analysis is illustrated in the third Chapter. The fourth, fifth and sixth Chapters present the work of this thesis, focusing on the strategy used + in ALICE to reconstruct the Ds mesons and discussing the results obtained in pp, Pb-Pb and p-Pb collisions, respectively. The measurements presented in this thesis were approved by the ALICE Collabo- ration and presented in various conferences. The proton-proton analysis has already + been published; the analysis of Ds v2 in Pb-Pb collisions is available on arXiv, wait- ing for a publication on a peer reviewed journal. The proton-nucleus and the other nucleus-nucleus results are still preliminary and will be published soon. v Abstract + Prompt Ds meson production in pp, p-Pb and Pb-Pb collisions at LHC with ALICE Il lavoro di questa tesi ha riguardato lo studio della produzione del mesone D+ in p s collisioni pp alla energia del centro di massa di s = 7 TeV e in collisioni p-Pb e p Pb-Pb a sNN = 5:02 TeV, attraverso il rivelatore ALICE al Large Hadron Collider del CERN. I quark pesanti costituiscono una sonda eccellente per le propriet`adel Plasma di Quark e Gluoni (QGP) che si crea in collisioni nucleo-nucleo ad alta ener- gia, attraverso la misura del fattore di modificazione nucleare (RAA) e dell'anisotropia azimutale degli adroni che originano dalla loro adronizzazione. A intermedio e basso + momento trasverso pT, lo studio del mesone Ds dovrebbe inoltre rivelare informazioni riguardo il meccanismo di adronizzazione del quark charm all'interno del mezzo. Se il quark charm adronizza per coalescenza con i quark pi`uleggeri presenti nel mezzo, + l'abbondanza relativa di mesoni Ds rispetto ai mesoni D non contenenti stranezza `eattesa essere maggiore in collisioni Pb-Pb rispetto a collisioni pp, nella regione ad intermedio-basso pT, grazie all'abbondanza di quark strani nel QGP. Le analisi presentate in questa tesi sono basate sulla ricostruzione completa della topologia di + + − + decadimento del canale Ds ! K K π , sfruttando la distanza del vertice di decadi- mento dal vertice di interazione primario. La sezione d'urto differenziale in pT per + + la produzione del mesone Ds e i rapporti di produzione del mesone Ds rispetto ai mesoni D non contenti stranezza sono stati misurati in collisioni pp e Pb-Pb. In particolare, l'analisi delle collisioni Pb-Pb ha portato alla misura del fattore di mo- + dificazione nucleare e, per la prima volta a LHC, del flow ellittico del mesone Ds per collisioni centrali, semi periferiche e periferiche. L'osservazione dell'aumento della produzione di particelle contenenti stranezza rispetto alla produzione di pioni con l'aumentare della molteplicit`adi particelle prodotte in collisioni pp e p-Pb ha sug- gerito di investigare la presenza di un simile incremento nella produzione relativa del + mesone Ds rispetto ai mesoni D non contenenti stranezza in collisioni p-Pb ad alta + + molteplicit`a. A questo proposito, la misura del rapporto di produzione Ds =D `e stata effettuata in funzione della molteplicit`adi particelle in collisioni p-Pb in diversi intervalli di pT. Nel primo Capitolo verr`aintrodotta la fisica degli ioni pesanti. Il secondo Capi- tolo `einvece interamente dedicato al ruolo delle osservabili per lo studio degli adroni pesanti nell'investigazione del QGP. Una descrizioni delle principali caratteristiche dell'esperimento ALICE e dei rivelatori utilizzati nelle analisi si pu`otrovare nel terzo Capitolo. Nel quarto, quinto e sesto capitolo verr`apresentato il lavoro di questa tesi, + introducendo la strategia usata in ALICE per la ricostruzione del mesone Ds e pas- sando poi alla discussione dei risultati in collisioni pp, p-Pb e Pb-Pb rispettivamente. Le misure presentate in questa tesi sono state approvate dalla Collaborazione ALICE e presentate in numerose conferenze. L'analisi dei dati in collisioni pp `estata + pubblicata. L'analisi del v2 del mesone Ds disponibile su arXiv, con l'obiettivo di una rivista peer review. I risultati delle analisi in collisioni p-Pb e il resto dei risultati delle analisi in collisioni Pb-Pb sono ancora preliminari e saranno pubblicati a breve. vii Contents 1 Quark-Gluon Plasma1 1.1 Strongly interacting matter........................1 1.2 Micro bang vs big bang: timescales of expansion, baryonic number..3 1.3 What theory tells us............................5 1.4 Collision Geometry and the Glauber Model...............7 1.5 Heavy-ion physics at the LHC.......................9 1.5.1 Particle multiplicity and energy density............. 10 1.5.2 Hadron multiplicities and chemical freeze-out.......... 11 1.5.3 Strangeness enhancement..................... 13 1.5.4 Collective flow and kinetic freeze-out............... 14 Radial flow............................. 14 Anisotropic flow.......................... 16 1.5.5 Chiral symmetry restoration................... 18 1.5.6 Jet quenching............................ 19 High-pT hadrons.......................... 21 Jets................................. 22 1.5.7 Quarkonium production...................... 24 1.5.8 Latest discoveries in small systems................ 27 2 Heavy flavours 31 2.1 The importance of being heavy...................... 31 2.2 Heavy-quark production in pp collisions................. 31 2.3 Heavy quarks in p-A collisions...................... 34 2.3.1 Cold nuclear matter effects.................... 34 2.3.2 Nuclear Parton Distribution Functions.............. 35 2.3.3 Experimental results in p-A collisions.............. 38 2.4 Heavy quarks in A-A collisions...................... 40 2.4.1 Collisional processes........................ 41 2.4.2 Gluon-radiation processes..................... 42 2.4.3 Heavy flavour hadronisation in the medium........... 43 2.4.4 Experimental results in A-A collisions.............. 46 3 The ALICE experiment at the LHC 53 3.1 The Large Hadron Collider........................ 53 3.2 The ALICE experiment.......................... 53 3.2.1 Magnet............................... 55 3.2.2 Inner Tracking System (ITS)................... 55 3.2.3 Time Projection Chamber (TPC)................. 56 3.2.4 Time-Of-Flight (TOF)....................... 58 3.2.5 V0 Detector............................. 59 3.2.6 Zero Degree Calorimeter (ZDC) Detector............ 60 3.3 The ALICE Trigger System and Data Aquisition............ 60 3.3.1 The Central Trigger Processor (CTP).............. 61 viii 3.3.2 The Data AcQuisition System (DAQ).............. 62 3.3.3 The High Level Trigger (HLT).................. 62 3.4 Machine-induced background....................... 62 3.5 Electromagnetic interactions........................ 63 3.6 Track and vertex reconstruction...................... 64 3.7 Secondary vertex reconstruction...................... 67 3.8 Simulations................................. 67 3.9 Centrality determination.......................... 68 3.10 The ALICE Offline
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