Hyperon production in p{Pb collisions with ALICE at the LHC. Didier Alexandre Thesis submitted for the degree of Doctor of Philosophy Nuclear Physics Group School of Physics and Astronomy University of Birmingham June 2016 Abstract This thesis discusses the production of the multi-strange, charged Ξ and Ω baryons in proton{lead (p{Pb) collisions at a centre-of-mass energy of 5.02 TeV. The trans- verse momentum, pT, distributions are analysed as a function of event multiplic- ity. A hydrodynamical model based on statistical physics reproduces the shapes of the multi-strange pT spectra, in conjunction with the spectra of lighter hadrons, in high multiplicity data. The good agreement is an indication of collective behaviour by all particles inside a system in thermal equilibrium, consistent with the picture of the build{up of a radially outward expansion due to an initially dense medium. These results are reminiscent of the observations made in lead{lead (Pb{Pb) colli- sions, which are explained by the formation of a Quark-Gluon Plasma. In addition, the pT{integrated yields of the hyperons are reported on, revealing a steady increase as a function of multiplicity. An enhancement with respect to non-strange hadrons is observed, and the Ξ/π and Ω/π ratios in high multiplicity p{Pb data approach those measured in central Pb{Pb collisions. The Ξ/π ratio is comparable with the calculations from a thermal model for strangeness saturation, whereas the Ω/π ratio deviates from that value by 2σ. i ii Author's contributions During the course of my degree, I have been a member of the ALICE collaboration. Therefore, my results cannot be solely attributed to my work. Instead, I benefited from the input of several other members. I made use of existing code written by David Dobrigkeit Chinellato, Michele Floris and Roberto Preghenella. I started my physics analysis work on strangeness, by working on the production 0 rates of Λ and KS. I made use of a macro written by David, in order to read Event Summary Data (ESD) files and extract the pT-distributions of these particles. Later, I adapted the existing code into one for the analysis of the doubly-strange Ξ hadrons, and then extended it to work for the triply-strange Ω, too. These particles became the focus of my main analysis project. David independently developed a code for the measurement of the multi-strange particles himself, which was very helpful to per- form cross-checks on my results. I performed the analysis that led to the final decision on the selection cuts and analysis settings for the multi-strange hadrons, extracted all pT-spectra and calculated their uncertainties. My results were approved by the collaboration to be presented in the Quark Matter conference in Darmstadt, in May 2014. I gave the corresponding talk in a parallel session and wrote the proceedings on my contribution [1]. In addition, I led the efforts towards the publication of the results in the Physics Letters B journal, shown in Appendix C [2], by taking the role of chair of the paper committee. I am grateful for all the discussions and advice pro- vided to me by my supervisor Roman Lietava, by Lee Barnby and by David during this long process. I also have to thank Michele for introducing me to the thermal model code, with which I performed several fits and extracted model descriptions for the discussion of the results in the paper and in this thesis. During my stay at CERN, I was also involved in several projects with the Central Trigger Processor (CTP) of ALICE. My main project consisted in performing tests on the firmware of the Local Trigger Unit (LTU), the interface between the CTP and the ALICE sub-detectors. I developed a C-written programme to run tests on iii the trigger protocol. This work is the subject of the last section of Chapter 3. I also helped in running trigger simulations with the LTU and test, together with the experts of several sub-detectors, that these were received correctly by their systems. In addition, I wrote a Python script to manage the trigger configuration file. All this work was supervised by Anton Jusko and Roman. During the beginning of the Run 2 of the LHC, I also did several CTP on-call shifts. Finally, I completed a course on the operation of the Experiment Control System, and served 72 hours in shifts in the ALICE run control room. iv To Marlene v vi Acknowledgements I owe the completion of this degree to many people, who in their own way, helped me bring it to its end. I am especially grateful to my supervisor Roman Lietava, who has always been available to discuss my work, give good advice, but most importantly, to teach me. I am also grateful to David Evans for the great opportunity to do this degree with the ALICE group of Birmingham, and for all his help during it. Special thanks go also to Lee Barnby for the many invaluable interactions we had, in particular, but not only, in the paper committe we were part of. I also thank all other members of our group, with whom it was always a pleasure to work: Katie Graham, Luke Hanratty, Peter Jones, Anton Jusko, Marian Krivda, Graham Lee, Arvinder Palaha, Plamen Petrov, Patrick Scott, Orlando Villalobos-Baillie and Nima Zardoshti. From the ALICE collaboration, I thank especially my friend David D. Chinellato. It was a privilege to be able to collaborate with him for the greater part of my degree. I am indebted to him for all that he taught me. I also thank him for the opportunity to work with him in Campinas, for a period of three months, during which I enjoyed all the discussions we had during the process of writing our paper. I also extend my gratitude to his colleagues Jun Takahashi and Giorgio Torrieri. I thank other members of the ALICE collaboration with whom I had the chance to have several helpful exchanges on the topic of strangeness, especially Domenico Colella, Michele Floris and Roberto Preghenella. At this stage, it feels important to remember the beginning of this journey. I am grateful to Fabrizio Salvatore for introducing me to the field of High Energy Physics six years ago. I enjoyed the company of my friends Pete Griffith, Luca Pescatore and Mark Levy, during our time spent at CERN, as well as during the summer schools we attended. I thank them for all the fun we had. I can also not forget the friends I made outside the field of physics during this time. There are several, however special thanks go to Deni, Fernanda and Luis from Brazil, for teaching me many things that cannot be expressed in equations. vii On a more personal level, I thank my father, who over the last eight years must have broken all records in phone calls to me. I also thank my late mother, who contributed a lot to who I am today. Finally, I thank Marlene, for her unconditional love and support through all these years. viii It is the Land of Truth (enchanted name!), surrounded by a wide and stormy ocean, the true home of illusion, where many a fog bank and ice, that soon melts away, tempt us to believe in new lands, while constantly deceiving the adventurous mariner with vain hopes, and involving him in adventures which he can never leave, yet never bring to an end. Immanuel Kant ix x Contents 1 Introduction 3 1.1 Quantum Chromodynamics . 4 1.1.1 Colour charge . 5 1.1.2 Confinement . 6 1.1.3 Asymptotic freedom . 6 1.1.4 The strong force potential . 8 1.1.5 Lattice QCD . 9 1.1.6 Chiral symmetry . 11 2 Heavy Ion Physics 13 2.1 Space-time evolution of a heavy-ion collision . 13 2.2 Experimental observables . 14 2.2.1 Strangeness Enhancement . 16 2.2.1.1 Experimental review . 19 2.2.2 Hydrodynamic flow . 20 2.2.2.1 Elliptic flow . 22 2.2.3 Jet suppression . 23 2.2.4 Charmonium suppression . 25 2.2.5 Direct photons . 27 2.2.6 Short-lived resonances . 28 2.3 Theoretical models . 29 2.3.1 The Statistical Hadronisation Model . 29 2.3.1.1 Heavy-ion collisions as grand canonical ensembles . 30 2.3.1.2 Strangeness suppression in small collision systems . 31 2.3.1.3 Comparisons with data . 32 2.3.1.4 The core-corona superposition of A-A collisions . 33 2.3.2 Transverse momentum distribution of hadrons . 36 2.3.2.1 The Boltzmann-Gibbs Blast-Wave model . 37 2.3.2.2 The L´evy-Tsallisfunction . 38 xi CONTENTS 2.4 Proton{lead collisions . 39 3 The ALICE Experiment at the LHC 43 3.1 The Large Hadron Collider . 43 3.2 The ALICE detector . 45 3.2.1 Inner Tracking System . 48 3.2.1.1 The Silicon Pixel Detector . 48 3.2.1.2 The Silicon Drift Detector . 50 3.2.1.3 The Silicon Strip Detector . 51 3.2.2 The Time Projection Chamber . 52 3.2.3 The V0 detector . 55 3.3 The Central Trigger Processor . 57 3.3.1 Operation . 57 3.3.2 Trigger classes . 59 3.3.3 The Local Trigger Unit (LTU) . 60 3.3.3.1 The LTU operation . 60 3.3.3.2 Snapshot Memory . 61 3.3.3.3 LTU tests in the CTP laboratory . 62 4 Selection of multi-strange hadrons in p{Pb collisions 65 4.1 Centrality in p-Pb collisions .