+ Λc baryon production measurements with the ALICE experiment at the LHC Jaime Norman University of Liverpool Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor of Philosophy December 2017 Abstract Quantum chromodynamics, the quantum field theory that describes the strong interaction, demonstrates a property known as asymptotic freedom which weakens the strong coupling constant αs at high energies or short distances. The measurement of particles containing heavy quarks, i.e. charm and beauty, in high-energy particle collisions is a stringent test of the theory of quantum chromodynamics in the regime where αs is small. In addition, asymptotic freedom leads to a phase transition of nuclear matter at high temperatures or energy densities to a phase known as the Quark-Gluon Plasma, where quarks and gluons are deconfined, and this state of matter can be studied in relativistic heavy-ion collisions. Particles containing heavy quarks, i.e. charm and beauty, have been proposed as probes of the properties of the Quark Gluon Plasma, where the measure- ment of mesons and baryons can offer insight into the transport properties of the medium and mechanisms related to the formation of hadrons during the transition back to ‘confined’ quark states. Proton-proton and proton- lead collisions offer a crucial benchmark for these measurements, and can also reveal important insights into particle production and interaction mechanisms. The goal of this thesis is to investigate the production of the charmed + baryon Λc in high-energy particle collisions with the ALICE detector at the Large Hadron Collider. The measurements presented will test pre- dictions utilising perturbative (small αs) and non-perturbative (large αs) + methods, will test possible cold-nuclear-matter modifications of the Λc yield in proton-lead collisions, and will set the stage for future measure- ments in lead-lead collisions. The measurements are carried out by recon- + − + structing the hadronic decay channel Λc ! pK π , making selections on its decay topology, extracting the signal via an invariant mass analysis, and finally correcting for its selection and reconstruction efficiency. A multivariate technique (Boosted Decision Trees) has been developed and is utilised in order to improve the signal extraction by optimally com- + bining discriminating variables related to the Λc decay topology. This technique has also been investigated as a possible approach to measuring + the Λc baryon in lead-lead collisions in the future, after the upgrade of the ALICE Inner Tracking System will make this measurement possible. + The transverse momentum dependence of the Λc baryon production cross section has been measured in proton-proton collisions at a centre-of-mass energy of 7 TeV and proton-lead collisions at a centre-of-mass energy per nucleon-nucleon collision of 5:02 TeV, in the transverse momentum range 2 < pT < 12 GeV=c, and is shown to be under-predicted by theoretical + 0 calculations. The baryon-to-meson ratio Λc /D has been measured to be consistent in proton-proton collisions and proton-lead collisions and under-predicted by theoretical calculations. The nuclear modification fac- tor RpPb is measured to be consistent with unity and in agreement with + the D meson RpPb, indicating no significant modification of the Λc yield in proton-lead collisions with respect to proton-proton collisions within the experimental uncertainties. Finally, Boosted Decision Trees have been shown to significantly improve the statistical precision with which the + measurement of the Λc baryon can be made in lead-lead collisions with the ALICE detector in the future. 4 Author's Contribution Throughout my PhD I have been working as part of the ALICE collaboration. + − + The work presented in this thesis extends the Λc ! pK π analysis in pp and p{ + Pb collisions to include a multivariate selection (using Boosted Decision Trees) of Λc candidates. This was carried out extending the framework developed by collaborators, and while I used much of the pre-existing framework all analysis work in this thesis, + including the Λc candidate selection, signal extraction, corrections and systematic uncertainties was performed by me. I also played a large role in the analysis performed in pp and p{Pb collisions using the standard, cut-based method, and developed the + strategy to merge the Λc analyses measured using different decay channels, though this is not presented in detail here. The preliminary results shown in this thesis were presented by myself at the `Strangeness in Quark Matter' conference in 2017 [1]. An extension of this work also involved a study to evaluate the statistical improve- + ment that will be gained from using a multivariate approach to extract the Λc signal in Pb{Pb collisions, after the upgraded ALICE Inner Tracking System is installed. This was carried out as an extension of and comparison to work performed for the Inner Tracking System Technical Design Report, and this work constitutes a chapter of this thesis. In addition to the work presented here, I helped review the analysis of the D meson p production cross section measurements at s = 5 TeV as part of the Analysis Review Committee, which have been presented as preliminary results at conferences. For my service task, I worked on a laser soldering technique that was being explored to bond the silicon chips of the Inner Tracking System upgrade to a circuit-board. I also had the opportunity to give overview talks on open heavy-flavour measurements at ALICE - at the `High pT physics in the RHIC-LHC era' workshop at Brookhaven National Laboratory, and at the `55th International Winter Meeting on Nuclear Physics' in Bormio, Italy [2]. All the work presented in this thesis is the result of my own work, except where explicit reference is made to the work of others. Acknowledgements Firstly, big thanks to my primary supervisor Marielle Chartier. From when I begun my undergraduate project to finishing my PhD, Marielle has supported and encouraged me throughout. Thanks also to my secondary supervisors, John Dainton for patiently talking through many ideas in this work, and Roy Lemmon for his feedback and help in shaping this thesis. Many thanks to Rossella Romita for first introducing me to the ALICE experiment and this analysis, and to Marcel Araujo Silva Figueredo, with whom the analyses in this thesis were finalised. It was a pleasure to work closely with both of you during the first couple of years of my PhD, and I learned a lot from both of you. During my time in ALICE I have been lucky to work in collaboration with many people, with whom I shared many interesting discussions. Thanks to those who I + have worked with directly on the Λc analysis, in particular to Chiara Zampolli and Andrea Alici for their help at the start of this analysis. Thanks to to Davide Caffarri and Zaida Conesa del Valle for the helpful discussions as well as the opportunity to present heavy-flavour results from ALICE at interesting conferences, and thanks to Andre Mischke, Jesus Guillermo Contreras Nuno, Francesco Prino and Martin Voelkl for their help in reviewing the analyses at different stages. Thanks to Andrea Dainese and Cristina Terrevoli for their useful discussions in shaping the studies presented in chapter 6 of this work. I would also like to thank Petra Riedler, Luciano Musa and Antoine Junique for the opportunity to work on the upgrade of the Inner Tracking System for my service task. Liverpool has become a second home over the past seven years. Thanks to ev- eryone I met during my undergraduate degree and my PhD, in particular K¯arlis Dreimanis, John Anders and David Morris for all the jokes and the drinks. Huge thanks to Jonathan Tinsley for proofreading sections of this thesis. Thanks also to all my non-physicist friends for giving me a welcome escape from physics when needed and for pretending to be interested in this work. Thanks to my family; my sister Meryn, and my parents Maggie and Bob - I would not be writing this if it wasn't for my parents consistent encouragement and support. And lastly to the `other' Alice, Alice Colquhoun, for sharing all the stress, the travelling, and enjoyment of the last few years. She has been much more than a 16 × 16 × 26 m3, 10; 000 tonne particle physics experiment to me, and for that I thank her. For Peter Norman, \science is either physics or stamp collecting." Contents List of Figures iii List of Tables vii 1 Introduction 1 1.1 The Standard Model . .2 1.2 Quantum Chromodynamics . .2 1.2.1 Confinement . .4 1.2.2 Deconfinement . .6 1.3 Quark-Gluon Plasma in heavy-ion collisions . 10 1.3.1 Kinematics of the collision . 12 1.3.2 Multiplicities and energy densities . 13 1.3.3 Thermal properties of the Quark-Gluon Plasma . 15 1.3.4 Nuclear modification factor . 16 1.3.5 Baryon-to-Meson Anomaly . 18 1.3.6 Collectivity . 20 1.4 Summary . 24 2 Heavy-flavour production - from pp to heavy-ion collisions 25 2.1 Theoretical description . 26 2.1.1 Heavy-flavour production in pp collisions . 26 2.1.2 Heavy-flavour production in p{Pb collisions . 33 2.1.3 Heavy-flavour production in Pb{Pb collisions . 36 2.2 Experimental Results . 42 2.2.1 Experimental results in pp and p{Pb collisions . 42 2.2.2 Experimental results in Pb{Pb collisions . 47 2.3 Aim of this thesis . 49 i CONTENTS 3 ALICE Experimental Setup 52 3.1 The Large Hadron Collider . 52 3.2 The ALICE detector . 54 3.2.1 The Inner Tracking System . 57 3.2.2 Time Projection Chamber . 59 3.2.3 Time-Of-Flight Detector .