Measurement Of Jet Fragmentation Functions And Of Their Moments In $pp$ Collisions At $\sqrt{s}=2.76$ TeV With ALICE At The LHC

A cross-over between ordinary nuclear matter and a state of deconfined quarks and gluons,the Quark Gluon Plasma(QGP),is predicted by lattice QCD calculations at low chemical potential and high temperature in the nuclear phase diagram.Ex-perimentally,ultra-relativistic heavy ion collisions are used to produce and study the hot and dense QGP medium.Produced in a hard scattering at the early stage of the collision a highly ener-getic parton is first expected to lose energy in the medium before fragmenting into a hadronic spray of particles called jet.A detailed study of the modification of the jet structure and of its fragmentation pattern in vacuum and in medium should provide insights into the QGP properties.The jet fragmentation functions describe the momentum distribution of hadrons inside a reconstructed jet.In proton-proton(pp)collisions their measurement is important for understanding the mechanisms of parton fragmentation.Such measurements also provide a test of perturbative Quantum Chromo Dynamics(pQCD)as well as a baseline for similar measurements in p-A collisions(revealing potential cold nuclear matter effects)or in A-A colli-sions(shedding light on the energy loss mechanisms in presence of a hot and dense medium).However,in heavy-ion collisions the presence of a large underlying event and of its event-by-event fluctuations makes the measurement of jet fragmentation functions a challenging task.The use of the fragmentation function moments has been proposed in[1]as a way to overcome this difficulty.The ALICE detector at the Large Hadron Collider(LHC)has unique tracking capabilities enabling to measure charged particles down to transverse momenta as low as 150?MeV/c.This allows assessing possible modifications of the jet structure and helps constraining the jet fragmentation functions.The ALICE Electromagnetic Calorimeter(EMCal)can also be used to measure hard probes of the initial collision,including jets,high pT photons,neutral mesons(?0,?,?),and electrons.For LHC Run 2(2015-2018)an additional detector was installed on the opposite side of the beam axis,the Di-Jet Calorimeter(DCal).It provides the angular coverage necessary to facilitate jet-jet,hadron-jet,and 7-jet correlations.In this thesis we will present the our measurements of charged-jet fragmentation functions in pp collisions at(?)= 2.76 TeV with ALICE.The first studies of fragmentation function moments will be discussed.Part of this work is also dedi-cated to the ALICE EMCal and DCal sampling electromagnetic calorimeters.The implementation of the DCal geometry in the ALICE offline software will be pre-sented.The document is organized as follows.The first chapter introduces and motivates the study of the QGP in heavy-ion collisions.Chapter 2 presents a few aspects of jet physics and motivates the study of jet fragmentation.Chapter 3 is dedicated to the ALICE experiment at LHC.Its sub-detectors are introduced and described with a main focus on those that are used for jet analyses studies.In Capter 4,after discussing about the basic principles of electromagnetic calorimeters,the ALICE EMCal and DCal detectors are presented.The DCal geometry togeather with the its implimentation in ALIROOT and the validation tests that we performed in order to validate it are discussed.In Chapter 5,after introducing the tools used in our physics analyses and focuses on the running conditions and quality checks of the data.In Chapter 6 we present our measurement of charged jet fragmentation functions with the ALICE experiment in pp Minimum Bias Collisions at the LHC at(?)= 2.76 TeV.The measurement covers(mini)-jet momenta from 5 to 60 GeV/c.Details about the underlying event subtraction as well as about the bin-by-bin and secondary particle corrections are guiven before to discuss the results.Finaly in Chapter 7,our measurement of jet fragmentation function moments is presented and discussed.