Measurement Of Underlying Event Actvity In Pp Collisons At 13 TeV With ALICE At The LHC

In recent decades,the Quark Gluon Plasma(QGP)has been attracting physicist’s strong interest.QGP is a new type of material phase state which is different to the daily hadronic matter,it once occurred in a short period of time after the ’Big Bang’,and deeply influenced the evolution of the universe and the formation of all matters existing in the universe.The partons,including quark,antiquark and gluon,are asy-mptotically free inside the QGP.The relativistic heavy ion collision makes the physicist’s desire of creating a’Little Bang’in the laboratory come true,then it is possible to study the formation and properties of the QGP systematically in the laboratory.From Super Proton Synchrotron(SPS)to Relativistic Heavy Ion Collider(RHIC),and then to the latest Large Hadron Collider(LHC),the QGP has been researched systematically under different collision energies in different collision systems,but there are still many unanswered questions.A parton traversing the QGP will lose part of energy due to the effect of strong interaction,then fragmentate to a jet,which is a collection of produced particles in the fragmentation process,this phenomenon is called jet quenching in the high energy physics experiment.RAA,which is used to reflect the energy loss of parton when traversing the QGP,is a very crucial observable in the high energy physics experiment.In the measurement of RAA,the pp collision is generally treated as a reference due to there is no evidence to prove the existence of QGP in pp collision.So it is particularly important to implement the precise measurement of pp collision.But unfortunately,in pp collision,except the most inreresting the hardest interac-tion,some soft processes and semi-hard processes are also included,the particles from these soft processes and semi-hard processes are called Underlying Event,UE for short.The UE make the precise measurement of pp collisions become very diffi-cult,because we can not distinguish rightly the particle which one is from the hard process and which one is from UE.The UE include all the particles except the part,icles from the hardest interaction,the components of UE are initial and final state radiation(ISR and FSR),multiple parton interaction(MPI)beam remnants,etc.In traditional UE measurement,according to the direction of the leading track,the whole topological space is divided into three distinct regions:Toward region,Transverse region and Away region.Toward region and Away region are mainly used to collected the particles from two back to back jets,the particles from UE are believed to be absolutely dominated in the Transverse region when the leading jet pT<50 GeV/c[1].Since the UE is isotropic,and three regions are with the same area,so the UE activity in the three regions present the same behaviours.In this thesis,based on the data taken by ALICE detector in 2016 for pp collisions at(?)= 13 TeV,the detailed information of UE activity in pp collisions at(?)=13 TeV are introduced,the newest results with respect to leading track pT are presented,and the comparisons with ATLAS results are also exhibited.In addition,this thesis also focus on the measurement of UE activity as a function of RT,the final results of RT probability distribution and charged particle<pT>distribution with respected to RT are given,they are the first results relative to the measurement of RT in the ALICE.In this thesis,The chapter 1 introduces the standard model and quantum chro-modynamics firstly,and then focuses on the formation and properties of QGP,the process of a heavy ion collision also will be introduced in detail in this chapter.In the chapter 2,the physical motivation of UE measurement and the physics concept of MPI are introduced firstly,the detailed information of two Monte Carlo models contributing to our analysis work also are going to be described in the next,then the highlights of UE measurements from RHIC to Tevatron,to LHC are presented,a new UE activity classifier,RT,will be introduced at last.The chapter 3 mainly describes the setup of the ALICE detector,the performances of subdetectors and the update program for RUN-3.Begin from chapter 4,the detailed information of UE activity measurements in pp collisions at(?)= 13 TeV are introduced.The detailed analysis procedures of UE activity as a function of leading track pT will be described in chapter 4,including data selection,event and track selection criterion,correction maps,systematical uncertainty estimation and final results discussion.In the chapter 5,the analysis process of UE activity with respect to RT are described,the unfolding method used to correct the RT smearing is also introduced in detail in this chapter.A summary and outlook will be given in the last chapter of this thesis,in chapter 6.