Study Of Dijet Momentum Imbalance In Pp And Pb-Pb Collisions At(?)= 5.02 TeV In ALICE

Quarks and gluons are considered as elementary particles.The interactions between quarks and gluons can be described by Quantum ChromoDynamics(QCD),namely quarks and gluons are confined within the nucleus in ordinary hadronic matter phase,and no free quarks and gluons can be extracted from hadrons.Lattice quantum chromodynamics(LQCD)predicts that under extreme conditions of high temperatures and high energy densities,a phase transition from hadronic matter to a deconfined state of quarks and gluons called Quark-Gluon Plasma(QGP)will occur.QGP is thought to exist in the very early stage after the big bang,and then the universe expands rapidly to form a stable hadronic phase.Studying the QGP creation and its properties can help us understand the evolution of the universe.Therefore,the search of the QGP creation and its properties has become one of the important topics in high-energy nuclear physics.The creation of QGP and the properties of QGP are studied by large experimental devices,such as Relativistic Heavy Ion Collider(RHIC)and Large Hadron Collider(LHC)in the laboratory.The LHC is located at the European Organization for Nuclear Research(CERN)and is the world’s largest particle accelerator so far.A Large Ion Collider Experiment(ALICE),is one of the major experiments at the LHC and its main objective is to study the QGP production and its properties in heavy ion collisions.In the laboratory,QGP phase exist only a very short time and therefore cannot be observed directly,by study the final state particles produced in the heavy ion collisions and their modifications,we can then interpolate the QGP formation and their properties.Before the collision,the colliding beam is accelerated by the superconducting magnetic field to a speed close to the relativistic speed of light and collide.The partons inside the nuclei will transfer momentums during the collisions.The hard scattering is called when the momentum transfer is much higher than the QCD scale.The outcoming partons produced from hard scattering are back-to-back which will eventually undergoes as two back-to-back dijets after hadronizations.Jet is a collimated spray of hardons in a finite size and originating from the hard scattered partons.The dijets produced from hard process have approximately equal transverse momentum and are emitted to back-to-back in azimuth(A(?)≈ π).In heavy ion collisions,QGP is expected to be created under extreme condition of high temperature and high energy density.The hard scattered partons will then transverse the medium after it’s created and suffer energy loss in the medium.Such a phenomenon is the so called jet quenching.The energy loss will change the direction and transverse momentum of dijets,resulting in the energy imbalance and the broadening of relative azimuth between the back-to-back jets.These jet modification can reflect the properties of the QGP.Therefore,jet is an important probe for studying the QGP,which is the initial idea to study dijets momentum imbalance in this thesis.So far,ATLAS and CMS have measured the relative azimuth and traverse momentum distribution and observed the broadening of the azimuthal correlation and momentum imbalance by comparing the results in pp and Pb-Pb collisions using dijets with high traverse momentum.From the previous measurements on the charged particles,we conclude that the interaction with QGP for low traverse momentum jets will be more complex and more interesting since low traverse momentum jets are more sensitive to the energy loss.Therefore,studying the low momentum jets can better explore the properties of QGP.In this analysis,we explore low traverse momentum jets reconstructed by ALICE detectors and study the dijet momentum imbalance in Pb-Pb collisions and compared to proton-proton collisions at the same collision energy,with the aim to understand low energy jet quenching mechanism.The data analyzed in this thesis was recorded by ALICE in 2015.In this thesis work,we have first examined the data quality with all the data used in this analysis,we then apply the physics selection on the events to perform dijet imbalance study.We have also performed Monte Carlo study to validate the generators used for the correction process.Finally,dijets momentum imbalance is measured in proton-proton and lead-lead collisions.From the results obtained,we observed the asymmetry of the dijet events in most central collisions.