| As we all know,protons and neutrons constitute the atomic nucleus,which we call nucleons.Protons and neutrons are composed of quarks and they interact with each other via gluons,so quarks and gluons are considered as elementary particles.The elementary particles and their interactions are described by Quantum Chromo-dynamics(QCD).The QCD theory indicates that in the nuclear matter phase,quarks and gluons are confined in the nucleus,and there are no free quarks or gluons.How-ever,under the extreme high temperature or high energy density conditions,the confined quarks and gluons will be deconfined from the nuclear matter and create a new phase with free quarks and gluons,called quark-gluon-plasma(QGP).QGP is believed to exist in the early universe after the Big Bang.After that,the early universe rapidly expand and cool down to form the stable nuclear matter,which is the current universe.Studying the formation process and the properties of QGP will help us to further understand the origin of the universe,the formation of matter and the evolution of various galaxies.Therefore,search for the generation and nature of QGP is a hot and important topic in today’s world.In the laboratory conditions,particles are accelerated and collided at the Rel-ativistic Heavy Ion Collider(RHIC)or the Large Hadron Collider(LHC)in order to study the QGP.By colliding heavy ions,one expects the high-temperature and high-density conditions for the formation of QGP achieved,so as to study the properties of QGP.This analysis relies on the Large Hadron Collider(LHC)located at the European Nuclear Research Center(CERN),which is currently the largest particle accelerator in the world.A Large Ion Collider Experiment(ALICE)is one of the main experiments running on LHC.Its main purpose is to study the formation of QGP and its properties by colliding heavy ions.We know that the existence time of QGP is extremely short,so we can not directly measure QGP.Experimentally,final state particles are used as a probe to study the creation of QGP and its properties by measuring the modifications of the probe.Jets used in our research is one of the important probes for studing QGP.In high-energy nucleon-nucleon collisions,the hard scattering process is a two-to-two process with large energy and transverse momentum transfer.It mainly occurs at the early stage of the collision,the parton generated by the hard scattering process will eventually fragmented into final-state hadrons.Due to Lorentz contraction,these hadrons tend to move in the direction of the hard scattering partons,collimated in a finite size cone,called as jets.Because jet is generated by the hard scattering process,it can be calculated by the perturbation QCD,the measurement of jets can help test the QCD model.In the nuclear-nuclear collisions,the hard scattering process is occurred before the formation of QGP,which means that the hard scattered partons will traverse the QGP transverse the QGP and interact with it.Theory predicts that the hard scattered partons will lose energy when they traverse the QGP transverse the QGP,resulting in the suppression of high transverse momentum particles.This phenomenon is called jet quenching.Because of jet quenching,the measured jet yield at high-pT will be suppressed and the jet structure will be modified.Therefore studying the jet productions will help us to understand the interaction mechanism between partons and the QGP.Based on the data collected by Alice experiment,this thesis studies the semi-inclusive recoil jets by using charged particle triggered jet correlations.The method of measuring recoil jets with charged particle triggers using different kinematics can effectively reduce the uncorrelated background associated with jets,so that the mea-surements of jets can be extend to the low transverse momentum range without the influence of underlying events.Charged particle triggers with two different trans-verse momentum intervals are selected to tag recoil jets.By subtracting the recoil jets associated with low-PT trigger particles from high-pT triggers,we removed the un-correlated background and obtain the background-free relative yield of semi-inclusive recoil jets △recoil.After the correction of the detector effect for △recoil.We have compared the corrected semi-inclusive recoil jet yield with Monte-Carlo predictions.We also studied the recoil jet yield with different resolution parameters and the ratio of jet yield using different jet resolutions.Such kind of ratio reflects the collimation information of jet splitting.Finally,we compared the results obtained in proton-proton collisions with that in the Pb-Pb collisions at same center-of-mass energy.We observed that the yield of the semi-inclusive recoil jets in the central Pb-Pb collisions was strongly suppressed with respect to that in proton-proton collisions,confirming the jet quenching phenomenon predicted by theory.By comparing the jet yield ratio using different jet resolution parameters in proton proton and Pb-Pb collisions,we observed a hint of jet energy redistribution to large radius in central Pb-Pb collisions,especially for jets with low transverse momentum. |
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