Study Of The Strangeness Production Mechanism In Pp Collision With ALICE At The CERN-LHC

The study of interactions among elementary particles is one of the main research objectives of heavy ion physics.Democritus(460?370 BC)suggested that the material is permanent.It is also believed that the material is not produced,the change is the combination among element parts of the material.Epicurus(341?270)believed that the world is eter-nal,it is composed by elementary particles and vacuum.The atom,as the smallest particle,is only characterized by weight,size and shape[1].Nowadays,mankind has a deeper understanding of elementary particles and properties among their interac-tions.The smallest particles are described by the Standard Model.There are four interactions among them.Humans built large scientific instruments to study properties of interactions among elementary particles.The CERN-LHC is the largest electronics facility in the European Organization for Nuclear Research.It has the world's largest particle collider with the highest collision energy.As an international collaboration,it is committed to the study of the origin of the mass and physical properties of ele-mentary particles.Such studies expand the human's understanding of the current world.A Larger ion collider experiment on the Large Hadron Collider is dedicated to characterize of a special substance,the quark-gluon plasma(QPG),This substance is thought to exist for a short time after the Big Bang,and it is the initial form of the current universe.To study of its properties has a great physical meaning.In this thesis dedicate to the study of production of the neutral strange particles in jets with the LHC-ALICE detector in proton-proton collisions at 7 TeV.The results show that the ratio of A to KSO is nearly three times higher in the lead-lead collisions than that in proton-proton collisions.A significant enhancement of the A to KSO ratio with respect to pp collisiols is also observed in proton-lead collisions.Two mechanisms are used to explain those phenomenons:the projection of radial flow on the transverse momentum(pr)spectrum,and the effect of jet fragmentation.The yield of strangenesses in jets(come from heard processes)is studied in this thesis.It is compared to that produced by soft processes.The first three chapters describe the physical and technical contexts of the analysis.The following three chapters describe the specific methods and steps of the data analysis.The discussion of the results and the outlook are given at final.The first chapter deals with the elementary theoretical knowledge of particle physics.Based on the standard model,the present description of the elementary particles and the elementary interaction is described.begin with the Lagrangian density operator of quantum chromodynamics,explain the elementary physics of the theory of quark confinement,asymptotic freedom,and other elementary theoretical knowledge.The QGP predicted by the lattice QCD,which is the main research object of the current heavy ion physics.and introducing the way of generating QGP by heavy ion collision is summarized.The collision geometry and space-time evolution of heavy ion collision and the description and explanation of the corresponding physical model are briefly introduced.Finally,the probes and tools of studying the extreme matter are described.The first chapter deals with the elementary theoretical knowledge of particle physics.Based on the standard model,properties of elementary particles and the corresponding interactions axe described.With the Lagrangian density operator of quantum chromodynamics,the theory of quark confinement,asymptotic freedom,and other elementary theoretical knowledges are described.The QGP,which is predicted by the lattice QCD,is the main research object of the heavy-ion physics.The way for generating the QGP in heavy-ion collision is summarized.The collision geometry and space-time evolution of heavy-ion collisions and the description and explanation of the corresponding physical models are briefly introduced.Finally,the QGP probes are described.Chapter 2 gives the understanding of jets in theory and experiments.In the high-energy physics experiments,it is found that some events have a relatively strong cluster in a similar direction.In order to simplify the complexity of the experimental final state,hadrons are reduced to a number of jets.The jet is raised to simplify the final state of the experiment.By measuring the jet is used to study the final state of the collisions.The jet is a physical observable that can be predicted with pertur-bative QCD.It is defined as a bunch of clusters from hard parton hadronization.However,in the experiment,due to the detector performance and track resolution,not all the particles have been reconstructed.In order to compare with the jet de-fined in theory,in the experiment the jet is defined as a cluster of particles(tracks)given by the jet algorithm.Chapter 3 introduces the ALICE analysis framework.Firstly,a discussion for how to accelerate the particle was given.It starts with protons and lead prepara-tion and how to accelerate them in the LHC ring.Then the ALICE detectors are described,high energy heavy-ion collisions can be reconstructed into an analytic event with the information recorded by the DAQ system.The related techniques of vertex reconstruction,track reconstruction and particle identification in ALICE experiment are described.The ALICE detector has the good resolution to study the momentum of particles from 100 MeV/c to 100 GeV/c.Finally,the software and its technical aspects are explained.The technical of how to effectuate the process of acquiring,storing,accessing and analyzing of high-energy physics experiment data in modern high-energy experiments is introduced.Chapter 4 mainly elaborates how to get the yield of neutral strange particles in jets in proton-proton collisions at 7 TeV with ALICE.It mainly describes how the ALICE experiment reconstructs the neutral strange particle decays to rebuild the neutral strange particles.Experimentally,due to the neutral strange particles are not charged,they can not ionize when pass through the gas detector,the trajectories of the paxticles can not be directly measured.The neutral strange particles have characteristics of strong production and weak decay.The decay of the secondary par-ticles is measured via its properties.After that,the explain of the jet reconstruction method in ALICE experiment.Since ALICE has the advantage for reconstructing charged particles.In this study,hybrid tracks were selected for the jet reconstruc-tion.Jet with the transverse momentum greater than 10 GeV/c were used to tag the hard scatterings.Finally,it is determined whether the neutral strange particles are generated in the jet by using the spatial angular distribution between the neutral strange particles and the jet.For neutral strange particles which come from the jet is hard to identify at present.There are two ideas for their characterization.The first is using the greater transverse momentum,the angle of the particle between jet is smaller the neutral strange particles produced by the hard scattering and should also be located in the jet region.This method is used in this thesis.The second one is to determine whether the neutral strange particles are particles in the jet by reconstructing the neutral strange particles and the primary particles used for jet reconstruction.The study is currently used as the contrast of the first method,but it is still under discussion.Chapter 5 shows the corrections for the strange particle yield in jets.It mainly includes the efficiency correction,underlying background subtraction and feed-down subtraction corrections.The corrections of experimental results are mainly obtained by using simulated data.This chapter is mainly based on the sample simulated by ALICE in 2010.For the correction efficiency,the difference between the particle generation and the particle reconstruction is mainly simulated.The effect of the detector on the particle reconstruction is finally used to correct the raw yield.In proton-proton collisions,there is a UE background for neutral strange particle.It is necessary to subtract this background,from the neutral strange particle yield in the jet cone obtained in Chapter 4.In the measured neutral strange particle particles?(?),a certain fraction is derived from the feed-down of multiple strange particles,which need to be subtracted.Chapter 6 mainly discusses uncertainties in the analysis.It mainly includes the statistic uncertainties the systematic uncertainties introduced by the analysis step.They will be discussed separately.The statistic uncertainty is calculated step by step according to the mathematical formula.For the systematic uncertainty,firstly we need to separate the relation between the variables,then change the variable in a given range and get the difference between default and new results.The final uncertainty is obtained by the sum of the square root.Eventually,the statistical uncertainty and the systematic uncertainty are displayed in the final result.Chapter 7 focuses on the final conclusion of this analysis.It mainly includes the discussion of the physical information reflected in the results.We found that the baryon to meson ratio in jets in proton-proton collisions is flat after 2 GeV/c.After that,the relevant simulation about Color reconnection has been showed.Finally,we propose the improvement of this analysis.Summary:It is the first time to measure the strangeness productions in jets and UE at LHC energy.which can provide the basis for the research on the parton fragmentation and baryon/meson production mechanism,which also could be the baseline for study the properties of QGP.The results show that the yield of the neutral strange particle baryon to the ratio for the inclusive case,which is not caused by the fragmentation of the jet.