Measurement Of π~0Production In P+p Collision Andπ~0v2in Pb+Pb Collision In2.76TeV By LHC/ALICE EMCal

For over thousand years people have thought about the fundamental particles from which all matter is made, starting with the gradual development of atomic theory. It began as a philosophi-cal concept in ancient Greece (Democritus), followed by the modern atomic theory proposed by John Dalton. It then entered the scientific mainstream in the early19th century by J. J. Thomson who discovered the electron by measuring the Mass-to-charge ratio of electron itself, People confirmed the model used to describe atom is called Rutherford model and in this model Ernest Rutherford demonstrated atom has a tiny but heavy nucleus. Besides the famous experiment by James Chadwick in finding the neutron and by Rutherford in α scattering experiment in con-firming the proton, more and more experiment results were announced on the deeper structure study the history of searching for structure of elementary particle getting started to set up on the experiment results. Then more and more experiment facilities set up on studying various aspect of elementary particle structure, In the deep inelastic collision experiment, we realized that nucleus should also has its sub-structure which now know it as quark and gluon. Hence one series of theory was build to describe the structure of nucleus or even much deeper structure. Among those theories, the most famous one is called Standard Model(SM) which summarized on electromagnetic, weak, and strong nuclear interactions. As part of standard model the strong force is one of the four fundamental interactions in particle physics. It describes the interaction between partons (quarks and gluons) which make up hadrons. The theory of the strong force is called quantum chromodynamics (QCD), which is a quantum field theory of the color charged partons. The force between color charges does not diminish while they are separated. This property causes the color charges to be confined in to hadrons, in ordinary matter. Quark-Gluon Plasma (QGP) is one phase of the QCD matter at extremely high temperature and/or pressure, where the partons are asymptotically free. In astronomy, the big bang theory not only suggests that the Big Bang in the early universe (high temperature) may create Quark-gluon plasma, but also expects that in the neutron star (high density) QGP may also exist. However, with the uni-verse going through long timescale evolution, the information of such created plasma has been fell off hence difficult to extract. On the other hand neutron Star is so far away from us that its detection is also challenging. Therefore, in the80s of last century, the Nobel Prize Winner T.D.Lee proposed idea that one could create high temperature and(or) density state of material in the laboratory by heavy ion collisions.By accelerating two heavy ion beams in opposite direction and then let them collided, one expects to realize the extreme high temperature and density matter in the laboratory. A great of results have been obtained at the Relativistic Heavy Ion Collider (RHIC) which was build at Brookhaven National Laboratory with a top center-of-mass energy200GeV. The results found at RHIC is a important baseline for the experiments take out at Large Hadron Collider (LHC) at CERN since90s of last century. A Large Ion Collider Experiment (ALICE) is one of the four main detectors at the LHC, which is dedicated in the heavy-ion collisions studied. The design top energy of it is up to (?)SNN=5.5TeV in Pb+Pb collision while (?)SNN=14TeV at p+p collision which is roughly14times larger than RHIC. Thus, relatively dense QCD matter will be created and last longer than in RHIC. It also provided us a better opportunity to study the properties of the QGP. Even We still can not measure QGP directly due to the limitation of the experiment facilities, several measurement method was also developed depending on the final measurable decay particle. Such as Jet quenching, High pT particle production, suppression of quarkonium, particle azimuthal correlation, collective phenomena etc. In the various final state particle, photon is treated as the golden particle who keeping the collision information in each stage of collision because of it generated in the whole stage of collision. It is also involved only in the electromagnetic interaction hence with lower interaction probability with others particle.With the theory background as explain above, during Ph.D study period my work is fo-cusing on the data analysis in ALICE experiment especially on the photon relative physics via EMCal detector. The thesis mainly includes the two topics:(1) Measurement of π0production in (?)SNN=2.76TeV proton+proton collisions data collected at2011. and (2) Measurement of π0u2in (?)SNN=2.76TeV Pb+Pb collision collected at2011.Before starting the data analysis, it is important to understand the experimental facilities we have. Hence in the chapter2, the experiment facilities of ALICE is introduced with start from an overview of ALICE detector. The sub-detector which involved in our analysis is presented from inner layer to out layer depending on their physical location. The first sub-detector need to be introduced is Inner Tracker System(ITS). It is a3layers silicon pixel technical base detector which dedicated in measuring the position of the charged tracks. By analysis the distribution of the tracks which deposited information in the ITS, the interaction vertex in the collision was fixed. The forward rapidity detector used to reconstruction the reaction event plane is introduced afterward. Finally but most important part of this chapter is the explained the detail composition of the π0detector which is Electromagnetic Calorimeter(EMCal), not only the material of its but also the performance of detector was introduced.The first work on analysis in the real data is on the data collected in2010proton+proton (?)SNN=2.76TeV collision. The analysis is presented by separated it to several analysis steps. After introducing the motivation of this part of work in the beginning, the event selection and cluster selection criteria are listed. Since detector can not100percent correctly reconstruct out what come across it. By comparing the different of the π0yield on the generated level to the one after go through the detector reconstruction, the π0reconstruction efficiency in EMCal is studied with the embedding technical. In the same time the raw yield extraction and Invariant yield calculation is also studied by employing different extraction method. By testing on dif-ferent combination of real data reconstruction procedure and efficiency calculation method, the invariant yield which count as central point for later systematic study is fixed. The systematic uncertainty in this analysis is studied by variant the different reconstruction procedure and cuts while keep the other stable, Hence the different between central point and yield after variant is treated as one source of systematic uncertainty. We hence present the minimum bias data sam-ple result with adding the systematic error on it. In order to extend the measurement of yield to higher transverse momentum range in the limitation of statistic, we analysis the EMCal trigger data sample which by selecting the particular event by online setting several criteria to accept particular events. With the same procedure the invariant yield of trigger data and systematic uncertainty are estimated. Finally the results from different transverse momentum are combined by scaling down the trigger yield to match the minimum bias yield with trigger rate calculation.The second work is on the analysis of Pb+Pb collisions data collected at2011. The motiva-tion of this part of work comes from the perspective of theory are introduced. In the introduction we explain the physics information expect to be reveals from this analysis in the three different transverse momentum ranges which is low, medium and high pT regions. Afterward the two main methods used to extract v2in the experiment are introduced. The first one which also assumed as common one is called dN/dΦ method, which this method after reconstruction π0in different transverse and its relatively azimuthal location with respect to event plane, the v2information thus can be extracted by fitting those relatively production distributions with certain models. The second method is called invariant mass method, In this method by calculating the different of background to total and signal to total yield ratio as the fitting model parameter, one fitted the cos(2△φ) as function of transverse momentum distribution to extract raw u2in-formation. The final step of extraction in both method1and method2are the raw u2extracted from above fitting divided the event plane resolution to get final result. The event plane count the different of the reconstructed event plane compared to the collision one which can not be measured. Since the event plane reconstruction in this analysis is a crucial item, the definition of it and how to reconstruction it is emphasized in the second part of this section. In the assumption of not perfect reconstruction of event plane, not only the event plane resolution which defined in the theory that should be studied but also the additional bias found in analysis due to cen- trality trigger bias should also be investigated. On other hand π0reconstruction strategy is also stressed in this chapter. After explaining the detail of event plane and π0reconstruction, the raw result thus come out by computing the correlation of this two parts in the azimuthal different. The sources of systematic uncertainty we take into count in this analysis are the:(1) the extract method difference, method1or method2as explain before.(2) The π0fitting models when extract its yield, the different is background description with Polynomial2or Polynomial1.(3) The detector used to reconstruct event plane, using VOA or VOC. By comparing the different of the result of u2calculated from different combination of three sources of systematic uncertainty as considered above, the final estimation of systematic uncertainty is calculated by combing the different source linearity in the assumption that they should be independent on each other. Final-ly the results of π0u2are presented as function of transverse momentum in6different centrality bins.