The Monte Carlo Study Of W Boson Polarization And T(?) Spin Correlation In T(?) Dileptonic Decay Channel

Since the appearance of human civilization,the exploration of nature has never stopped.Until 1970s in the last century,as the foundation of Standard Model,we had a comprehensive and systematic understanding of our natural world.Till now,many experiments have already shown impressive agreements with the Standard Model's predictions.Though the Standard Model has a great success,still many questions need solving,and the Higgs particle which is predicted by the Standard Model is still not discovered in experiments.To discover the Higgs particle and prove the Standard Model accurately in a much higher energy scale are important tasks for present particle physics.A great proton-proton collider LHC(Large Hadron Collider)with 14-TeV center of mass energy and 10^-34cm^-2/s luminosity is under construction at CERN(European Organization for Nuclear Research).When finished,it will be the largest hadron collider in the world.ATLAS(A Toroidal LHC ApparatuS)is a huge general-purpose particle detector which will be installed on one colliding point of LHC.While aims mainly at the discovery of the Higgs particles,the ATLAS detector will also try to search for much heavier W-like and Z-like bosons,super-symmetric particles,and to study the structure of the fundamental fermions,CP violation,and so on.The ATLAS detector is composed by an inner detector,a magnetic system,a calorimetric system, a muon spectrometer,a trigger and data acquisition(DAQ)system.While running at low luminosity,there will be 8 million top quark events produced each year on LHC,it's really a big "top factory".Many researches on top quark can be carried out,such as accurate measurement of top mass,W polarization measurement in top quark decay channels.During the related top works,study of top quark pairs(t(?))is an interesting and meaningful work.From the Standard Model's prediction,almost all the top quarks decay to W bosons and b quarks,and according to the different final particles of W decay,t(?)events can be divided into 3 kinds:single lepton plus jets events (t(?)→w⁺w^-b(?)→(lv)(jj)b(?)),dilepton events(t(?)→w⁺w^-b(?)→(lv)(lv)b(?)), fully hadronic decay events(t(?)→w⁺w^-b(?)→(jj)(jj)b(?)).The dilepton events are especially interesting for its two isolated leptons.For the W boson in top decay chain,its polarization information is precisely predicted by the Standard Model,it is a sensitive probe of new physics in top production and decay.The W polarization measurement complements top spin studies, provides a direct test of tWb vertex understanding,and more particularly of its V-A structure.There are three possible helicities for a spin-one W boson,-1,0,+1,that will be called left-handed(F_L),longitudinal(F₀)and right-handed(F_R).Any deviation of F_L, F_R,F₀ from their theoretical values could point to a non-Standard-Model.The polarization of top quark and spin correlation of t(?)are also precisely predicted by Standard Model,we can get top' s spin information by studying its children.The same with W polarization,t(?)spin correlation is a sensitive probe to new physics.In my thesis,the main work is the studies of W polarization and t(?)spin correlation in dileptonic channel using Monte Carlo(MC)data samples.My thesis starts from the MC data production,and then presents a whole process from channel reconstruction to the measurements of W polarization and t(?)spin correlation in experiment,and then to the analysis of systematic errors.Reasonable results are given at last.In this thesis,my work is as following:reconstruct the t(?)dileptonic channel independently;give an experimental method to extract the W polarization parameters and t(?)spin correlation parameters;for the possible systematic errors in the measurements,I give a reasonable estimation.There are 6 parts in my thesis:first,"The History of High Energy Physics and LHC",which tells the development of high energy physics and gives a simple introduction of LHC;second,"The ATLAS detector",which gives an introduction of the ATLAS detector;third,"Top Physics and Data Production of t(?)Decay",in which top physics researches and MC production of t(?)sample are described;fourth, "The Reconstruction of t(?)Dileptonic Decay Channel",in which the reconstruction of t(?)dileptonic decay channel are described in detail;fifth,"W polarization Study in t(?)Dileptonic Decay Channel",in which the measurement of W polarization is detailedly presented and the possible systematic errors are estimated; sixth,"t(?)Spin Correlation Study in t(?)Dileptonic Decay Channel",in which the measurement of t(?)correlation are detailedly presented and the possible systematic errors are estimated.