在ATLAS实验中利用top夸克对单轻子道事例对w玻色子极化和top夸克对自旋关联的研究

摘要

The particle physics came out and developed with the exploration of the composition of the nature world.The Standard Model which was developed in early 1970s provides us a comprehensive and systematic theory to understand the particle physics.Many of the prophecies from the Standard Model have been proved by the experiments.Although the Standard Model has made such great successes, there are still many question need to be solved.The LHC, which is the largest high energy collider up to now, is constructed to provide the physics which can be used to test the Standard Model, and find the Higgs particle.It has 6 detectors, which are ATLAS, CMS, ALICE, LHCb, LHCf and TOTEM.Accurate study on the final reconstructed object performance is done to support all physics study.This thesis firstly presents my work on the electron identification efficiency study at ATLAS, which is used to improve the agreement of the electron performance between real data and simulated Monte Carlo. The LHC is a real top quark factory, as about 8 million top quark pairs will be produced per year.The top quark physics is an ideal physics channel to test the Standard model and find new physics, as the large mass and long life of the top quark.This thesis presents the measurements of the helicity fractions of the W boson and ttbar spin correlation in top quark pair semi-lepton decay, using the 4.7fb-1 data collected by the ATLAS experiment at LHC with proton-proton collision at the center of mass equal to 7TeV.In those analyses, I focus on the tt semi-leptonic channel (one W from top quark decays into leptons and another W from the second top quark decays into two light quarks).After the event selection, to further suppress W+jets and multi-jet backgrounds, I further require two jets to be tagged as bottom quarks.A complete reconstruction algorithm of the top quark pair system is described in the thesis.The physics variables are then calculated using the fully reconstructed tt system.The distributions are obviously biased by the hadronization/showering and detector effects, which are modelled by the matrices named the response matrices.The response matrices are calculated from Monte Carlo simulations.The unfolded angular distributions of final particles are then fitted by theory function to extract the Whelicity fractions and spin correlation.By using an unfolding method based on the migration probability from truth bin to reconstructed bin, the distributions of the angle are unfolded back to their parton level.After fitting the distributions with theory function, the W helicity fractions are measured:t(t) spin correlation differential result is also measured by using the unfolding method.Both of these two measurement results have good agreements with the Standard Model within the statistical and systematic errors.

目录

声明

TABLE OF CONTENTS

LIST OF FIGURES

Abstract

Chapter 1．Introduction

Chapter 2．The Standard Model and top quark physics

2．1．The particle physics

2．2．The Standard Model

2．2．1．The fundamental particles

2．2．2．The electroweak interaction

2．2．3．The quantum chromodynamies

2．3．The top quark physics

2．3．1．The top quark Production

2．3．2．The top quark decay

2．3．3．The top quark properties

Chapter 3．The LHC and ATLAS experiment

3．1．The LHC

3．2．The ATLAS experiment

3．2．1．The magnet system

3．2．2．The inner detector

3．2．3．The calorimeters

3．2．4．The muon spectrometer

3．2．5．Trigger and Data Acquisition

Chapter 4．The collision data and Monte Carlo samples

4．1．The collision data

4．2．The MC samples

Chapter 5．The electron identification efficiency measurement

5．1．The measurement motivation

5．2．The identification criteria

5．3．The measurement method-Tag and Probe method

5．4．The measurement results-Efficiency and Scale factor

5．5．The trigger dependence of electron id efficiency measurement

5．6．The pile-up dependence of the ID efficiency measurement

5．7．Summary

Chapter 6．The Objects selection

6．1． Electron

6．1．1．The electron identincation

6．1．2．The electron trigger

6．1．3．The offline electron selection

6．1．4．The electron efficiencies and scale factors

6．1．5．The electron energy scale and resolution smearing

6．2．Muon

6．2．1．The muon identification

6．2．2．The muon trigger

6．2．3．The offiine muon selection

6．2．4．The muon momentum scale and resolution smearing

6．2．5．The muon efficiencies and scale factors

6．3．Jet

6．3．1．The jet calibration

6．3．2．The jet energy scale and uncertainties

6．3．3．The jet energy resolution

6．3．4．The jet quality and pile-up rejection

6．3．5．The Jet reconstruction efficiency

6．4．The b-tagging of jet

6．5．The EmissT Description and Performance

Chapter 7．The event reconstruction

7．1．The event selection

7．2．The event reconstruction

7．3．The W+jets background estimation

7．4．The QCD background estimation

7．5．The Event yield and control plots

Chapter 8．The Unfolding method

8．1．Introduction

8．2．The response matrix

8．3．The iteration method

Chapter 9．The Wboson polarization measurement

9．1．The scan method in unfolding technique

9．2．The validation of the unfolding technique

9．2．1．The binning choice

9．2．2．The effects of iteration

9．2．3．The un-bias test

9．2．4．The I/O test

9．2．5．The closure test based on SM matrix

9．3．The difierential measurement

9．4．The inclusive measurement

9．5．The systematic uncertainties

Chapter 10．The ttbar spin correlation measurement

10．1．The validation of the unfolding technique

10．1．1．The binning choice

10．1．2．The effects of the iteration

10．1．3．The un-bias test

10．1．4．The I/O test

10．1．5．The closure test based on SM matrix

10．2．The differential measurement

Chapter 11．Summary and outlook

BIBLIOGRAPHY

ACKNOWLEDGEMENTS

LIST OF PUBLISHED PAPERS