ATLAS探测器上的底夸克标记校准和希格斯粒子衰变到底夸克对的观测

摘要

This thesis firstly talks about the b-jet tagging efficiency calibration with the tag-and-probe method in the ATLAS experiment with the data collected in2015and2016.The efficiency measurement method,the object selection,the event and probe jet selection and the statistical and systematic uncertainties are discussed,as well as the results.The b-tagging efficiencies have been measured as a function of the jet transverse momentum,pseudorapidity and of the average number of pile-up collisions.The efficiency measured in data have been compared to those predicted from simulation,and simulation-to-data efficiency scale factors have been determined.The efficiency scale factors are close to unity,with total uncertainty ranging between2％and12％. Then the evidence of the Standard Model(SM)Higgs boson decaying to bottom quarks in the associated production mode with vector bosons with36.1fb-1data collected at√s=13TeV during the LHC Run-2(until2016)will be introduced,and followed by the observation of VH,H→bb from the study including also the additional44fb-1data collected in2017. For the study based on36fb-1data,the event classification,the discriminating technique between signal and background,the main background and their modelling,the signal properties and the interpretation of results are discussed.For the study based on80fb-1data,the small difference from the previous study and the observation result are discussed.Both results provide a strong evidence of such process,with statistical significances of3.5and4.9standard deviations respectively.The signal strength,defined as the ratio between the measured cross section times branching ratio and the SM prediction,is measured with the2015-2017dataset to beμbbVH=1.6+0.27-0.25=1.16±0.16(stat.)+0.21-0.19(syst.),indicating good agreement with the SM.The combination of the results of the VH,H→b(b)results based on80fb-1of13TeV collisions with the results with the results of the same analysis performed on data collected at lower(7and8TeV)centre-of-mass energies in2011and2012and same decay mode with other Higgs boson production modes(vector-boson fusion,associated production with a top-quark pair)have led to the observation of the H→bb decay with a significance of5.4standard deviations,and a signal strengthμbb=1.01±0.20=1.01±0.12(stat.)±0.16-0.15(syst.).A combination of the results of the VH,H→b(b)results based on80fb-1of13TeV collisions with the results of searches for the same production mode on the same amount of data in the diphoton and ZZ*→4l Higgs boson final states has led to the observation of VH production with a significance of5.3standard deviations,and a signal strengthμVH=1.13+0.24-0.23=1.13±0.15(stat)+0.18-0.17(syst.)in good agreement with the SM.

目录

声明

Abstract

Contents

Chapter 1 Introduction

Chapter 2 Theoretical and Experimental Overview of the Higgs Boson

2．1 The Standard Model of Particle Physics

2．1．1 The Standard Model particles and interactions

2．1．2 The Standard Model lagrangian

2．1．3 The Higgs mechanism

2．2 Higgs Boson Phenomenology at the LHC

2．2．1 Higgs boson production

2．2．2 Higgs boson decays

2．3 Higgs Boson Measurements at the LHC

2．3．1 Higgs boson discovery

2．3．2 Higgs boson characterisation：bosonic decays

2．3．3 Higgs boson characterisation：mass

2．3．4 Higgs boson characterisation：spin and parity

2．3．5 Higgs boson characterisation：fermionic decays

2．3．6 Higgs boson characterisation：production modes

2．3．7 Higgs boson characterisation：overall picture

2．4 Drawbacks of the Standard Model

Chapter 3 The ATLAS Experiment at the LHC

3．1 The Large Hadron Collider

3．1．1 Proton acceleration in the LHC

3．1．2 Proton collisions in the LHC

3．1．3 History of the LHC and evolution of the beam parameters

3．2 The ATLAS detector

3．2．1 The ATLAS coordinate system

3．2．2 The Inner Derector

3．2．3 The calorimeter

3．2．4 The hadronic calorimeter

3．2．5 The muon spectrometer

3．2．6 The ATLAS data taking and the ATLAS trigger system

Chapter 4 Object Reconstruction and Performance

4．1 Charged particle track reconstruction

4．2 Primary vertex reconstruction

4．3 Electron and photon reconstruction

4．4 Jet reconstruction

4．5 Muon reconstruction

4．6 Tau lepton reconstruction

4．7 Missing transverse energy reconstruction

Chapter 5 b-jet identification in the ATLAS experiment：algorithms and performance

5．1 b-tagging algorithms in ATHENA release 20.7

5．2 b-jet efficiency calibration with ATHENA release 20.7 and 2015-2016 data

5．2．1 Data and simulated event samples

5．2．2 Efficiency measurement method

5．2．3 Object selection

5．2．4 Event and probe jet selection

5．2．5 Multivariate event discriminant

5．2．6 Statistical uncertainties

5．2．7 Systematic uncertainties

5．2．8 Results

Chapter 6 Evidence of VH，H→b(b) production

6．1 Overview of the EPS 2017 analysis

6．2 Data sample

6．3 Simulated samples

6．4 Event selection

6．4．1 Common event selection

6．4．2 0-lepton channel specific selection

6．4．3 1-lepton channel specific selection

6．4．4 2-lepton channel specific selection

6．5 Event classification

6．6 Multivariate analysis

6．7 Multi-jet background estimation

6．8 Systematic uncertainties

6．8．1 Experimental uncertainties

6．8．2 Simulated background modelling uncertainties

6．8．3 Multi-jet background uncertainties

6．8．4 Signal modelling uncertainties

6．9 Statistical Analysis

6．9．1 Fit inputs

6．9．2 Nuisance parameters：normalisation and systematic uncertainties

6．9．3 Nuisance parameters：smoothing and pruning

6．10 Results

6．11 Cross-check：the diboson analysis

6．12 Cross-check：the dijet-mass analysis

Chapter 7 Observation of VH production and H→b(b) decays，and future prospects

7．1 Analysis of 2017 data and observation of H→b(b) decays and of VH production

7．2 Observation of H→b(b) decays by the CMS Collaboration

7．3 VH simplified template cross section measurements with H→b(b) decays

7．3．1 The Simplified Template Cross Section framework

7．3．2 Simplified Template Cross Section measurements with V H，H→b(b)

7．4 Prospects for the V H，H→b(b) measurement at the HL-LHC

Chapter 8 Conclusion

Liste des figures

Liste des tables

Bibliography