High-precision Top Quark Mass Related Research

The discovery of the Higgs particle is undoubtedly one of the most important advances in particle physics in recent years.Its experimental data are in line with the predictions of Standard Model（SM）,and no New physics has been found so far.In order to explore new physics beyond the standard model,it is necessary to study the properties of the top quark,especially its mass.For example,the Coleman-Weinberg potential of the Higgs particle is closely related to the mass of the top quark.When the higgs potential is extrapolated to a larger energy scale(such as the Planck energy scale:MP～1.22×10¹⁹ GeV),the mass of the top quark will have a great influence on the Electroweak vacuum stability.At large energy scales,if the top quark has a large mass,the quartic running coupling coefficient of the Higgs boson in Standard Model is likely to be less than zero,and its meaning that the electroweak vacuum of Standard Model is unstable.In this paper,we aim to study how to select a reasonable mass scheme and accurately measure the mass of the top quark by threshold scan.In chapter 1,we first give a detailed introduction to the three main decay modes of the top quark（Fully-leptonic decay,Fully-hadronic decay and Semi-leptonic decay）.Then,we described two main methods for measuring the the top quark mass in reality:the method of kinetic reconstruction and the method of theoretical and experimental comparison.Although we can also measure the the top quark mass under these two main measurement methods,they are far from the measurement accuracy we need at present,so it’s necessary to find a new measurement method.In addition,we introduced the current measurement results of the top quark mass and closely relate the top quark mass to the electroweak vacuum stability and draw a two-dimensional phase diagram with the top quark mass and higgs mass as the y-coordinate and x-coordinate respectively.This two-dimensional phase diagram divides the electroweak vacuum into three regions:EW vacuum unstable region,EW vacuum metastable region and EW vacuum stable region.In this way,we can determine the state of the electroweak vacuum by the mass of the top quark and the mass of the higgs particle measured in experiments.So,it’s a very meaningful thing to accurately measure the top quark mass.In chapter 2,according to the definition of the top quark mass,we make a comprehensive summary for the top quark pole mass,short-distance mass and threshold mass,especially the ambiguity of the top quark pole mass.In addition,inspired by the definition of other masses（such as （?） mass and PS mass）,we tried to define a new mass,that’s the （?） mass.In chapter 3,we explained the theoretical calculation method for production cross section.First of all,we calculated the LO and NLO production cross sections of the e⁺e^-→ y*（?） process in detail.Then,we discuss the divergence problem in e⁺e^-→y*→ （?）g process and prove strictly that there is only soft divergence but no collinear divergence.At the end of chapter 3,we introduced the calculation method of （?） bound state production cross section,and present the relevant literature to the reader.In chapter 4,we mainly talked about the threshold scanning measurement scheme of the top quark mass.In this chapter,we first give a brief introduction to the used QQbar_threshold program package,which includes the installation method and use method of the QQbar_threshold program package.Then,we discuss the stability of （?） threshold and the impaction of higher order correction on the cross section under different mass schemes.Finally,we discussed the error in the measurement of the top quark and analyze the impaction of the change of single parameter on the mass of the top quark and it threshold and production cross section.For example,we illustrated the impaction of two-parameter variation on （?） production cross section and threshold,and discussed the impaction of the Yukawa coupling of Higgs boson and top quark on （?） threshold and production cross section,and gave a rough calculation result.In chapter 5,which is the last chapter of this paper,we sort out the research content and overall framework.