Search For Long-lived Axion-like Particles With Far Detectors At Future Lepton Colliders

Many beyond standard model theories have predicted the existence of long-lived particles（LLPs）,so searching for LLPs at colliders is one important way to discover and test new physics.After LLPs are produced at colliders,they travel some distance and then decay inside the detector,producing a signal with unique characteristics.In recent years,the study of LLPs at colliders has received extensive attention.In this thesis,we explore long-lived axion-like particles（ALPs）using far detectors that may be installed at future electron-positron colliders.ALPs are pseudo-Nambu-Goldstone bosons produced after a global U（1）symmetry breaking,which is introduced to solve the strong CP problem in the strong interaction.Their couplings with the standard model particles are proportional to 1/Λ,whereΛis the energy scale of the global U（1）symmetry breaking,and their masses and couplings with standard model particles are independent of each other.In this thesis,we first review the experimental research status of ALPs at colliders and the phenomenological research status at electron-positron colliders,and then introduce the structure of near detectors and the operation modes of next-generation circular and linear electron-positron colliders.At future electron-positron colliders,new detectors can be installed at a position far from the Interaction Point（IP）,where LLPs produced can travel long distances to decay inside the far detector.In principle,such new detectors can enhance the discovery potential of LLPs.After describing eight far detector designs with different geometric parameters and explaining the reasons for each design,we discuss the discovery potential of the different designs of far detectors to detect long-lived ALPs in detail.In this thesis,we consider the signal process e^-e⁺→γa,a→γγ,in which an ALP is produced in association with a photon and then decays into two photons,and then we simulate collision events with a center-of-mass energy of 91.2 Ge V by relevant programs.This thesis presents a search strategy for long-lived ALPs with far detectors.The number of signal events finally observed is proportional to the signal production cross section,the integrated luminosity of the collider,and the average decay probability of long-lived ALPs inside the far detector.In this work,a more accurate method of calculating the average decay probability is adopted,and the relationships between average decay probability and decay length of the ALP at different far detectors are shown.It is found that as the decay length increases,the average decay probability increases rapidly and reaches a peak value.Then,the average decay probability decreases steadily as the decay length continuously increases.This thesis also presents the polar angle distributions of ALPs with different masses,and it is found that most of the long-lived ALPs move along a direction perpendicular to the incoming beam.The signal cross section considered in this study depends on the coupling coefficients of ALP to two photons C_γγand ALP to a photon and a Z-boson C_γZ.Since it is difficult for standard model particles to reach the far detector,the background is assumed to be zero.Sensitivity reaches on the parameter spaces spanned by the ALP’s mass and couplings at different far detectors and with various integrated luminosities of 16 ab^–1,150 ab^–1and 750 ab^–1 are given in three scenarios:C_γZ=0,C_γZ=C_γγ,and both C_γZ and C_γγcan be freely changed.The results of this study show that,reducing the distance between the far detector and the IP,increasing its volume,and installing it in the direction perpendicular to the incoming beam,help to improve the discovery potential of the far detector when searching for long-lived ALPs.The results of this thesis provide an important reference for the optimization and construction of far detectors at future electron-positron colliders.