Studies On Probes Of Photon And Muon In Ultra-high Energy Induced Nuclear Reactions

Investigating origin of mass and microstructure of matter are frontier fields in physics research, so that large numbers of theoretical studies and high energy experiments have been developed. The two major branches of ultra-high energy nuclear induced reactions are ultra-high energy heavy ion collision experiments and ultra-high energy cosmic ray experiments,respectively.The underlying theory of strong interactions,Quantum Chromo-Dynamics(QCD),predicts that nuclear matter undergoes a phase transition to a new state under extreme conditions of hadronic densities and/or temperature.Such a state is called quark-gluon plasma(QGP),where quarks and gluons are deconfined.Even if the QGP is formed,it can only exist for a very short period of time and then undergoes a phase transition to QCD state after expansion and cooling of the system.According to "Big bang" theory,the initial universe after the big bang would be likely to undergo a phase transition from QGP to QCD and then the final universe was formed after very instant evolutions.In ultra-high energy heavy ion collision experiments,heavier and heavier nucleus are accelerated,beam energy becomes higher and higher,energy density of final state of the system becomes higher and higher.So it's possible that "Little bang" condition can be reached by the artificial accelerator experiments.Meanwhile,high energy cosmic ray measurement experiments have been established for direct measurements of cosmic particle.In this way people hope to find answers of origin of the cosmic ray and its accelerations mechanism,which help people to understand interactions among the celestial bodies and the evolution processes of the universe.Theories of QCD phase transition and QGP formation lead to development of the ultra-high energy heavy ion collision experiments.Due to the very short lifetime of QGP(typically at a time scale of 1 fm/c)and complicated influences on it caused by nuclear effects,it becomes the most important goal that measuring precisely the signatures of QGP formation in ultra-high energy heavy ion collisions.Electromagnetic probes,photons and dileptons,produced at the initial phase of the collision system,are considered as rather pure signatures to explore properties of the QGP.The specific advantage of photons and dileptons is that because of the very nature of their interactions,they tend to leave the system without any distortion of their energy-momentum. Thus they carry the information of initial behavior of the system,and also the information of thermodynamical properties of the QGP.Therefore,the electromagnetic signatures are important probes to discover and study the QGP.In cosmic ray measurement experiments gamma and muon are also important means to study the origin and acceleration mechanism of the cosmic rays. This dissertation focuses on probes of photon and muon and their detections in ultra-high energy nuclear induced reactions and the Earth's atmosphere shower simulation.The first chapter is an introduction for the ultra-high energy induced nuclear reactions. Firstly history and status of the ultra-high energy nuclear induced reaction experiments are reviewed.Then dynamics,soft physics and several theoretical models in ultra-high energy nuclear induced reactions are discussed.Finally a lot of possible signatures of QGP formation are introduced.Photons are produced at different evolution stages,and their production rates and momentum distributions are dependent on the momentum distributions of quarks,anti-quarks and gluons in QGP,and these distributions are determined by thermodynamical properties of the QGP.So photons produced in the QGP can reflect well the thermodynamical information of the system at the time of their production.Variances of invariant mass spectrum of the dileptons are related to the variances of initial quark distribution of the system.So the initial temperature of the system can be determined by the invariant mass spectrum.We give a detailed description for photon and muon physics,and also discusses their production mechanisms in the second chapter. At last,we present some experimental results on photon and muon physics in ultra-high energy induced nuclear reaction experiments.Collisions at LHC/ALICE will create a system which has higher energy density,larger volume and longer time scale than the counterparts in BNL/RHIC,and these offer good conditions to detect signatures of the QCD phase transition and the QGP formation.ALICE can measure and identify photons over a broad p_T range with photon spectrometer(PHOS),which is an electromagnetic calorimter of high granularity.In the third chapter we introduce the ALICE experiment and physics performance of the PHOS.Trigger decision criterion of the ALICE/PHOS is studied with Monte Carlo simulation.In order to estimate its trigger efficiency and trigger rate, we simulate p-p collision events with PYTHLA and Pb+Pb collision events with HIJING within AliROOT environment.Finally we present our simulation results.This study is meaningful to the experimental data decision,trigger and saving for the ALICE/PHOS,and is also important to parameters' setting of front-end electronics of the PHOS.Resistive plate chamber(RPC)has been applied in several major ultra-high energy induced nuclear reaction experiments performed both with cosmic rays and at accelerators.Noticeable advantages of using RPC are that it is less expensive,very flexible for designing external signal readout and its good space and time resolutions for charged particles.The RPC detector are mostly used to detect muons.In the fourth chapter we give a study on readout and cross-talks for multi-strip RPC with PSpice simulation.An electrical model for multi-strip RPC is presented, and comparison between simulation results and cosmic ray test data is carried on.Based on the model,the influences of the RPC's design parameters on the readout are studied with PSpice simulation.Cross-talks(CT)phenomenon is observed and the relative amplitudes of the cross-talks are studied for different design parameters.The simulation results show a good agreement with the test data and also are in accordance with theoretical conclusions.The development of RPC detector is not only important to the CERN/LHC,but also is important to the other high energy experiments.The electrical computing model and qualitative conclusions presented in the dissertation are meaningful of guidance for extensive application of the RPC detectors in high energy experiments.The origin and acceleration mechanism of the high energy cosmic rays are unsolved problems in cosmic ray physics.In addition,the influences on electronic manufacture,biology and climate caused by cosmic rays attract people's attention.So,large number of theoretical studies and cosmic ray measurement experiments have been developed.Primary cosmic ray particle penetrates into the Earth's atmosphere and interacts with air molecules,then produces an atmosphere shower.Computer simulation is one of the most convenient and effective tools to quantitatively analyze such atmosphere showers.In the fifth chapter,Geant4 Monte Carlo simulation package EASS(the Earth's Atmosphere Shower Simulation)is developed,and its structure and characteristics are well introduced.The Earth's atmosphere shower and influences of geomagnetic field on it are studied with EASS.The well known International Geomagnetic Reference Field(IGRF) as the internal geomagnetic filed and Tsyganenko 1989 model for the external geomagnetic field are introduced into the EASS package used to study precisely the propagation of high energy cosmic ray particles in the Earth's atmosphere.The preliminary results of energy distribution, longitudinal and lateral developments with or without geomagnetic field are presented.The simulation results with EASS show an agreement with theoretical and experimental conclusions. Compared with other programs on cosmic ray physics,the EASS shows four noticeable advantages: 1)the Earth's atmosphere model is defined precisely,2)geomagnetic field models are planted successfully,3)complete physical processes,4)modular structure of the EASS package, and easily upgrade for much higher energy.The work of this chapter is useful to help people effectively study the Earth's atmosphere shower physics,and can give valuable suggestions to the high energy cosmic ray measurement experiments.In the sixth chapter we give a conclusion for our work.An overview of research status of soft physics in ultra-high energy nuclear induced reaction experiments is presented from BNL/AGS and CERN/SPS to BNL/RHIC and CERN/LHC in appendix of the dissertation.For the AGS and SPS experiments,no direct or not enough determinable evidences for the QGP formation were found.At BNL/RHIC experiment a strongly interacting QGP(called sQGP)was discovered and T.D.Lee called this is "a historical event", and "it will be produced at LHC".In this part,five parts of soft physics are reviewed and discussed in detail,and they are collision geometry,particle production,correlation & fluctuation, collective expansion and hadronization,respectively.At last an outlook on soft physics for the LHC is presented.