Multi-Pion HBT Correlations In The Heavy-Ion Collisions At LHC Energy

Pion multiplicity in the heavy-ion collisions at the Large Hadron Collider(LHC)energies(the center-mass energy of nucleon-nucleon?TeV)can reach several thousands.In this case,the Bose-Einstein condensation of identical pions may appear in the particle-emitting sources,and can suppress the Hanbury Brown and Twiss(HBT)correlations of the identical pions.Recently,the ALICE Collaboration at European Organization for Nuclear Research(CERN)observed significant suppressions of the three-and four-pion correlations in Pb-Pb collisions at(?)=2.76 TeV.This may indicate the pion emissions in the collisions have a considerable coherence.In order to explain the experimental measurements,we investigate systematically the three-and four-pion HBT correlations and the normalized three-and four-pion correlation functions in the collisions,based on the expanding pion gas model with identical pion Bose-Einstein condensation and the granular source model with coherent pion-emission droplets.The dissertation is organized as follows:Chapter 1 briefly introduces the physics background of relativistic heavy-ion colli-sions,recent experimental measurements of multi-pion HBT correlations,and our research motivations.Chapter 2 introduces the basic theories involved in the dissertation.They are quan-tum statistics theory of two-pion HBT correlation,Bose-Einstein condensation of identical pion gas,granular source model of pion emission in relativistic heavy-ion collisions.Chapter 3 investigates the three-and four-particle HBT correlations of identical pions in an expanding pion gas(EPG)model with Bose-Einstein condensation.The Bose-Einstein condensation and its influence on the multi-pion correlations are studied for the particle-emitting sources with different sizes,temperatures,and particle numbers.The momentum dependence of this influence is investigated.The multi-pion correla-tion functions in the EPG model are analyzed in different multi-pion average transverse-momentiun intervals and compared with the experimental data for Pb-Pb collisions at(?)=2.76 TeV.It is found that the multi-pion correlation functions and cumulant cor-relation functions are sensitive to the condensation fraction of the EPG sources in the low average transverse-momentum intervals of the three and four pions.The model results of the multi-pion correlations are consistent with the experimental data in a considerable degree,which gives a source condensation fraction between 16%and 47%.Chapter 4 calculates the normalized three-and four-pion HBT correlation functions for the EPG sources with Bose-Einstein,condensation and investigates the variations of the multi-pion correlation functions with the condensation of particle-emitting source in different multi-particle average momentum intervals.The reason of the enhancement of normalized multi-pion correlation functions in a high multi-particle relative momentum region is studied.It is found that the intercepts of the normalized correlation functions at zero multi-particle relative momentum are sensitive to source condensation and parti-cle momentum.The normalized three-and four-pion correlation functions in low average transverse-momentum intervals are sensitive to source condensation.However,this depen-dence becomes weak in an intermediate average transverse-momentum interval because the particles with high momenta are likely emitted from excited states incoherently in the EPG model,even if the source has a considerable condensation fraction.In a wide momentum range,the normalized correlation functions for low source temperatures are enhanced at large multi-particle relative momenta because of a rapid increase of two-pion chaoticity parameter with increasing particle momentum.It is hoped that the significant enhancement of the normalized four-pion correlation function will be identified through future analyses of experimental data.Chapter 5 studies the three-and four-pion HBT correlations and the normalized three-and four-pion correlation functions in a granular source model with coherent pion-emission droplets.It is found that the intercepts of multi-pion correlation functions at the multi-particle relative momentum near zero are sensitive to the droplet number in granular source.They decrease with decreasing droplet number.In different multi-particle average transverse-momentum intervals,the model results of three-and four-pion correlation functions are in basic agreement with the experimental data measured by the ALICE Collaboration in central Pb-Pb collisions at(?)=2.76 TeV at the LHC.The normalized four-pion correlation function is sensitive to the momentum dependence of emission coherence.Its rise in moderate relative-momentum region is a signal that high-momentum pions are more possibly emitted chaotically compared to low-momentum pions which may condense in the ground state due to identical boson condensation.Chapter 6 gives the summary and innovation points of the dissertation.An outlook of further researches is also presented.