Study On The Electromagnetic Field In Small System Of High Energy Heavy Ion Collision

In recent years,people have realized that there maybe is a CP violation effect in quantum chromodynamics(QCD).This effect could be observed as chiral magnetic effect(CME)once coupled to a strong magnetic field.The high energy heavy-ion collision could give an excellent environment for studying CME,because not only the quark-gluon plasma(QGP)with high temperature could be generated but also an extremely strong magnetic field is produced.CME predicts the generation of an electric current along the direction of the external magnetic field in QGP.Thus a charge separation effect of final hadron distribution could be observed in heavy ion collisions.The contribution of CME to the charge separation effect related not only with the strength of magnetic field,but also with the angle between direction of magnetic field and reaction plane.In the heavy ion small collision system with p+A or d+A collision,people used to believe that the direction of magnetic field is determined by the distribution of spectators mainly,while the direction of reaction plane is computed from the space distribution of participants.Due to the decoupling of angular correlation between the reaction plan and direction of magnetic field,this charge separation effect should be vanished.However,the experiment data show a quite different result compare to theory predictions.This is so called puzzle of CME in small system collisions.Aiming at this puzzle in this thesis,by employing a new physical charge profile model inside a proton and a neutron,we calculated the property of electromagnetic field produced in small system in RHIC and LHC energy systematically,including the space profile of electromagnetic field and its dependence on impact parameter b.Especially,we have studied the angular correlation function between direction of magnetic field and reaction plane carefully.Conflict with people’s previous expectations,our results show a significant angular correlation between them.This correlation comes mainly from the very close distance between the location of projectile proton and the location of C.M.S of hot medium produced in small collision system.Then the contribution of projectile proton to the magnetic field is the main source after average over contributions from other participants.So the strong angular distribution priority of projectile could contribute significantly to the angular correlation between direction of magnetic field and reaction plane.This discovery breakthrough people’s previous thought about magnetic filed produced in small collision system.It can help us to clarify contribution of CME to the observation in small system collision experiments.